# Instability of non-constant harmonic maps for the dimensional equivariant wave map system

Size: px
Start display at page:

Download "Instability of non-constant harmonic maps for the dimensional equivariant wave map system"

## Transcription

1 Instability of non-constant harmonic maps for the dimensional equivariant wave map system Raphaël Côte Abstract In this paper we study (1+2)-dimensional equivariant wave maps to a surface N. This is, after reduction, solutions u : R t R r R to the following initial value problem : { utt u rr 1 r u r = f(u) r 2, (u, u t ) t=0 = (u 0, u 1 ). (f depends on N). We consider the existence of a finite energy harmonic map Q (a stationary solution), and show that when it exists, Q is instable in the energy space. Our result applies in particular to the case of wave maps to the sphere S 2, and to the critical Yang-Mills equations in dimension 4. 1 Introduction 1.1 Recall of known results Let us introduce a function g C 1, such that g(0) = 0, and f = g g. We consider the following initial value problem on function u : R t R + r R : u tt u = f(u) r 2, (1) (u, u t ) t=0 = (u 0, u 1 ). (We denote u = u rr + u r /r = 1/r(ru r ) r the radial Laplacian in R 2 ). This problem has the following geometric interpretation. Let N be a surface of revolution with polar coordinates (ρ, θ) [0, ) S 1. Let ds 2 be the metric on N : ds 2 = dρ 2 + g 2 (ρ)dθ 2. (2) A wave map is a function U : R 1+2 N satisfying the system : { U α + Γ α βγ (U) au β a U γ = 0, (U, U t ) t=0 = (U 0, U 1 ). Mathematical Subject Classification : 35L15. Keywords : Instability, wave map, criticality. (3) 1

2 (Γ α βγ are the Christoffel symbols for (N, ds2 ), α = ρ or θ). Denote (r, φ) the usual polar coordinates on R 2. We are concerned with the corotationnal equivariant case, that is, we impose that U has the form ρ = u(t, r), θ = φ. The wave map system (3) then simplifies to a single nonlinear scalar equation for u : R t R + r R, which is (1) (see the book by Shatah and Struwe [4] for further details). Of course, any result on u has a reformulation for U. At least formally, one has conservation of one quantity, that is energy : ( ) E(u) = u 2 t + u2 r + g2 (u) r 2 rdr = E(u 0, u 1 ). (4) (In fact E(u) = U t 2 L 2 + U 2 L 2 ). Let us introduce : { H = (u, v) (u, v) 2 H def = E(u, v) = (v 2 + u 2r + g(u)2 r 2 ) } rdr <. (5) H appears as an energy space, in which it is natural to study the solutions to (1). In [6], Shatah and Tahvildar-Zadeh proved local in time existence and uniqueness of solutions to (1) arising from initial data in the energy space : Local existence in H [6, Theorem 1.1]. Let (u 0, u 1 ) H. Then there exist T > 0 and a unique solution u to Problem (1) such that : (u, u t ) L ([0, T ), H), u L q ([0, T ), Ḃ1/2 10/3,10/3 (R+, rdr)). Let us define the usual notation : E(u, a, b) def = b a ( ) u 2 t + u2 r + g2 (u) r 2 rdr. One way to express the finite speed of propagation is the fact that the energy is decreasing on light cones : R 0, τ R, E(u(t), 0, R τ ) E(u(t + τ), 0, R). (6) One should also notice that Problem (1) has a natural scaling : u(t, r) is a wave map u λ (t, r) def = u(λt, λr) is a wave map. A straightforward computation gives : ( ) E(u λ ) = λ 2 u 2 t + λ 2 u 2 r + λ 2 g2 (u) (λr) 2 rdr = E(u). 2

3 The energy remains unaffected by scaling : Problem (1) is thus said to be (scaling-)critical for the energy. The main remaining open question for this problem is global well-posedness. The result by Shatah and Tavildar-Zadeh [6] solves in particular the case of small energy data : there exists a constant ε 0 > 0 such that if E(u) = E(u 0, u 1 ) < ε 0, then u is global in time. Another direct consequence of the proof of Theorem 1.1 of [6] is the following condition for blow-up : u blows-up at time T only if lim inf t T E(u(t), 0, T t) ε 0. (7) Remind that due to radial symmetry, concentration of energy can only happen at point r = 0. Under some assumptions on N, the energy can not concentrate, and this is enough to ensure global existence in time : in [5], Shatah and Tavildar- Zadeh proved non-concentration under the assumption g 0 (geodesical convexity). This condition was later laxed by Grillakis to g(ρ) + g (ρ)ρ > 0, and finally by Struwe in [7], to g > 0 for ρ > 0. On the other hand, in [1], Bizoń and al. gave strong evidence of blow-up for system (1), in the case N = S 2, and g = sin. In [7], Struwe proved further that if u does blow up (say at time t = 0), then for a subsequence t n, there exists a scaling parameter λ(t n ) such that λ(t n ) t n and : u(t n + t/λ(t n ), r/λ(t n )) Q(r) in H 1 loc (R1+2 ), (8) where Q is a non-constant harmonic map, i.e. a stationary solution to (1) : Q = f(q) r 2. (9) This proves in particular that if there is no harmonic map, then there is no blow up : this is the case when g > 0 for ρ > 0. Furthermore, [7, Theorem 1] has two corollaries : 1. Let Q be a non-constant harmonic map with least energy. Suppose E(u) E(Q) : then u is global in time. 2. Blow-up (at T ) is characterized by lim inf t T E(u(t), 0, T t) E(Q). In particular, if the initial data is such that E((u 0, u 1 ), 0, R) E(Q), then the corresponding wave map u(t) is defined at least up to time t = R (remind that the energy is decreasing on cones). These results can be reformulated by saying that one can choose ε 0 = E(Q). 3

4 1.2 Statement of the results For a single function u, not depending on time, we shall often note u H instead of (u, 0) H, as well as E(u, a, b) for E((u, 0), a, b). From now on, Q will denote a non-constant harmonic map, i.e. Q satisfies (9). Q is important for the qualitative study of (1). Indeed, notice that our first criterion (7) for blow-up states the impossibility to apply the local wellposedness result. Due to [7], it is in fact equivalent to the formation of a singularity with the well-defined blow-up profile Q, which is a descriptive result. As we shall see, the existence of Q is equivalent to the vanishing of g at some point. The problematic is now : if Q exists, does blow-up occur? In particular, for initial data in a neighborhood of Q, does one has blow-up? Our goal in this paper is to prove the instability of Q in the energy space. This result should be related to previous works on instability of stationary states for other equations. In particular, we refer to [3] for the critical nonlinear Schrödinger equation : { iut + u + u 4/N u = 0, (t, x) R R N, u(0, x) = u 0 (x), x R N, and [2] for the critical generalized Korteweg-de Vries equation : { ut + (u xx + u 5 ) x = 0, (t, x) R R, u(0, x) = u 0 (x), x R. In this context, proving instability for Q should be viewed as a first step toward understanding the blow-up mechanism. More precisely we can build wave maps, which are in an arbitrary neighborhood of Q at time 0, and which change profile to a certain Q(λ ), λ 1 (in fact λ > 1) : Theorem 1. [Geometric instability of Q in H] Suppose g vanishes at some point C > 0 (isolated zero). Then there exists a non-constant finite energy harmonic map Q for Problem (1). Moreover, for any λ 0 > 1, there exist a sequence of initial data (u 0 n, u 1 n) such that (u 0 n, u 1 n) (Q, 0) H 0 as n, (10) and if we denote the arising wave maps u n (solutions to (1)), u n is defined at least up to some time t n such that (u n, u nt )(t n ) (Q(λ 0 ), 0) H 0 as n. (11) We shall call this change of profile (from Q to Q(λ 0 )) geometric instability. Notice, as proven in Corollary 4, that one must have t n. As a direct consequence, we have : 4

5 Corollary 1. Let Q be a finite energy harmonic map for Problem (1). Then Q is unstable in H. For some constant C(λ 0 ) > 0 independent of n : sup (u n, u nt ) (Q, 0) H C(λ 0 ). t [0,t n] As Problem (1) has no symmetry besides scaling, this result also proves orbital instability. Of course, as mentioned earlier, this result can be rewritten as geometrical instability of harmonic maps for the wave map system (3). Theorem 1 deals in particular with the following two special cases. Corollary 2 (Wave maps to the 2-sphere). It corresponds to the case g = sin : C = π. The equation takes the form (cf. [1] for more details) : sin 2u u tt u = 2r 2. And the harmonic solution is explicit : Q(r) = 2 arctan(r). Then Q is geometrically unstable. Q can be seen as the minimal (in the sense of energy) connection between the north and south poles. Theorem 1 is here in agreement with the numerical investigation of [1]. Corollary 3 (Critical Yang-Mills equation in dimension 4). It corresponds to g(ρ) = (1 ρ 2 ) : C = 1, which gives the equation : u tt u = 2u(1 u2 ) r 2. Then the harmonic map Q(r) = tanh(ln r) = r2 1 r 2 +1 is geometrically unstable. Here, one should think of a slight modification of our setting, as ũ = u+1 is the wave map - with g replaced by g(ρ) = g(ρ 1) = ρ(2 ρ) : the main difference will be that ρ [ 1, ). The proof is organized as follows : in Section 2.1 we characterize harmonic maps and their properties. In Section 2.2, we study special, infinite energy wave maps, which are in fact related to harmonic maps : the self similar solutions. Then, in Section 2.3, we regularize these solutions to obtain wave maps with initial data in a neighborhood of Q. Finally in Section 2.4, using finite speed of propagation, we exhibit a family of initial data satisfying Theorem 1. Acknowledgments. I thoroughly thank my advisor Frank Merle for numerous fruitful discussions, and the Department of Mathematics of Princeton University, where this work has been done. 5

6 2 Proofs 2.1 On stationary solutions to (1) Recall that the existence of a non constant harmonic map implies that g vanishes at some point (besides 0) : it is a consequence of [7]. In the following, we suppose that there exists a least positive real number C such that g(c ) = 0. Without loss of generality, we can suppose that g(ρ) > 0 for ρ (0, C ). Finally, as to avoid degeneracy of the energy, we assume that g has only isolated zeros. We shall denote G(ρ) = ρ 0 g(ρ ) dρ : G is increasing. Proposition 1. For any g such that g(c ) = 0, there exists a non constant finite energy harmonic map Q, i.e. a solution to (9). Furthermore : 1. Regularity for Q. Any harmonic map Q is of class C 2 and satisfies one of the equations : rq r = g(q), or rq r = g(q). As a consequence, Q is monotone, and joins 2 consecutive zeros of g. 2. Variational characterization of Q. Suppose that Q joins 0 to C. Then Q is of minimal energy for this property : for a function v, such that v(0) = 0, and v(r) C as r, then E(v) E(Q) = v(r) = Q(λr) for some λ > 0. We will use many times the following simple inequality : Lemma 1 (Pointwise inequality). For a finite energy function v : E(v, α, β) = β α ( ) vr 2 + g2 (v) β r 2 rdr 2 g (v)v r dr α 2 G(v(β)) G(v(α)). (12) Proof. First, let us prove the existence of a harmonic map. Let Q be a maximal (in the sense of Cauchy-Lipschitz) solution to Suppose Q is defined on (a, b). Then : b a ( ) Q 2 r + g2 (Q) r 2 rdr = 2 rq r = g(q), Q(1) = C /2. (13) b a Q r g(q)dr = 2(G(Q(b)) G(Q(a))). 6

7 Now, as u = 0 and u = C are solutions, by uniqueness, we always have Q(r) (0, C ) for r > 0. This proves both that Q defined for r [0, ) and that Q is of finite energy 2G(C ). If we differentiate (13), we obtain : Q = 1 r (rq r) r = f(q) r 2. As Q r (1) 0, Q is not constant : it is the desired harmonic map. Let us now prove the properties for any harmonic map. We denote r = e x, so that r / r = / x, and thus equation 9 writes : Q xx = f(q). (14) The Theorem of Cauchy-Lipschitz allows us to solve (14) ; this ensures that Q is C 2 where it is defined. Multiply (14) by Q x and integrate between a and b : [Q 2 x] b a = [g 2 (Q(x))] b a. (15) On another side, for any finite energy function v(r), in view of (12), G(v) satisfies the Cauchy criterion as α, β 0, and as α, β. Thus G(v) admits limits G(v)(0) and G(v)( ), at r = 0 and r respectively. As G is increasing and continuous, it is a homeomorphism, and in fact v admits limits in 0 and, v(0) and v( ). Of course, as v is of finite energy, we must have : g(v(0)) = g(v( )) = 0. Therefore, Q admits limits at r = 0 and at (or in the x variable, at ± ), that are zeros of g. In (15), fix b, and let a. The left hand side has a limit, so that Q x has a limit l at. If l 0, then Q has no limit at, a contradiction. Hence Q x 0, and finally we get Q 2 x (b) = g2 (Q(b)). We already know that Q joins two zeros of g, say α and β : if they are not consecutive, then for a certain c, g(q(c)) = 0, so Q x (c) = 0. By uniqueness, Q = Q(c) = constant : a contradiction, so a and b are two consecutive zeros. The same argument shows that for all x R, Q(x) (a, b), and that g(q) has a constant sign : as a consequence, Q x does not vanish, hence Q is monotone. We then deduce (for sign reasons) that either : x, Q x = g(q) or x, Q x = g(q). Finally, let us prove the the minimizing property. Up to rescaling of v, we can suppose that v(1) = Q(1). Now, using (12) : E(Q, 0, 1) = 2 G(Q(1)) G(Q(0)) = 2 G(v(1)) G(v(0) E(v, 0, 1). 7

8 In the same way, E(Q, 1, ) E(v, 1, ), and we obtain E(v) E(Q). Thus E(v) = E(Q) (note that this already shows that Q is minimizing). More precisely, using 12, we have for all r : E(v) = E(v, 0, r) + E(v, r, ) ( G(v(r)) G(0) + G(C ) G(v(r)) ) E(Q) = E(v). So there is in fact equality everywhere. As a consequence, v is non-decreasing (so as not to lose any energy), and takes its values in (0, C ) for r > 0. The energy equality also gives : v 2 r = g 2 (v)/r 2. As before, using the fact that v is non-decreasing, we get : v r = g(v)/r. As v(1) = Q(1), v = Q. From now on, Q will denote a harmonic map joining 0 to C. As g > 0 on (0, C ) : rq r = g(q). (16) Proposition 2 (Decomposition). There exists α 0 > 0 and an increasing function δ : [0, α 0 ] R +, with δ(α) 0 as α 0, such that the following is true. Suppose v is a function of finite energy, with v(0) = 0 and v(r) C as r and such that : E(v) = E(Q) + α < E(Q) + α 0. Then there exist λ R +, ɛ H, such that : v(r) = Q(λr) + ɛ(r), ɛ H δ(α). Proof. Follows from the variational characterization of Q : cf. Appendix A. Let u be a wave map with energy lower than E(Q) + α 0. Such a decomposition exists for all u(t) : this gives two functions λ(t) and ɛ(t, r) (the proof of Proposition 2 shows that they are continuous functions of t) such that u(t, r) = Q(λ(t)r) + ɛ(t, r). Now blow up occurs only when there is concentration of at least E(Q) energy. As ɛ(t, r) has a small energy, it cannot lead to blow up : only the Q(λ(t)r) can do it, which it equivalent to λ(t). On the other side, if λ(t), then the initial blow-up criterion is fulfilled, and the result of [7] applies. Finally u blows up at time T if and only if λ(t) as t T. In this setting, Theorem 1 gives the existence of a wave map whose scaling parameter λ(t) goes from λ(0) = 1 to λ(t n ) = λ 0 > 0 : this can be seen as a first step toward existence of blow up. Another consequence of Proposition 2 is the following : 8

9 Corollary 4. Let T > 0 and ε > 0. Then there exists η > 0 such that for all initial data (u 0, u 1 ) satisfying (u 0, u 1 ) (Q, 0) H η, the arising wave map u is defined at least up to time T and : sup (u, u t ) (Q, 0) H ε. t [0,T ] In particular, this shows that one can not expect instability of Q for bounded times, and in Theorem 1, t n. The proof is postponed to Appendix A. 2.2 Self-similar solutions on light cones The proof of Theorem 1 relies on the study of self-similar solutions. A self similar solution is a solution u to (1), defined for t < 0, with the ansatz : ( ) r u(t, r) = v. t It of course blows up at time t = 0. Let us first exhibit self similar wave maps. Corollary 5. Define, for ξ < 1 : P α (ξ) = Q ( 2αξ ξ 2 Then P α generates a self-similar solution (on the light cone r < t ). Proof. It is mainly computations with change of variables. We plug the ansatz u(t, r) = v( r/t) (and r/t = ξ) in (1), and write the equation for v(ξ) : u t = r t 2 v (ξ), So v satisfies : ( r 2 u tt = r2 t 4 v (ξ) 2r t 3 v (ξ), u r = 1 t v (ξ), u rr = 1 t 2 v (ξ). t 4 1 t 2 ) ( v (ξ) + 2r t ) v (ξ) = f(v(ξ)) rt r 2. Now we multiply by t 2, replace r/t by ξ, and again multiply by ξ 2 : we get the equation for v in the ξ variable : ξ 2 (ξ 2 1) d2 v dξ 2 + ( 2ξ 3 ξ ) dv dξ ). = f(v). (17) 9

10 We would like to make the first ( order term vanish. For this, let us now do the change of variable χ = ln ξ/(1 + ) 1 ξ 2 ) i.e. ξ = 1/ cosh χ (diffeomorphism for ξ (0, 1), that is r < t ), then : dv dξ = 1 ξ dv 1 ξ 2 dχ, d 2 v dξ 2 = 2ξ 2 1 dv ξ 2 (1 ξ 2 ) 3/2 dχ + 1 ξ 2 (1 ξ 2 ) d 2 v dχ 2. We plug this last relation in (17), the equation simplifies to d 2 v dχ 2 = f(v). But this is simply equation (14), whose solutions are ( ) 2αξ Q(2α exp χ) = Q 1 +, 1 ξ 2 according to Proposition 1 (recall x = exp r). This computation motivates the definition, for b > 0 : ( ) S(b; r) def 2r = P 1/b (br) = Q 1 + for r 1 1 b 2 r 2 b. (18) Indeed if we define ( ) ( ) S(b; t, r) def r r = P 1/b = S b;, (19) t b t S(b; t, r) satisfies system (1) with initial data (at time t = 1/b) : { S(b; 1 b, r) = S(b; r), S t (b; 1 b, r) = brs r(b; r). (20) S(b; t, r) is defined in the interior of the cone {(t, r) r b t } ; it blows up at time t = 0, and its life-span is 1/b. Observe that from the proof of Corollary 5, it follows that S(b; r) satisfies the equation : (b 2 r 2 1) S(b; r) + b 2 f(s(b; r)) rs r (b; r) = r 2. One important thing to notice is the following : Claim. { A 0, limb 0 (S(b; r), brs r (b; r)) (Q, 0) H([0,A]) 0, but b > 0, S(b, r) H = +. This is due to the specific form of P α. If the convergence were in H(R + ), we would obtain a family of blowing up wave maps, whose initial data would 10

11 converge to Q (in H). But these blowing up wave maps are always of infinite energy. Indeed, if we compute at point ξ = 1 (c.f. (45)) : ( S r b; 1 ) ( ) 1 b b ε Q b ε, (as Q (1/b) 0), and Sr 2 (b) C b ε gives a logarithmic divergence when integrating in rdr, and thus an infinite energy. However, if we were to forget this infinite energy tail, S(b; t, r) = S(b; r/(b t ) has the profile Q(λ(t)r) with λ(t) = 1/(b t ). We recover the fact that λ(t) as t 0, that is, blow-up. Furthermore 1/ t is the self-similar blow-up rate. Notice that this rate can never be the blow-up rate of a finite energy wave map (cf. (8)). In higher dimension, self-similar solutions are some examples of blowing up wave maps with smooth initial conditions (see [4, ch. 7]). In dimension 2, no blowing up wave map of finite energy is known. The next step is to regularize S(b; r) to obtain initial conditions in H. 2.3 Construction of approximation of self-similar profiles in the energy space For simplicity in writing expressions in throughout this section, let us first introduce the following notations. Let c 0 < 1. Notation. We define : C(c 0 ), a constant that may change form line to line, but which does only depend on c 0 < 1 (as c 0 1, C(c 0 may tend to + ). the interval I(b, r) = [r, 2r/(1 + 1 b 2 r 2 )], the functions h(r) = sup ρ [Q(r),C ] g(ρ) = sup θ [r, ) g(q)(θ), and h 2 (r) = 1 r r 2 0 h2 (s)sds. Observe that if g is decreasing in a neighborhood of C, (which is always the case if g (C ) < 0), h(r) = g(q)(r) in this neighborhood. In any case, h decreases to 0 as r. In particular, h 2 (r) 0 as r. This gives the existence of a constant A 10 such that for α A : h(α) 0.01, and h 2 (α) This section is devoted to the proof of the following proposition : Proposition 3. Let α, b > 0 such that bα c 0 < 1, and α A. There exist C 2 functions S 0 (α, b; r) and S 1 (α, b; r), defined for r R +, and satisfying : { S 0 (α, b; r) = S(b; r) if 0 r α, S 0 (21) (α, b; r) = Q(r) if r α(1 + h(α)), { S 1 (α, b; r) = brs r (b; r) if 0 r α, S 1 (22) (α, b; r) = 0 if r α(1 + h(α)). 11

12 With the following estimate : (S 0 (α, b; r), S 1 (α, b; r)) (Q(r), 0) 2 H C(c 0)(b 2 α 2 h 2 (α) + h(α)). (23) Remark. 1. These modified profiles are simply truncated self-similar wave maps (at point α), that were smoothly reconnected to Q (at point α(1 + h(α)). 2. As we shall see through the proof, the contribution before truncation is in h 2 (α), and the contribution for joining is in h(α), which is often worse (one should think as h 2 h 2 ). So the global contribution is in h(α). Proof. The proof goes as follows. As the values of (S 0 (α, b; r), S 1 (α, b; r)) are given on [0, α], we only have to compute the desired estimate on this interval : this is done in lemma 2 (pointwise estimate) and 3 (H estimate). On [α, α(1+ h(α))], we need both the construction of a smooth reconnection, and the estimates that goes along with it : this is lemma 4. The part [α(1+h(α)), ) does not add any contribution. The proofs are mainly computational, and are postponed to Appendix B. Lemma 2. If br c 0, then : S(b; r) Q(r) C(c 0 )b 2 r 2 h 2 (r), (24) S r (b; r) Q r (r) C(c 0 )b 2 rh(r). (25) Lemma 3. For bα c 0 and α A, then : S(b; r) Q(r) 2 H([0,α]) C(c 0)b 4 α 4 h 2 (α), (26) brs r (b; r) 2 L 2 ([0,α]) C(c 0)b 2 α 2 h 2 (α), (27) Q H([α; )) 2 C Q(α) h(α). (28) Lemma 4 (Joining lemma). Let v : I = [0, a] [b, ) R. Then there exists ṽ : R + R extending v, as smooth as v, such that : ṽ 2 H v 2 H(I) + (v(b) v(a))2 b + a b a + 2 max(v(a), v(b))2 ln b a. (29) To construct the S i (α, b; r), we only have choice on the interval r [α, α(1 + h(α))]. For S 0 (α, b; r) we use the joining Lemma 4 to obtain a smooth reconnection. For S 1 (α, b; r), the density of smooth functions in L 2 allows us to have a smooth reconnection such that : S 1 (α, b; r) 2 L 2 ([α,α(1+h(α))]) h(α). 12

13 Let us now turn to estimate (23). First, by estimate (27) we have : S 1 (α, b; r) 2 L 2 C(c 0 )b 2 α 2 h 2 (α) + h(α). (30) Let us now focus on S 0 (α, b; r) Q(r). For r α(1 + h(α), there is no contribution. On [0, α], we use lemma 3 : it gives the bound C(c 0 )b 4 α 4 h 2 (α) C(c 0 )b 2 α 2 h 2 (α). (31) On [α, α(1 + h(α))], in order to use estimate (29), we compute (with Rolle s theorem, using h(α) 0.01)) : ( ) S(b; α) Q(α(1 + h(α)) = Q 2α 1 + Q (α(1 + h(α))) 1 b 2 α 2 α (1 + h(α)) 1 b 2 α2 sup Q r (ϑ) α(1 + h(α))h(α)/α h(α). So the second term on the right hand side of (29) can be estimated by (α 10): 2 α(2 + h(α)) 4h(α) 4h(α). αh(α) And for the third term of (29), it remains to notice that S(b; α) 2, Q(α(1 + h(α)) 2 C 2 and the well known : ln(1 + h(α)) h(α). Let s also allow h(α) for the time-derivative estimate on this interval (using the density of regular function in L 2 ). So the contribution of S 0 (α, b; r) Q(r) H on [α, α(1 + h(α))] is bounded by : ϑ α (2C 2 + 4)h(α). (32) Summing up the contributions (31) and (32), we get : S 0 (α, b; r) Q(r) H C(c 0 )b 2 α 2 h 2 (α) + Ch(α). (33) (33) and (30) give the estimate (23). 2.4 Finite speed of propagation and conclusion Before proving Theorem 1, let us remind a consequence of the finite speed of propagation for system (1). Proposition 4. Let (u 0, u 1 ) and (v 0, v 1 ) be a couple of initial data (at time t = 0) with finite energy on the interval [0, R). Let u, v be the respective solutions to (1) arising from them. Suppose that : r [0, R), (u 0, u 1 )(r) = (v 0, v 1 )(r). Suppose that u is defined up to time T > 0, and let T 0 = min{r, T }. Then v does not blow up before time T 0, and for t [0, T 0 ] we have : r [0, R t ), u(t, r) = v(t, r). 13

14 Remark. Of course, if (u 0, u 1 ) and (v 0, v 1 ) coincide on (a, b), with a > 0, then at time t [(a b)/2, (b a)/2], u(t) and v(t) coincide on the interval r (a + t, b t ). Proof. It relies on the proof of Theorem 1.1 of [6] : more precisely, this Theorem is a direct consequence of the following claim, which is what is proved indeed in [6] Claim : There exists ε 0 > 0 such that the following is true. Let (u 0, u 1 ), initial data at time t 0, have energy less than ε 0 on B(R 0, C) = (R 0 C, R 0 + C). Then exists a unique solution u to (1) on the full cone of dependence K(R 0, C) = {(t, r) r R 0 < C t t 0 }. Proposition 4 is also a direct consequence of this claim. Let T 1 0 be the greatest time for which u and v coincide on the truncated cone {(t, r) t [0, T 1 ] and 0 r < R t}. First, T 1 > 0 : indeed, we divide [0, R] into finitely many overlapping intervals (a n, b n ) such that on every interval, the energy of (u 0, u 1 ) is less than ε 0. Thanks to the claim, on every cone of dependence associated to (a n, b n ), v (exist and) coincides with u. As the (a n, b n ) overlap and are in finite number, there exists δ > 0 such that u and v coincide on the truncated cone {(t, r) t [0, δ] and 0 r < R t}. Thus, T 1 δ > 0. Now, if T 1 < T 0 : u(t 1 ) = v(t 1 ), u t (T 1 ) = v t (T 1 ) on [0, R T 1 ), we can repeat the same argument at time T 1 to obtain a greater time for the truncated cone on which u and v coincide, and this contradicts the maximality of T 1. Hence T 1 = T 0. Proof of Theorem 1. The idea is the following. We know the evolution of S(b) : given initial data S i (α, b; r), we deduce from Proposition 4 what happens at time t on the space interval r [0, α t]. The problem is now to choose α = α(b) large enough so that our control takes place for large enough times (so that the scaling parameter λ changes), but not too large so that the initial data is close to (Q, 0) in H : there we need the estimates of Proposition 3. Choose a fixed c 0 < 1, and set : λ 0 = 1 1 c 2 0 > 1 and for b > 0, α = α(b) = c 0 b. 14

15 Define R(b, t; r) as the wave map arising from the regularized initial conditions with α(b) = 2c 0 /b : { R(b; 0, r) = S 0 (c 0 /b, b; r), R t (b, 0, r) = S 1 (34) (c 0 /b, b; r). First, (R(b; 0, r), R t (b; 0, r)) coincide with (S(b, 1/b, r), S t (b, 1/b, r)) on the interval [0, c 0 /b], and is of finite total energy. Thus, Proposition 4 ensures that R(b; t, r) is defined at least up to time T = c 0 /b (as S(b; t, r) is defined on an interval of length 1/b 2c 0 /b), and : [ t 0; c 0 b ], r c 0 b t, R(b; t, r) = S In particular, for t(b) = c 0 (1 c 0 )/b : (b; 1b + t, r ) ( ) r = S b;. 1 bt r c2 0 b, R(b; t(b), r) = S (b; λ 0r). (35) Now, we use the estimate (23) (for t = 0) and we obtain : (R(b; 0, r), R t (b; 0, r) (Q(r), 0) 2 H C(h(c 0 /b)+h 2 (c 0 /b)) 0 as b 0. This is the first condition (10). It remains to estimate what happens at time t(b) = c 0 (1 c 0 )/b, i.e. to bound : (R(b; t(b), r), R t (b; t(b), r)) (Q(λ 0 r), 0) H. Consider separately the contributions on [0, c 2 0 /b] and on [c2 0 /b, ). On the first interval, estimates of Lemma 3 give the bound (R(b; t(b), r), R t (b; t(b), r) (Q(λ 0 r), 0) 2 H([0,c 2 0 /b]) Ch 2(c 2 0/b). (36) On the second interval, we work out separately (R(b; t(b), r), R t (b; t(b), r) H([c 2 0 /b, )) and Q(λ 0 r) H([c 2 0 /b, )). (37) Let us focus on the first term of (37). In view of the initial conditions (34), E(R(b)) E(Q) + C(h 2 (c 0 /b) + h(c 0 /b)). On the other side, we use our control on the interval [0, c 0 /b] : (S(b; λ 0 r), bλ 0 rs(b; λ 0 r)) H([0,c 2 0 /b]) Q H([0,c 2 0 /b]) E(Q) 2(C Q(c 2 0/b))h(c 2 0/b). 15

16 (First inequality because S(b; λ 0 r) Q(r) and the pointwise inequality, second inequality due to (28)). Hence we have (as h is decreasing) : (R(b; t(b), r), R t (b; t(b), r)) H([c 2 0 /b, )) = E(R(b)) (S(b; λ 0 r), bλ 0 rs(b; λ 0 r)) H[0,c 2 0 /b] E(Q) + C(h(c 2 0 /b) + h 2(c 2 0 /b)) (E(Q) Ch(c2 0 /b)) C(h(c 2 0/b) + h 2 (c 2 0/b)). For the second term of (37), using (28), we obtain : (38) Q(λ 0 r) H([c 2 0 /b, )) 2(C Q(λ 0 c 2 0 /b))h(c2 0 /b) = o(h(c2 0 /b)). (39) Finally adding up (36), (38) and (39), we have : (R(b; t(b), r), R t (b; t(b), r)) (Q(λ 0 r), 0) H C(h(c 2 0 /b) + h 2(c 0 /b) + h 2 (c 2 0 /b)) 0 as b 0. This exactly the second condition (11). To conclude, choose a sequence (b n ) decreasing to 0, and define the sequence : u n (t, r) = R(b n ; t, r), t n = (1 c 0 )c 0 /b n. The previous computations show that the initial data (u n (0), u nt (t)) (Q, 0) in H and that (u n (t n ), u nt (t n )) (Q(λ 0 ), 0) in H as n. This is true for any c 0 < 1, and thus for any λ 0 = 1/(1 c 2 0 ) > 1. The instability in H is then straightforward. Remark. 1. As (R(b; 0, r), R t (b, 0, r) (Q, 0) H 0, E(R(b)) E(Q) : for b small enough, R(b) admits a decomposition and a scaling factor λ b (t) for all t up to an possible blow-up time. Theorem 1 simply says that λ b (c 0 /b) λ The proof of Theorem 1 also gives the time to leave a neighborhood of Q. Suppose we start in a δ-neighborhood of Q : then the corresponding b is such that h(c 0 /b) + h 2 (c 0 /b) δ. We have to wait time c 0 /2b to leave the neighborhood, that is approximately h 1 (δ). Let us give an example. In the case of Corollary 2 (g = sin), studied in [1], we have : C = π, and g (π) = 1, so h(r) 1/r, h 2 (r) ln r/r 2, so Q b Q H 1/ b. Thus the time to exit a δ-neighborhood is O(1/δ 2 ). In the case of Corollary 3 (g(ρ) = (1 ρ 2 ), we have : C = 1, g (1) = 2, so h(r) 1/r 2, h 2 (r) 1/r 2, so Q b Q H 1/b, and the time to exit a δ-neighborhood is O(1/δ). 16

17 Appendix A : Decomposition of a wave map with energy close to E(Q) Proposition 2 (Decomposition). There exist α 0 > 0 and an increasing function δ : [0, α 0 ] R +, with δ(α) 0 as α 0, such that the following is true. Suppose v is a function of finite energy, with v(0) = 0 and v(r) C as r and such that : E(v) = E(Q) + α < E(Q) + α 0 Then there exist λ R + and ɛ H such that : v(r) = Q(λr) + ɛ(r), and ɛ H δ(α). Proof. Recall that G(s) = s 0 g, E(Q) = 2G(C ), and that G is increasing. Denote D the unique point such that G(Q(D )) = G(C )/2 (or equivalently E(Q, 0, D ) = E(Q)/2. We proceed by contradiction. Suppose that there exist δ 0 > 0 and a sequence of finite energy functions v n, such that and : E(v n ) E(Q) + 1 n, v n(0) = 0, v n (r) C as r, n N, λ > 0, v n Q(λ ) H δ 0. (40) Set w n (r) = v n (r/λ n ), where λ n is such that w n (D ) = Q(D ) (this is possible because v n is continuous). Using scaling invariance, we have : E(w n ) = E(v n ) E(Q) + 1 n. (w nr ) is L 2 (rdr)-bounded, and so, on (0, ), (w n ) is locally C 1/2 -continuous, and so locally equicontinuous. Furthermore : G(w n (a)) G(w n (b)) b a g(w n (ρ)w nr (ρ) dρ E(w n, a, b). Apply with a 0, b = r, and then a = r, b. As g is not uniformly 0 outside [0, C ], we deduce that for N large enough, the (w k ) k N are uniformly bounded in C 0 ([0, )) by a constant K. So one can apply Ascoli s theorem : for any compact set X of (0, ), (w n X ) n has a compact closure in C 0 (X). Let us construct a diagonal extraction. Let m N, and suppose we already constructed an extraction φ 1 φ m : N N to obtain a converging subsequence w φ1 φ m(n) [1/m,m] w in C 0 ([1/m, m]) as n. 17

18 We can then construct φ m+1 so that the convergence of w φ1 φ m φ n+1 (n) takes place in C 0 ([1/(m+1), m+1]) (by applying Ascoli s theorem on [1/(m+ 1), m + 1]). Now, define : ϕ(n) = φ 1 φ n (n). For any m, and n m, w ϕ(n) is a subsequence of w φ1 φ m(n), and hence converges in the space C 0 ([1/m, m]). Let us denote again w C 0 (R + ) the common limit. For convenience, we can consider the subsequence as the initial sequence, and thus drop the ϕ. We obtained : m N, w n w C 0 ([1/m, m]). We can also suppose that w nx w x weakly in L 2 (rdr). In particular, w is continuous, w(d ) = Q(D ). By weak limit : w 2 r rdr lim inf w n 2 r rdr. And by Fatou lemma (of course, there is a.e. convergence) : g 2 (w)dr/r lim inf g 2 (w n )dr/r. So that E(w) E(Q). Moreover w(d ) = Q(D ). As w is of finite energy, w admits limits in r = 0 and r, where g vanishes : using the pointwise inequality between 0, D and, we deduce that the only possibility for these limits are 0 and C. Let us now prove that : r D, w(r) Q(D ), and r D, w(r) Q(D ). (41) We prove only one of these inequalities, the second one can be deduced in the same way. We again proceed by contradiction. Let r < D such that w(r) > Q(D ) : denote ε = G(w(r)) G(C )/2 > 0. Due to uniform convergence on compact sets, there exists N such that for n N, G(w n (r)) G(C )/2 ε. Then : E(Q) + 1 n E(w n) E(w n, 0, r) + E(w n, r, D ) + E(w n, D, ) 2 [ G(w n (r)) + G(w n (r)) G(C )/2 + G(C )/2 ] 2(G(C ) + 2ε) E(Q) + 4ε. This is impossible if n 1/(4ε), and proves (41). Thus, we conclude that w(0) = 0 and w C as r. Together with E(w) E(Q) and w(d ) = Q(D ), the variational characterization of Q allows to conclude that w = Q. Let us now prove that w n Q H = E(w n Q) 0. First, E(w n ) E(Q), so that lim w nr L 2 (rdr) = Q r L 2 (rdr), and the weak convergence w nr Q r is in fact strong-l 2 (rdr) : w nr Q r L 2 (rdr) 0. (42) 18

19 Let us now consider g 2 (w n Q)dr/r. Let ε > 0. Define c, d > 0 such that E(Q, 0, c) ε/16 and E(Q, d, ) ε/16. The convergence of w n to Q in C 0 ([c, d]) gives the existence of N 4/ε such that n N, d c g 2 (w n Q) dr r ε/2. (because g is continuous at 0). Again by convergence in C 0 ([c, d]), we can choose N such that : n N, G(w n (c)) G(Q(c) + G(w n (d)) G(Q(d)) ε/16. Hence (pointwise inequality), E(w n, c, d) E(Q, c, d) ε/8 E(Q) ε/4. In view of N 4/ε, we obtain So that for n N : g 2 (w n Q) dr r E(w n, 0, c) + E(w n, d, ) ε/2. c 0 + d c + E(w n, 0, c) + ε/2 + E(w n, d, ) ε. d This together with (42) proves that w n Q H = v n Q(λ n ) H 0 as n : a contradiction with (40). Corollary 4. Let T > 0 and ε > 0. Then there exists η > 0 such that for all initial data (u 0, u 1 ) satisfying (u 0, u 1 ) (Q, 0) H η, the arising wave map u is defined at least up to time T and : sup (u, u t ) (Q, 0) H ε. t [0,T ] Proof. We will choose η > 0 small enough later. First, let us notice that we can use the decomposition. Observe indeed that the energy E(u 0, u 1 ) E(Q) + Cη 2 E(Q) + α 0, if η is so small that Cη 2 α 0. Hence, u 0 has limits at r = 0 and as r, that are zeros for g. As 0 and C are isolated zeros of g, we necessarily have u 0 (0) = 0, u 0 (r) C as r, if we choose η > 0 small enough. The local existence Theorem gives a maximal time T > 0 of existence : we can consider u(t) the arising wave map. By conservation of energy E(u) E(Q) + Cη 2, we get : u 2 t rdr Cη2. (43) In particular, as u(t, 0) and lim r u(t, r) are always zeros for g, we obtain that for all t < T, u(t, 0) = u 0 (0) = 0 and lim r u(t, r) = lim r u 0 (r) = C (see [7, Lemma 1]). Therefore, we can apply Proposition 2 : for all t < T, there exists λ(t), ɛ(t, r) such that u(t, r) = Q(λ(t)r) + ɛ(t, r), and ɛ(t) H δ(cη 2 ). 19

20 We can choose λ(0) = 1. Now, as noticed earlier, blow-up for u is characterized by λ(t). From now on, we will consider T T maximal such that : t < T, λ(t) 2 + ε. Then (if δ(cη 2 ) is small enough) : 0 1 a u(t) Q H C λ(t) 1. (44) Now fix a (0, 1/2), such that for r [1 a, 1 + a], Q r (r) Q r (1)/2. Using (43), we have : t 1+a t 1+a u t drdt u 2 t rdr dr dt Cηt. r On the other side : t 1+a 0 1 a u t drdt = = 1+a 1 a 1+a 1 a 0 1 a (u(t, r) u(0, r))dr (Q(λ(t)r) Q(r) + ɛ(t, r) ɛ(0, r)) dr. Now, as ɛ(t) L C( ɛ(t) H ) c(η) (for some function c such that c 0 at 0) : And : 1+a 1 a 1+a 1 a (ɛ(t, r) ɛ(0, r))dr 2c(η). Q(λ(t)r) Q(r)dr 1 1+a 2 λ(t) 1 Q r (1)rdr C λ(t) 1. 1 a Combining our two expressions for the integral, we deduce : λ(t) 1 C(ηt + c(η)). And finally, using (44) and u t 2 L η : (u(t), u t (t) (Q, 0) H C(ηt + η + c(η)). It is enough to choose η α 0 /C so that C(ηT + η + c(η)) < ε. Indeed, the previous computations are then valid up to T. Now if T < T, we have that for all t [0, T ], λ(t) 1 + ε < 2 + ε, therefore the solution can be continued (blow-up hasn t occurred) and the continuity of λ(t) contradicts the maximality of T. Remark. The proof gives a more accurate result : the time to leave a η neighborhood is at least O(1/η). This is coherent with the computations for the sequence u n of Theorem 1. 20

21 Appendix B Here, we prove the computational lemmas needed for Proposition 3. Proof of Lemma 2. Let us note ϑ = 2r 1+. We compute explicitly : 1 b 2 r2 2br 3 S b (b; r) = Q r (ϑ)(1 + ) b 2 r 2 b 2 r 2 So if we plug in br 0.01 and Q r = g(q)/r : Now S b (b; r) C(c 0 )Q r (ϑ)br 3 C(c 0 )g(q)(ϑ)br 2. S(b; r) Q(r) b 0 S b (b; r) db C(c 0 ) sup (g(q))(θ) b 2 r 2. θ I(r,b) For the second estimate, we again compute explicitly : ( 2 S r (b; r) = Q r (ϑ) b 2 r + 2r 2 (1 + b 2 ) r. (45) 1 b 2 r 2 ) 2 1 b 2 r 2 Now, we can compute : 4r 3 b S r,b (b; r) = Q rr (ϑ)( 1 + ) 3 1 b 2 r 2 (1 b 2 r 2 ) ( 2br 2 2b 2 r ) 1 b 2 r 2 Q r (ϑ) ( (1 b 2 r 2 ) 3/2 1 + ) 3. (46) 1 b 2 r 2 Recall Q r = g(q)/r and Q rr = ((g 1)g)(Q)/r 2. We plug in again br c 0 < 1, and we get : S r,b (b; r) C(c 0 )rb ( ((1 g )g)(q)(ϑ) + g(q)(ϑ) ) So when we integrate in b, we have : S r (b; r) Q r (r) b 0 C(c 0 )b 2 r C(c 0 )b 2 r S r,b (b; r) db sup θ I(b,r) ( = C(c 0 )((1 + 1 g )g)(q)(ϑ)br. (g + 1 g g)(q)(θ) 1 + sup g (Q)(θ) θ I(b,r) And as Q takes its values in [0, C ], we get the second estimate. ) sup g(q)(θ). θ I(b,r) 21

22 Proof of Lemma 3. We integrate the pointwise estimate of the previous lemma. α ) S(b; r) Q(r) 2 H([0,α]) ((S(b; = r) Q(r)) 2r (S(b; r) Q(r))2 + 0 r 2 rdr ( α (D0 h(r)b 2 r ) ( 2 h(r)b 2 r 2 ) 2 ) + rdr r 0 α C(c 0 )b 4 h 2 (r)r 3 dr C(c 0 )b 4 α 2 α 2 h 2 (α). 0 For the time-derivative estimates, we use directly (45), and we plug in br bα c 0 < 1. This leads to : so that : S r (b; r) Q r (ϑ) (2 + C(c 0 )b 2 r 2 ) C(c 0 )h(r)/r, α brs r (b; r) 2 L 2 [0,α] C(c 0)b 2 h 2 (r)rdr. For the third bound, we compute, using rq r = g(q) : Q H([α; )) = 2 α 0 C g(q)q r dr = 2 g 2 C Q(α) h(α). Q(α) Proof of Lemma 4. Let us first do the computations for an affine interpolation : For r [a, b], ṽ(r) = (v(b) v(a)) r a b a + v(a), i.e. ṽ r(r) = Then : b a b v(b) v(a). b a ṽr 2 rdr = 1 ( ) v(b) v(a) 2 (b 2 a 2 ) 1 b + a (v(b) v(a))2 2 b a 2 b a, a ṽ 2 dr max(v(b), v(a))2 r b a dr r max(v(b), v(a))2 ln b a. This already gives an extension in H. For a smoother extension, it is enough to regularize ṽ r (locally near a and b) with a small variation of the L 2 norm, while keeping constant ṽ(b) ṽ(a) = b a ṽrdr. This is possible thanks to the density of smooth functions with given mean among L 2 functions with the same given mean. 22

23 References [1] P. Bizoń, T. Chmaj, and Z. Tabor. Formation of singularities for equivariant (2 + 1)- dimensional wave maps into the 2-sphere. Nonlinearity, 14(5): , [2] Y. Martel and F. Merle. Instability of solitons for the critical generalized Korteweg-De Vries equation. Geometric and Functional Analysis, 11(1):74 123, [3] F. Merle. Blow-up phenomena for critical nonlinear Schrödinger and Zakharov equations. In Proceedings of the International Congress of Mathematicians, volume III, pages Documenta Mathematica, [4] J. Shatah and M. Struwe. Geometric Wave equations, volume 2. Courant Lectures Notes in Mathematics, [5] J. Shatah and A. S. Tahvildar-Zadeh. Regularity of harmonic maps from the Minkowski space into rotationally symmetric manifolds. Communications on Pure and Applied Mathematics, 45(8): , [6] J. Shatah and A. S. Tahvildar-Zadeh. On the Cauchy problem for equivariant wave maps. Communications on Pure and Applied Mathematics, 47(5): , [7] M. Struwe. Equivariant wave maps in two space dimensions. Communications on Pure and Applied Mathematics, 56(7): , Raphaël Côte Département de Mathématiques et Applications École normale supérieure 45, rue d Ulm, Paris Cedex 05, France 23

### DETERMINATION OF THE BLOW-UP RATE FOR THE SEMILINEAR WAVE EQUATION

DETERMINATION OF THE LOW-UP RATE FOR THE SEMILINEAR WAVE EQUATION y FRANK MERLE and HATEM ZAAG Abstract. In this paper, we find the optimal blow-up rate for the semilinear wave equation with a power nonlinearity.

### BIHARMONIC WAVE MAPS INTO SPHERES

BIHARMONIC WAVE MAPS INTO SPHERES SEBASTIAN HERR, TOBIAS LAMM, AND ROLAND SCHNAUBELT Abstract. A global weak solution of the biharmonic wave map equation in the energy space for spherical targets is constructed.

### Equivariant self-similar wave maps from Minkowski spacetime into 3-sphere

Equivariant self-similar wave maps from Minkowski spacetime into 3-sphere arxiv:math-ph/99126v1 17 Oct 1999 Piotr Bizoń Institute of Physics, Jagellonian University, Kraków, Poland March 26, 28 Abstract

### Long-term dynamics of nonlinear wave equations

Long-term dynamics of nonlinear wave equations W. Schlag (University of Chicago) Recent Developments & Future Directions, September 2014 Wave maps Let (M, g) be a Riemannian manifold, and u : R 1+d t,x

### Recent developments on the global behavior to critical nonlinear dispersive equations. Carlos E. Kenig

Recent developments on the global behavior to critical nonlinear dispersive equations Carlos E. Kenig In the last 25 years or so, there has been considerable interest in the study of non-linear partial

### Dynamics of energy-critical wave equation

Dynamics of energy-critical wave equation Carlos Kenig Thomas Duyckaerts Franke Merle September 21th, 2014 Duyckaerts Kenig Merle Critical wave 2014 1 / 22 Plan 1 Introduction Duyckaerts Kenig Merle Critical

### BLOWUP THEORY FOR THE CRITICAL NONLINEAR SCHRÖDINGER EQUATIONS REVISITED

BLOWUP THEORY FOR THE CRITICAL NONLINEAR SCHRÖDINGER EQUATIONS REVISITED TAOUFIK HMIDI AND SAHBI KERAANI Abstract. In this note we prove a refined version of compactness lemma adapted to the blowup analysis

### THE WAVE EQUATION. d = 1: D Alembert s formula We begin with the initial value problem in 1 space dimension { u = utt u xx = 0, in (0, ) R, (2)

THE WAVE EQUATION () The free wave equation takes the form u := ( t x )u = 0, u : R t R d x R In the literature, the operator := t x is called the D Alembertian on R +d. Later we shall also consider the

### Blow-up on manifolds with symmetry for the nonlinear Schröding

Blow-up on manifolds with symmetry for the nonlinear Schrödinger equation March, 27 2013 Université de Nice Euclidean L 2 -critical theory Consider the one dimensional equation i t u + u = u 4 u, t > 0,

### Sharp Upper Bound on the Blow up Rate for critical nonlinear Schrödinger Equation

Sharp Upper Bound on the Blow up Rate for critical nonlinear Schrödinger Equation Frank Merle,, Pierre Raphael Université de Cergy Pontoise Institut Universitaire de France Abstract We consider the critical

### The harmonic map flow

Chapter 2 The harmonic map flow 2.1 Definition of the flow The harmonic map flow was introduced by Eells-Sampson in 1964; their work could be considered the start of the field of geometric flows. The flow

### 2 A Model, Harmonic Map, Problem

ELLIPTIC SYSTEMS JOHN E. HUTCHINSON Department of Mathematics School of Mathematical Sciences, A.N.U. 1 Introduction Elliptic equations model the behaviour of scalar quantities u, such as temperature or

### An introduction to Mathematical Theory of Control

An introduction to Mathematical Theory of Control Vasile Staicu University of Aveiro UNICA, May 2018 Vasile Staicu (University of Aveiro) An introduction to Mathematical Theory of Control UNICA, May 2018

### Formation of singularities for equivariant (2 + 1)-dimensional wave maps into the 2-sphere

INSTITUTE OF PHYSICS PUBLISHING Nonlinearity 14 (2001) 1041 1053 NONLINEARITY PII: S0951-7715(01)18802-X Formation of singularities for equivariant (2 + 1)-dimensional wave maps into the 2-sphere Piotr

### Université de Cergy-Pontoise. Insitut Universitaire de France. joint work with Frank Merle. Hatem Zaag. wave equation

The blow-up rate for the critical semilinear wave equation Hatem Zaag CNRS École Normale Supérieure joint work with Frank Merle Insitut Universitaire de France Université de Cergy-Pontoise utt = u + u

### Partial Differential Equations

Part II Partial Differential Equations Year 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2015 Paper 4, Section II 29E Partial Differential Equations 72 (a) Show that the Cauchy problem for u(x,

### Presenter: Noriyoshi Fukaya

Y. Martel, F. Merle, and T.-P. Tsai, Stability and Asymptotic Stability in the Energy Space of the Sum of N Solitons for Subcritical gkdv Equations, Comm. Math. Phys. 31 (00), 347-373. Presenter: Noriyoshi

### LECTURE 15: COMPLETENESS AND CONVEXITY

LECTURE 15: COMPLETENESS AND CONVEXITY 1. The Hopf-Rinow Theorem Recall that a Riemannian manifold (M, g) is called geodesically complete if the maximal defining interval of any geodesic is R. On the other

### The Dirichlet s P rinciple. In this lecture we discuss an alternative formulation of the Dirichlet problem for the Laplace equation:

Oct. 1 The Dirichlet s P rinciple In this lecture we discuss an alternative formulation of the Dirichlet problem for the Laplace equation: 1. Dirichlet s Principle. u = in, u = g on. ( 1 ) If we multiply

### Solutions to the Nonlinear Schrödinger Equation in Hyperbolic Space

Solutions to the Nonlinear Schrödinger Equation in Hyperbolic Space SPUR Final Paper, Summer 2014 Peter Kleinhenz Mentor: Chenjie Fan Project suggested by Gigliola Staffilani July 30th, 2014 Abstract In

### A CONVEX-CONCAVE ELLIPTIC PROBLEM WITH A PARAMETER ON THE BOUNDARY CONDITION

A CONVEX-CONCAVE ELLIPTIC PROBLEM WITH A PARAMETER ON THE BOUNDARY CONDITION JORGE GARCÍA-MELIÁN, JULIO D. ROSSI AND JOSÉ C. SABINA DE LIS Abstract. In this paper we study existence and multiplicity of

### arxiv:math-ph/ v2 22 Jun 2001

Formation of singularities for equivariant 2+1 dimensional wave maps into the two-sphere arxiv:math-ph/115v2 22 Jun 21 Piotr Bizoń, Tadeusz Chmaj, and Zbis law Tabor Institute of Physics, Jagellonian University,

### Part III. 10 Topological Space Basics. Topological Spaces

Part III 10 Topological Space Basics Topological Spaces Using the metric space results above as motivation we will axiomatize the notion of being an open set to more general settings. Definition 10.1.

### Measure and Integration: Solutions of CW2

Measure and Integration: s of CW2 Fall 206 [G. Holzegel] December 9, 206 Problem of Sheet 5 a) Left (f n ) and (g n ) be sequences of integrable functions with f n (x) f (x) and g n (x) g (x) for almost

### A Variational Analysis of a Gauged Nonlinear Schrödinger Equation

A Variational Analysis of a Gauged Nonlinear Schrödinger Equation Alessio Pomponio, joint work with David Ruiz Dipartimento di Meccanica, Matematica e Management, Politecnico di Bari Variational and Topological

### ON WEAKLY NONLINEAR BACKWARD PARABOLIC PROBLEM

ON WEAKLY NONLINEAR BACKWARD PARABOLIC PROBLEM OLEG ZUBELEVICH DEPARTMENT OF MATHEMATICS THE BUDGET AND TREASURY ACADEMY OF THE MINISTRY OF FINANCE OF THE RUSSIAN FEDERATION 7, ZLATOUSTINSKY MALIY PER.,

### Math The Laplacian. 1 Green s Identities, Fundamental Solution

Math. 209 The Laplacian Green s Identities, Fundamental Solution Let be a bounded open set in R n, n 2, with smooth boundary. The fact that the boundary is smooth means that at each point x the external

### CUTOFF RESOLVENT ESTIMATES AND THE SEMILINEAR SCHRÖDINGER EQUATION

CUTOFF RESOLVENT ESTIMATES AND THE SEMILINEAR SCHRÖDINGER EQUATION HANS CHRISTIANSON Abstract. This paper shows how abstract resolvent estimates imply local smoothing for solutions to the Schrödinger equation.

### u xx + u yy = 0. (5.1)

Chapter 5 Laplace Equation The following equation is called Laplace equation in two independent variables x, y: The non-homogeneous problem u xx + u yy =. (5.1) u xx + u yy = F, (5.) where F is a function

### DIFFERENTIAL GEOMETRY, LECTURE 16-17, JULY 14-17

DIFFERENTIAL GEOMETRY, LECTURE 16-17, JULY 14-17 6. Geodesics A parametrized line γ : [a, b] R n in R n is straight (and the parametrization is uniform) if the vector γ (t) does not depend on t. Thus,

### NONLINEAR ELLIPTIC EQUATIONS WITH MEASURES REVISITED

NONLINEAR ELLIPTIC EQUATIONS WITH MEASURES REVISITED Haïm Brezis (1),(2), Moshe Marcus (3) and Augusto C. Ponce (4) Abstract. We study the existence of solutions of the nonlinear problem (P) ( u + g(u)

### Laplace s Equation. Chapter Mean Value Formulas

Chapter 1 Laplace s Equation Let be an open set in R n. A function u C 2 () is called harmonic in if it satisfies Laplace s equation n (1.1) u := D ii u = 0 in. i=1 A function u C 2 () is called subharmonic

### A LOWER BOUND ON BLOWUP RATES FOR THE 3D INCOMPRESSIBLE EULER EQUATION AND A SINGLE EXPONENTIAL BEALE-KATO-MAJDA ESTIMATE. 1.

A LOWER BOUND ON BLOWUP RATES FOR THE 3D INCOMPRESSIBLE EULER EQUATION AND A SINGLE EXPONENTIAL BEALE-KATO-MAJDA ESTIMATE THOMAS CHEN AND NATAŠA PAVLOVIĆ Abstract. We prove a Beale-Kato-Majda criterion

### Functional Analysis. Franck Sueur Metric spaces Definitions Completeness Compactness Separability...

Functional Analysis Franck Sueur 2018-2019 Contents 1 Metric spaces 1 1.1 Definitions........................................ 1 1.2 Completeness...................................... 3 1.3 Compactness......................................

### An introduction to semilinear elliptic equations

An introduction to semilinear elliptic equations Thierry Cazenave Laboratoire Jacques-Louis Lions UMR CNRS 7598 B.C. 187 Université Pierre et Marie Curie 4, place Jussieu 75252 Paris Cedex 05 France E-mail

### arxiv: v1 [math.ap] 25 Jun 2008

On the existence of smooth self-similar blow-up profiles for the wave-map equation arxiv:86.4148v1 [math.ap] 5 Jun 8 Pierre Germain October, 17 Abstract Consider the equivariant wave map equation from

### Analysis in weighted spaces : preliminary version

Analysis in weighted spaces : preliminary version Frank Pacard To cite this version: Frank Pacard. Analysis in weighted spaces : preliminary version. 3rd cycle. Téhéran (Iran, 2006, pp.75.

### THE FORM SUM AND THE FRIEDRICHS EXTENSION OF SCHRÖDINGER-TYPE OPERATORS ON RIEMANNIAN MANIFOLDS

THE FORM SUM AND THE FRIEDRICHS EXTENSION OF SCHRÖDINGER-TYPE OPERATORS ON RIEMANNIAN MANIFOLDS OGNJEN MILATOVIC Abstract. We consider H V = M +V, where (M, g) is a Riemannian manifold (not necessarily

### at time t, in dimension d. The index i varies in a countable set I. We call configuration the family, denoted generically by Φ: U (x i (t) x j (t))

Notations In this chapter we investigate infinite systems of interacting particles subject to Newtonian dynamics Each particle is characterized by its position an velocity x i t, v i t R d R d at time

### 1 Lyapunov theory of stability

M.Kawski, APM 581 Diff Equns Intro to Lyapunov theory. November 15, 29 1 1 Lyapunov theory of stability Introduction. Lyapunov s second (or direct) method provides tools for studying (asymptotic) stability

### ON THE INVERSE FUNCTION THEOREM

PACIFIC JOURNAL OF MATHEMATICS Vol. 64, No 1, 1976 ON THE INVERSE FUNCTION THEOREM F. H. CLARKE The classical inverse function theorem gives conditions under which a C r function admits (locally) a C Γ

### Existence, stability and instability for Einstein-scalar field Lichnerowicz equations by Emmanuel Hebey

Existence, stability and instability for Einstein-scalar field Lichnerowicz equations by Emmanuel Hebey Joint works with Olivier Druet and with Frank Pacard and Dan Pollack Two hours lectures IAS, October

### Minimal Surface equations non-solvability strongly convex functional further regularity Consider minimal surface equation.

Lecture 7 Minimal Surface equations non-solvability strongly convex functional further regularity Consider minimal surface equation div + u = ϕ on ) = 0 in The solution is a critical point or the minimizer

### Multisolitons for NLS

Multisolitons for NLS Stefan LE COZ Beijing 2007-07-03 Plan 1 Introduction 2 Existence of multi-solitons 3 (In)stability NLS (NLS) { iut + u + g( u 2 )u = 0 u t=0 = u 0 u : R t R d x C (A0) (regular) g

### Threshold behavior and non-quasiconvergent solutions with localized initial data for bistable reaction-diffusion equations

Threshold behavior and non-quasiconvergent solutions with localized initial data for bistable reaction-diffusion equations P. Poláčik School of Mathematics, University of Minnesota Minneapolis, MN 55455

### Global well-posedness for semi-linear Wave and Schrödinger equations. Slim Ibrahim

Global well-posedness for semi-linear Wave and Schrödinger equations Slim Ibrahim McMaster University, Hamilton ON University of Calgary, April 27th, 2006 1 1 Introduction Nonlinear Wave equation: ( 2

### Contents: 1. Minimization. 2. The theorem of Lions-Stampacchia for variational inequalities. 3. Γ -Convergence. 4. Duality mapping.

Minimization Contents: 1. Minimization. 2. The theorem of Lions-Stampacchia for variational inequalities. 3. Γ -Convergence. 4. Duality mapping. 1 Minimization A Topological Result. Let S be a topological

### Problem Set 5: Solutions Math 201A: Fall 2016

Problem Set 5: s Math 21A: Fall 216 Problem 1. Define f : [1, ) [1, ) by f(x) = x + 1/x. Show that f(x) f(y) < x y for all x, y [1, ) with x y, but f has no fixed point. Why doesn t this example contradict

### Conservation law equations : problem set

Conservation law equations : problem set Luis Silvestre For Isaac Neal and Elia Portnoy in the 2018 summer bootcamp 1 Method of characteristics For the problems in this section, assume that the solutions

### 9 Radon-Nikodym theorem and conditioning

Tel Aviv University, 2015 Functions of real variables 93 9 Radon-Nikodym theorem and conditioning 9a Borel-Kolmogorov paradox............. 93 9b Radon-Nikodym theorem.............. 94 9c Conditioning.....................

### Asymptotic stability for solitons of the Gross-Pitaevskii and Landau-Lifshitz equations

Asymptotic stability for solitons of the Gross-Pitaevskii and Landau-Lifshitz equations Philippe Gravejat Cergy-Pontoise University Joint work with F. Béthuel (Paris 6), A. de Laire (Lille) and D. Smets

### ERRATUM TO AFFINE MANIFOLDS, SYZ GEOMETRY AND THE Y VERTEX

ERRATUM TO AFFINE MANIFOLDS, SYZ GEOMETRY AND THE Y VERTEX JOHN LOFTIN, SHING-TUNG YAU, AND ERIC ZASLOW 1. Main result The purpose of this erratum is to correct an error in the proof of the main result

### On Universality of Blow-up Profile for L 2 critical nonlinear Schrödinger Equation

On Universality of Blow-up Profile for L critical nonlinear Schrödinger Equation Frank Merle,, Pierre Raphael Université de Cergy Pontoise Institut Universitaire de France Astract We consider finite time

### An introduction to semilinear elliptic equations

An introduction to semilinear elliptic equations Thierry Cazenave Laboratoire Jacques-Louis Lions UMR CNRS 7598 B.C. 187 Université Pierre et Marie Curie 4, place Jussieu 75252 Paris Cedex 05 France E-mail

### Metric Spaces and Topology

Chapter 2 Metric Spaces and Topology From an engineering perspective, the most important way to construct a topology on a set is to define the topology in terms of a metric on the set. This approach underlies

### Scientific Computing WS 2018/2019. Lecture 15. Jürgen Fuhrmann Lecture 15 Slide 1

Scientific Computing WS 2018/2019 Lecture 15 Jürgen Fuhrmann juergen.fuhrmann@wias-berlin.de Lecture 15 Slide 1 Lecture 15 Slide 2 Problems with strong formulation Writing the PDE with divergence and gradient

### Stability and Instability of Standing Waves for the Nonlinear Fractional Schrödinger Equation. Shihui Zhu (joint with J. Zhang)

and of Standing Waves the Fractional Schrödinger Equation Shihui Zhu (joint with J. Zhang) Department of Mathematics, Sichuan Normal University & IMS, National University of Singapore P1 iu t ( + k 2 )

### Lecture 2: Isoperimetric methods for the curve-shortening flow and for the Ricci flow on surfaces

Lecture 2: Isoperimetric methods for the curve-shortening flow and for the Ricci flow on surfaces Ben Andrews Mathematical Sciences Institute, Australian National University Winter School of Geometric

### On Chern-Simons-Schrödinger equations including a vortex point

On Chern-Simons-Schrödinger equations including a vortex point Alessio Pomponio Dipartimento di Meccanica, Matematica e Management, Politecnico di Bari Workshop in Nonlinear PDEs Brussels, September 7

### A REMARK ON AN EQUATION OF WAVE MAPS TYPE WITH VARIABLE COEFFICIENTS

A REMARK ON AN EQUATION OF WAVE MAPS TYPE WITH VARIABLE COEFFICIENTS DAN-ANDREI GEBA Abstract. We obtain a sharp local well-posedness result for an equation of wave maps type with variable coefficients.

### Exact Solutions of the Einstein Equations

Notes from phz 6607, Special and General Relativity University of Florida, Fall 2004, Detweiler Exact Solutions of the Einstein Equations These notes are not a substitute in any manner for class lectures.

### Singularities and Laplacians in Boundary Value Problems for Nonlinear Ordinary Differential Equations

Singularities and Laplacians in Boundary Value Problems for Nonlinear Ordinary Differential Equations Irena Rachůnková, Svatoslav Staněk, Department of Mathematics, Palacký University, 779 OLOMOUC, Tomkova

### Uniqueness of ground state solutions of non-local equations in R N

Uniqueness of ground state solutions of non-local equations in R N Rupert L. Frank Department of Mathematics Princeton University Joint work with Enno Lenzmann and Luis Silvestre Uniqueness and non-degeneracy

### Construction of concentrating bubbles for the energy-critical wave equation

Construction of concentrating bubbles for the energy-critical wave equation Jacek Jendrej University of Chicago University of North Carolina February 13th 2017 Jacek Jendrej Energy-critical wave equations

### i=1 α i. Given an m-times continuously

1 Fundamentals 1.1 Classification and characteristics Let Ω R d, d N, d 2, be an open set and α = (α 1,, α d ) T N d 0, N 0 := N {0}, a multiindex with α := d i=1 α i. Given an m-times continuously differentiable

### MATHS 730 FC Lecture Notes March 5, Introduction

1 INTRODUCTION MATHS 730 FC Lecture Notes March 5, 2014 1 Introduction Definition. If A, B are sets and there exists a bijection A B, they have the same cardinality, which we write as A, #A. If there exists

### WELL-POSEDNESS FOR HYPERBOLIC PROBLEMS (0.2)

WELL-POSEDNESS FOR HYPERBOLIC PROBLEMS We will use the familiar Hilbert spaces H = L 2 (Ω) and V = H 1 (Ω). We consider the Cauchy problem (.1) c u = ( 2 t c )u = f L 2 ((, T ) Ω) on [, T ] Ω u() = u H

### Notes on uniform convergence

Notes on uniform convergence Erik Wahlén erik.wahlen@math.lu.se January 17, 2012 1 Numerical sequences We begin by recalling some properties of numerical sequences. By a numerical sequence we simply mean

### L p Spaces and Convexity

L p Spaces and Convexity These notes largely follow the treatments in Royden, Real Analysis, and Rudin, Real & Complex Analysis. 1. Convex functions Let I R be an interval. For I open, we say a function

### Stability of Feedback Solutions for Infinite Horizon Noncooperative Differential Games

Stability of Feedback Solutions for Infinite Horizon Noncooperative Differential Games Alberto Bressan ) and Khai T. Nguyen ) *) Department of Mathematics, Penn State University **) Department of Mathematics,

### Brownian Motion. 1 Definition Brownian Motion Wiener measure... 3

Brownian Motion Contents 1 Definition 2 1.1 Brownian Motion................................. 2 1.2 Wiener measure.................................. 3 2 Construction 4 2.1 Gaussian process.................................

### A LOCALIZATION PROPERTY AT THE BOUNDARY FOR MONGE-AMPERE EQUATION

A LOCALIZATION PROPERTY AT THE BOUNDARY FOR MONGE-AMPERE EQUATION O. SAVIN. Introduction In this paper we study the geometry of the sections for solutions to the Monge- Ampere equation det D 2 u = f, u

### Supplementary Notes for W. Rudin: Principles of Mathematical Analysis

Supplementary Notes for W. Rudin: Principles of Mathematical Analysis SIGURDUR HELGASON In 8.00B it is customary to cover Chapters 7 in Rudin s book. Experience shows that this requires careful planning

### ORBITAL STABILITY OF SOLITARY WAVES FOR A 2D-BOUSSINESQ SYSTEM

Electronic Journal of Differential Equations, Vol. 05 05, No. 76, pp. 7. ISSN: 07-669. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu ftp ejde.math.txstate.edu ORBITAL STABILITY OF SOLITARY

### Variational Theory of Solitons for a Higher Order Generalized Camassa-Holm Equation

International Journal of Mathematical Analysis Vol. 11, 2017, no. 21, 1007-1018 HIKAI Ltd, www.m-hikari.com https://doi.org/10.12988/ijma.2017.710141 Variational Theory of Solitons for a Higher Order Generalized

### A TWO PARAMETERS AMBROSETTI PRODI PROBLEM*

PORTUGALIAE MATHEMATICA Vol. 53 Fasc. 3 1996 A TWO PARAMETERS AMBROSETTI PRODI PROBLEM* C. De Coster** and P. Habets 1 Introduction The study of the Ambrosetti Prodi problem has started with the paper

### Convergence and sharp thresholds for propagation in nonlinear diffusion problems

J. Eur. Math. Soc. 12, 279 312 c European Mathematical Society 2010 DOI 10.4171/JEMS/198 Yihong Du Hiroshi Matano Convergence and sharp thresholds for propagation in nonlinear diffusion problems Received

### Piecewise Smooth Solutions to the Burgers-Hilbert Equation

Piecewise Smooth Solutions to the Burgers-Hilbert Equation Alberto Bressan and Tianyou Zhang Department of Mathematics, Penn State University, University Park, Pa 68, USA e-mails: bressan@mathpsuedu, zhang

### OPTIMAL POTENTIALS FOR SCHRÖDINGER OPERATORS. 1. Introduction In this paper we consider optimization problems of the form. min F (V ) : V V, (1.

OPTIMAL POTENTIALS FOR SCHRÖDINGER OPERATORS G. BUTTAZZO, A. GEROLIN, B. RUFFINI, AND B. VELICHKOV Abstract. We consider the Schrödinger operator + V (x) on H 0 (), where is a given domain of R d. Our

### B. Appendix B. Topological vector spaces

B.1 B. Appendix B. Topological vector spaces B.1. Fréchet spaces. In this appendix we go through the definition of Fréchet spaces and their inductive limits, such as they are used for definitions of function

### An asymptotic ratio characterization of input-to-state stability

1 An asymptotic ratio characterization of input-to-state stability Daniel Liberzon and Hyungbo Shim Abstract For continuous-time nonlinear systems with inputs, we introduce the notion of an asymptotic

### UNIQUENESS OF POSITIVE SOLUTION TO SOME COUPLED COOPERATIVE VARIATIONAL ELLIPTIC SYSTEMS

TRANSACTIONS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 00, Number 0, Pages 000 000 S 0002-9947(XX)0000-0 UNIQUENESS OF POSITIVE SOLUTION TO SOME COUPLED COOPERATIVE VARIATIONAL ELLIPTIC SYSTEMS YULIAN

### arxiv: v1 [math.ap] 10 Apr 2013

QUASI-STATIC EVOLUTION AND CONGESTED CROWD TRANSPORT DAMON ALEXANDER, INWON KIM, AND YAO YAO arxiv:1304.3072v1 [math.ap] 10 Apr 2013 Abstract. We consider the relationship between Hele-Shaw evolution with

### 1.2 Fundamental Theorems of Functional Analysis

1.2 Fundamental Theorems of Functional Analysis 15 Indeed, h = ω ψ ωdx is continuous compactly supported with R hdx = 0 R and thus it has a unique compactly supported primitive. Hence fφ dx = f(ω ψ ωdy)dx

### Energy method for wave equations

Energy method for wave equations Willie Wong Based on commit 5dfb7e5 of 2017-11-06 13:29 Abstract We give an elementary discussion of the energy method (and particularly the vector field method) in the

### 2. Dual space is essential for the concept of gradient which, in turn, leads to the variational analysis of Lagrange multipliers.

Chapter 3 Duality in Banach Space Modern optimization theory largely centers around the interplay of a normed vector space and its corresponding dual. The notion of duality is important for the following

### Finite-dimensional spaces. C n is the space of n-tuples x = (x 1,..., x n ) of complex numbers. It is a Hilbert space with the inner product

Chapter 4 Hilbert Spaces 4.1 Inner Product Spaces Inner Product Space. A complex vector space E is called an inner product space (or a pre-hilbert space, or a unitary space) if there is a mapping (, )

### 16 1 Basic Facts from Functional Analysis and Banach Lattices

16 1 Basic Facts from Functional Analysis and Banach Lattices 1.2.3 Banach Steinhaus Theorem Another fundamental theorem of functional analysis is the Banach Steinhaus theorem, or the Uniform Boundedness

### Continuous Functions on Metric Spaces

Continuous Functions on Metric Spaces Math 201A, Fall 2016 1 Continuous functions Definition 1. Let (X, d X ) and (Y, d Y ) be metric spaces. A function f : X Y is continuous at a X if for every ɛ > 0

### Riemann integral and volume are generalized to unbounded functions and sets. is an admissible set, and its volume is a Riemann integral, 1l E,

Tel Aviv University, 26 Analysis-III 9 9 Improper integral 9a Introduction....................... 9 9b Positive integrands................... 9c Special functions gamma and beta......... 4 9d Change of

### THEOREMS, ETC., FOR MATH 515

THEOREMS, ETC., FOR MATH 515 Proposition 1 (=comment on page 17). If A is an algebra, then any finite union or finite intersection of sets in A is also in A. Proposition 2 (=Proposition 1.1). For every

### LECTURE 10: THE ATIYAH-GUILLEMIN-STERNBERG CONVEXITY THEOREM

LECTURE 10: THE ATIYAH-GUILLEMIN-STERNBERG CONVEXITY THEOREM Contents 1. The Atiyah-Guillemin-Sternberg Convexity Theorem 1 2. Proof of the Atiyah-Guillemin-Sternberg Convexity theorem 3 3. Morse theory

### MATH 722, COMPLEX ANALYSIS, SPRING 2009 PART 5

MATH 722, COMPLEX ANALYSIS, SPRING 2009 PART 5.. The Arzela-Ascoli Theorem.. The Riemann mapping theorem Let X be a metric space, and let F be a family of continuous complex-valued functions on X. We have

### Regularity and compactness for the DiPerna Lions flow

Regularity and compactness for the DiPerna Lions flow Gianluca Crippa 1 and Camillo De Lellis 2 1 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy. g.crippa@sns.it 2 Institut für Mathematik,

### Applications of the compensated compactness method on hyperbolic conservation systems

Applications of the compensated compactness method on hyperbolic conservation systems Yunguang Lu Department of Mathematics National University of Colombia e-mail:ylu@unal.edu.co ALAMMI 2009 In this talk,

### Topological properties

CHAPTER 4 Topological properties 1. Connectedness Definitions and examples Basic properties Connected components Connected versus path connected, again 2. Compactness Definition and first examples Topological

### DYNAMICAL CUBES AND A CRITERIA FOR SYSTEMS HAVING PRODUCT EXTENSIONS

DYNAMICAL CUBES AND A CRITERIA FOR SYSTEMS HAVING PRODUCT EXTENSIONS SEBASTIÁN DONOSO AND WENBO SUN Abstract. For minimal Z 2 -topological dynamical systems, we introduce a cube structure and a variation

### Problem List MATH 5143 Fall, 2013

Problem List MATH 5143 Fall, 2013 On any problem you may use the result of any previous problem (even if you were not able to do it) and any information given in class up to the moment the problem was