Wellposedness and inhomogeneous equations

Transcription

1 LECTRE 6 Wellposedness and inhomogeneous equations In this lecture we complete the linear existence theory. In the introduction we have explained the concept of wellposedness and stressed that only wellposed evolution equations are truly relevant for the description of systems in the sciences. So far we have characterized those operators that generate C semigroups and solved the Cauchy problems associated with them. We now prove that a Cauchy problem for a closed linear operator A is wellposed if and only if A generates a C semigroup. In this sense, semigroup theory provides the natural framework for linear evolution equations. As a second main topic we treat inhomogeneous problems where one adds a given forcing (or control) function f to the differential equation. Such equations are solved by means of Duhamel s (or the variation of parameters) formula that involves the C semigroup solving the homogeneous problem. We then apply these results to the linear wave equation. Here we will reformulate the given equation (having second order in time) as an evolution equation of first order, which belongs to the class we have studied so far. We first repeat our basic linear evolution equation. Let A be a closed operator on X. For each given u D(A), we consider the Cauchy problem u (t) = Au(t), t, u() = u. (6.1) Recall that a solution of (6.1) is a function u C 1 (R +, X) such that u(t) D(A) for all t R and u satisfies (6.1). Observe that then Au C(R +, X) and thus u C(R +, [D(A)]). We next introduce the concept of wellposedness. Definition 6.1. The Cauchy problem (6.1) is called wellposed if (a) D(A) is dense in X, (b) for each u D(A) there is a unique solution u = u( ; u ) of (6.1), (c) whenever u,n, u D(A) and u,n tend to u in X as n, then u( ; u,n ) converge to u( ; u ) uniformly on compact subsets of R + (continuous dependence on initial data). We can now establish the announced characterization of wellposedness in terms of the given operator A. Theorem 6.2. Let A be a closed linear operator. Then (6.1) is wellposed if and only if A generates a C semigroup T ( ). In this case, the function u = T ( )u solves (6.1) for each given initial value u D(A). Proof. If A is a generator, then T ( )u is the unique solution of (6.1) according to Proposition 2.8, D(A) is dense in X by Proposition 3.3 and the solution depends continuously on the initial data since T ( ) is locally bounded. 53

2 Conversely, let (6.1) be wellposed. We define the operator T (t) : D(A) X by T (t)x = u(t; x) for x D(A) and t using uniqueness. For x, y D(A) and α, β C, the function v given by v(t) = αu(t; x) + βu(t; y) for t solves (6.1) with initial value αx + βy since A is linear. niqueness now yields v(t) = u(t; αx + βy) = T (t)(αx + βy), so that T (t) is linear for every t. We claim that for each t > there is a c > such that T (t)x c x for all x D(A) and all t [, t ]. In fact, if this assertion were wrong, there would exist t >, a sequence (x n ) n in D(A) and a sequence (t n ) n in [, t ] such that x n = 1 and T (t n )x n =: c n as n. Set y n := 1 c n x n D(A) for every n N. The initial values y n tend to as n, but the norms u(t n ; y n ) = 1 c n T (t n )x n = 1 do not converge to. This contradicts the wellposedness of (6.1) and thus T ( ) is locally bounded. So we can extend each single operator T (t) to a continuous linear operator on D(A) = X (also denoted by T (t)) having the same operator norm. Clearly, T () = I. Since t T (t)x X is continuous on R + for every x D(A), D(A) = X and T ( ) is locally bounded, the strong continuity of T ( ) on X follows by approximation. Furthermore, let t, s and x D(A). Then u(s; x) belongs to D(A) so that v(t) := T (t)u(s; x) = u(t; u(s; x)) for t is the unique solution of (6.1) with initial value u(s; x). On the other hand, u(t + s; x) = T (t + s)x for t solves this problem, too. Because solutions are unique, we obtain T (t)t (s)x = T (t + s)x which gives the semigroup property by approximation. Let B be the generator of T ( ). By definition, we have A B. Since D(A) is dense in X and T (t) D(A) D(A) for all t, Proposition 4.1 shows that D(A) is a core of B. So for any x D(B), there are x n D(A) such that x n x and Ax n = Bx n Bx in X as n. The closedness of A now implies x D(A) and A = B. One cannot drop the continuous dependence on initial data in Theorem 6.2, as seen by the next simple example. Example 6.3. Let B be a closed, densely defined, unbounded operator on a Banach space Y. Set X = Y Y and A = ( ) B with dense domain D(A) = Y D(B). For (x, y) D(A) one has the unique solution u(t) = ( ) x+tby y of (6.1) with u() = (x, y). But for t > the map T (t) : (D(A), X ) X, (x, y) u(t) is not continuous, since T (t) ( ) ( y = tby ) y. We further note that λi A is not surjective for every λ C, see Example II.6.5 of [EN99]. Actually, it can be shown that (6.1) has a unique solution for a closed operator A and each x D(A) if and only if the operator A 1 on X 1 = [D(A)] given by A 1 x = Ax with D(A 1 ) = { x D(A) Ax D(A) } generates a C semigroup on X 1, see Proposition II.6.6 in [EN99]. Moreover, if ρ(a) and (6.1) has a unique solution for each x D(A), then A is a generator (and in particular densely defined), see Theorem II.6.7 in [EN99]. For u X one calls the orbit T ( )u the mild solution of (6.1), see Definition 6.8. In Exercise 6.3 it is shown in which sense the function T ( )u solves (6.1). 54

3 We now come to the second main topic of this lecture. In the remainder of this lecture, let J R be a closed interval containing and having non-empty interior. Further, let u X, f C(J, X) and A be a closed linear operator. We study the inhomogeneous Cauchy problem or inhomogeneous evolution equation u (t) = Au(t) + f(t), t J, u() = u. (6.2) It is convenient to allow for finite time intervals J here since possibly f is not given for all times. (As noted in the introduction this situation occurs when treating nonlinear problems.) Moreover, for later use we include backward time. Our solution concept for (6.1) directly extends to (6.2). Definition 6.4. A function u : J X is a solution of (6.2) if u belongs to C 1 (J, X), u(t) D(A) for all t J and (6.2) holds. This definition implies that the initial value u of a solution must belong to D(A). Observe that a solution of (6.2) is contained in C(J, [D(A)]). Our solutions are often called classical or strict solutions in the literature. We first derive Duhamel s formula for the solutions of (6.2). Since the case f = is included, it is natural to assume from the beginning that A is a generator. The next proof is just a variant of the uniqueness part of the proof of Proposition 2.8. Proposition 6.5. Let A generate the C semigroup T ( ), u D(A), and f C(J, X). If J R +, we require that T ( ) can be extended to a C group. If u solves (6.2), then u is given by u(t) = T (t)u + In particular, solutions of (6.2) are unique. T (t s)f(s) ds, t J. (6.3) Proof. For simplicity, we concentrate on the case that J R +. Let t J with t > and set v(s) = T (t s)u(s) for s t, where u solves (6.2). sing Lemma 2.9, one shows that v is continuously differentiable with derivative v (s) = T (t s)u (s) T (t s)au(s) = T (t s)f(s) for all s t. By integration we deduce T (t s)f(s) ds = v(t) v() = u(t) T (t)u. We point out that by Duhamel s formula (6.3) one can define a function u C(J, X) for any given u X and f C(J, X). This leads to a weaker solution concept, which turns out to be very useful. Definition 6.6. Let A generate the C semigroup T ( ), u X and f C(J, X). If J R +, we require that T ( ) can be extended to a C group. Then the function u C(J, X) given by u(t) = T (t)u + is called the mild solution of (6.2). T (t s)f(s) ds, t J, 55

4 Proposition 6.5 says that every solution of (6.2) is a mild one, whereas the next example shows that the converse implication may fail. Example 6.7. Let X = C (R), A = d ds with D(A) = C1 (R) and let ϕ X be non-differentiable. The operator A generates the C group T ( ) given by T (t)g = g( + t), see Example 3.5. Clearly, T (t)ϕ / D(A) for all t R. Set f(s) = T (s)ϕ for s R. The function f belongs to C(R, X) and the mild solution of (6.2) with u = is given by u(t) = T (t s)t (s)ϕ ds = tt (t)ϕ for t R. Hence, u(t) / D(A) for t, i.e., u does not solve (6.2). We will derive conditions on f and u such that the mild solution of (6.2) in fact solves (6.2). At first, we treat the case u =. Lemma 6.8. Let A generate the C semigroup T ( ) and f C(J, X). J R +, we require that T ( ) can be extended to a C group. Define v(t) = T (t s)f(s) ds, t J. Then the following assertions are equivalent. (a) v C 1 (J, X). (b) v(t) D(A) for all t J and Av C(J, X). If (a) or (b) are valid v solves (6.2) with u =. Proof. We concentrate on the case that J R +. The general case can be treated similarly. Since f is locally bounded, the function v belongs to C(J, X) and v() =. To show the asserted equivalence, we need a few preparations. We fix t J and take h > such that t ± h J. We then define D 1 (h) := 1 h (T (h) I)v(t), D± 1 2 (h) := (v(t ± h) v(t)), I + (h) := 1 h +h t T (t + h s)f(s) ds, I (h) := 1 h ±h t h T (t s)f(s) ds, assuming that t > for D 2 (h) and I (h). (These formulas are asymmetric since T ( ) is not assumed to be a group.) Observe that D 1 (h) = D + 2 (h) I+ (h), (6.4) D 2 (h) = 1 h (T (h) I)v(t h) + I (h) h = 1 h A T (τ)v(t h) dτ + I (h), (6.5) where we use Lemma 3.2 in the last equality. We start by investigating I ± (h). Employing the continuity of f and Lemma 2.9, we obtain I + (h) f(t) = 1 +h (T (t + h s)f(s) f(t)) ds h t T (t + h s)f(s) f(t) max t s t+h as h +. Similarly, one sees that I (h) f(t) as h If

5 First, assume that v satisfies (a). Hence, D 2 ± (h) v (t) as h +. Equality (6.4) then implies that D 1 (h) converges to v (t) f(t) as h +. As a result, v(t) belongs to D(A) for all t J and Av = v f is continuous. Second, let (b) hold so that D 1 (h) Av(t) as h +. From (6.4) we now infer that v is differentiable from the right and ( d dt )+ v = Av + f. Moreover, (6.5) and (b) yield D 2 (h) = 1 h = 1 h h h Av(t) + f(t) T (τ)av(t h) dτ + I (h) T (τ)av(t) dτ + 1 h h T (τ)(av(t h) Av(t)) dτ + I (h) as h +, thanks to Remark 2.1 (d) and (g) and the continuity of Av. Summing up, v is differentiable with v = Av + f C(J, X) so that (a) is true. In both cases we have also shown that v solves (6.2) with u =. The above lemma now implies that we obtain solutions of (6.2) if the initial value u belongs to D(A) and if the inhomogeneity has either more time regularity (i.e., f C 1 (J, X)) or more space regularity (i.e., f C(J, [D(A)])). Theorem 6.9 (Existence result for inhomogeneous evolution equations). Let A generate the C semigroup T ( ), u D(A) and J R be a closed interval containing. If J R +, we require that T ( ) can be extended to a C group. Assume either that f C 1 (J, X) or that f C(J, [D(A)]). Then the mild solution u given by (6.3) is the unique solution of (6.2) on J. Proof. niqueness was already shown in Proposition 6.5. By Proposition 2.8, the function T ( )u is contained in C 1 (J, X) C(J, [D(A)]) and solves (6.2) with f =. It remains to show that the map t v(t) = T (t s)f(s) ds defined in Lemma 6.8 also belongs to C 1 (J, X) C(J, [D(A)]) and solves (6.2) with u =, since then u = T ( )u + v solves (6.2). Hence, we have to verify (a) or (b) in Lemma 6.8. Let f C 1 (J, X). Since v(t) = T (s)f(t s) ds for all t J, it follows that v C 1 (J, X) and hence (a) in Lemma 6.8 is satisfied. Let f C(J, [D(A)]). Since A is closed and commutes with T (t s) on D(A), we obtain v(t) D(A) and Av(t) = T (t s)af(s) ds so that Av C(J, X). In this case, (b) in Lemma 6.8 is fulfilled. We want to apply the above results to the wave equation on a bounded open set R d with Dirichlet boundary conditions. This equation describes the displacement w(t, x) of a vibrating body at a time t R and at a point x. Here we consider the system tt w(t, x) = w(t, x) b(x) t w(t, x) + g(t, x), x, t R, w(t, x) =, x, t R, w(, x) = w (x), t w(, x) = w 1 (x), x, 57 (6.6)

6 for the given initial displacement w and initial velocity distribution w 1. The functions g and b are also given. Let us sketch the physical background of this equation. (This is not meant to be an honest derivation.) In the differential equation we assume for simplicity that the mass density of the body is equal to 1 everywhere. The differential equation in (6.6) then describes the balance of forces at the space point x and at the time t. The acceleration tt w(t, x) is equal to the sum of the forces on the right-hand side, where w(t, x) describes the mechanical force due to tension, b(x) t w(t, x) is a damping proportional to the velocity t w(t, x) and g(t, x) corresponds to an external force. The term w(t, x) can be justified if one assumes that the material is perfectly elastic, homogeneous (with material constant 1) and if only small deflections occur. So far we have implicitly assumed that w C 2 (J ) so that (6.6) would hold in a pointwise sense. For the analysis of the problem it is much more convenient to reformulate (6.6) as an evolution equation (of second order in time) in the space L 2 (). To this aim, we use the Dirichlet Laplacian D introduced in Example Recall that D acts in L 2 (), that D( D ) H 1 () and that ( D u v) L 2 = u v dx for u D( D ) and v H 1 (). (6.7) Let A u = u with D(A ) = H 2 () H 1 (). As indicated after Example 5.1, we have A D if C 1 and A = D if C 2. (Here the regularity assumptions are not optimal.) However, we will stick to the case of a general bounded open R d and to the operator D given by (6.7). sing this setup, we rewrite (6.6) as w (t) = D w(t) bw (t) + g(t), t J, w() = w, w () = w 1, (6.8) where w and w denote the first and second derivatives of w with respect to t. The unknown w now is a function from J to L 2 () and similarly for g. The Dirichlet boundary conditions and the differential operator are incorporated in the operator D in a somewhat generalized form. We are looking for solutions w C 2 (J, L 2 ()) C 1 (J, H 1 ()) of (6.8), where we are given initial values w D( D ) and w 1 H 1 (). with w(t) D( D ) for all t J For simplicity we assume that b L (). Moreover, we take g C(J, L 2 ()). We note that the time interval J is dictated by g. If g =, we will take J = R. Observe that a solution w of (6.8) satisfies the integrated equation tt w(t) ϕ dx + w(t) ϕ dx + b t w(t) ϕ dx = g(t) ϕ dt for all t J and each ϕ H 1 (), see (6.7). We will come back to such weak formulations in later lectures. We call the Cauchy problem (6.8) with g = wellposed if the following conditions hold. 58

7 (a) For all w D( D ) and w 1 H 1 () there is a unique solution w = w( ; w, w 1 ) of (6.8) with g =. (b) Let w,n, w D( D ) and w 1,n, w 1 H 1 (), n N. Assume that w,n tend to w in H 1 () and that w 1,n tend to w 1 in L 2 () as n. Then w(t; w,n, w 1,n ) converge to w(t; w, w 1 ) in H 1 () and w (t; w,n, w 1,n ) converge to w (t; w, w 1 ) in L 2 () as n, both locally uniformly in t J. Comparing the above concept with Definition 6.1 you may miss the density condition. In fact, we already know from Example 5.13 that D( D ) H 1 () is dense in H 1 () L 2 () which is the appropriate density assumption in view of (b) above. To solve (6.8) and to show its wellposedness, we want to use the theory established in this lecture. As in the case of ordinary differential equations, we thus introduce the new state ( ) ( ) u1 (t) w(t) u(t) = =. u 2 (t) t w(t) The state space will be X = H 1 () L 2 () endowed with the scalar product (u v) = ( (u 1, u 2 ) (v 1, v 2 ) ) = ( u 1 v 1 + u 2 v 2 ) dx. This choice fits well to condition (b) above, where solutions are required to converge in the corresponding norm on X given by u 2 = u dx + u 2 2 dx. (As noted in Example 5.13 this norm is equivalent to the usual norm on X.) Physically one can interpret u(t) 2 as the total energy of the solution u(t) = (w(t), w (t)) modulo constants, where w(t) 2 2 dx corresponds to the potential energy and w (t) 2 dx to the kinetic energy. We further define the operator ( ) I A = with D(A) = D( D b D ) H 1 () (6.9) in X, where b denotes the bounded multiplication operator ϕ bϕ on L 2 (). Finally, we put u = (w, w 1 ) and f = (, g) C(J, X). We recall from Example 5.13 that for b = the operator A is skewadjoint in X. We next show the announced result describing in which sense (6.8) is equivalent to the evolution equation (6.2) for the operator matrix A defined in (6.9). Lemma 6.1. Let R d be open and bounded and b L (). Let u = (w, w 1 ) D(A) and g C(J, L 2 ()), and set f = (, g) C(J, X). Then the following assertions hold. (a) The problem (6.8) has a solution w if and only if the problem (6.2) with A from (6.9) has a solution u. If this is case, we have u = (w, t w). Also the uniqueness of solutions to these two problems is equivalent. 59

8 (b) The problem (6.8) with g = is wellposed if and only if the problem (6.1) with A from (6.9) is wellposed. Proof. (a) Let w C 2 (J, L 2 ()) C 1 (J, H 1 ()) solve (6.8). Then u := (w, w ) belongs to C 1 (J, X), u(t) D(A) for all t J and ( u w (t) = ) ( (t) w w = ) (t) (t) D w(t) bw = Au(t) + f(t) (t) + g(t) holds for all t J. Moreover, u() = (w(), w ()) = u. Thus, u solves (6.2). Conversely, let u = (u 1, u 2 ) solve (6.2) for A. We set w = u 1 obtaining w C 1 (J, H 1 ()) and w(t) D( D ) for all t J. It further follows ( w ) ( ) ( ) (t) w(t) u u 2 (t) = A + f(t) = 2 (t) u 2 (t) D w(t) bu 2 (t) + g(t) for all t J. As a consequence, w = u 2 C 1 (J, L 2 ()) and u = (w, w ), so that w C 2 (J, L 2 ()), (w(), w ()) = (w, w 1 ) and w solves (6.8). This equivalence also yields that the solutions to (6.2) for our A are unique if and only if the solutions to (6.8) are unique. (b) It follows from Example 5.13 that D(A) is dense in X. Part (a) then easily implies the equivalence of the two wellposedness assertions. Combining the above lemma with Example 5.13 and the previous theorems, we can now solve the undamped wave equation with b =. The damping b will be treated in the next lecture by a perturbation argument. Proposition Let R d be open and bounded, J R be a closed interval containing, (w, w 1 ) D( D ) H 1 () and either g C 1 (J, L 2 ()) or g C(J, H 1 ()). Then the wave equation (6.8) with b = has a unique solution. Moreover, problem (6.8) with b = and g = is wellposed. Proof. Thanks to Example 5.13, the operator A defined in (6.9) with b = is skewadjoint on X = H 1 () L 2 () and thus generates a (unitary) C group on X by Stone s Theorem 5.7. The assertions now follow from Theorems 6.2 and 6.9 and Lemma 6.1. We finally consider the free Schrödinger equation given by t u(t, x) = i u(t, x), x R d, t R u(, x) = u (x) x R d. (6.1) Here we look for solutions u C 1 (R, L 2 (R d )) C(R, H 2 (R d )). To obtain such solutions we introduce in L 2 (R d ) the operator A given by Au = i u with D(A) = H 2 (R d ). We say that (6.1) is wellposed if the Cauchy problem (6.1) for this operator A is wellposed in L 2 (R d ). Since A is skewadjoint by Example 5.1, Stone s Theorem 5.7 and Theorem 6.1 immediately yield the next result. Proposition The free Schrödinger equation (6.1) is wellposed. 6

9 Exercises Exercise 6.1. Let X = C (, 1), Au(s) = s(1 s)u (s) for s (, 1) and u D(A) = { u C 2 (, 1) X Au X }. Show that A is densely defined, dissipative, invertible and generates a contraction semigroup on X. Exercise 6.2. Let X be a Banach space and A generate a C semigroup T ( ) on X such that T (t) I in B(X) as t. Show that λr(λ, A) I in B(X) as λ and deduce that A is bounded. Exercise 6.3. Let X be a Banach space, A generate a C semigroup T ( ) on X, f C(R +, X) and u X. An integrated solution u of u (t) = Au(t) + f(t), t, u() = u (6.11) is a function u C(R +, X) such that u(s) ds D(A) and u(t) = A Show that the mild solution u(s) ds + u + v(t) = T (t)u + is the unique integrated solution of (6.11). f(s) ds for all t. T (t s)f(s) ds 61

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