On groups of Hölder diffeomorphisms and their regularity
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1 On groups of Hölder diffeomorphisms and their regularity Joint work with Armin Rainer David Nenning University of Vienna Valencia, October 17, 217
2 Diffeomorphism groups generated by time-dependent vector fields
3 Diffeomorphism groups generated by time-dependent vector fields C 1 (Rd, R d )... Space of continuously differentiable functions vanishing at infinity together with their first derivative.
4 Diffeomorphism groups generated by time-dependent vector fields C 1(Rd, R d )... Space of continuously differentiable functions vanishing at infinity together with their first derivative. For u : I R d R d such that u(, x) is measurable for all x R d, u(t, ) C 1 1 u(t, ) C the ODE for all t I, 1 dt <, t Φ(t) = x + u(r, Φ(r)) dr s admits a unique solution for each fixed s I, x R d ; denoted as Φ u (t, s, x) = x + φ u (t, s, x).
5 Diffeomorphism groups generated by time-dependent vector fields C 1(Rd, R d )... Space of continuously differentiable functions vanishing at infinity together with their first derivative. For u : I R d R d such that u(, x) is measurable for all x R d, u(t, ) C 1 1 u(t, ) C the ODE for all t I, 1 dt <, t Φ(t) = x + u(r, Φ(r)) dr s admits a unique solution for each fixed s I, x R d ; denoted as Φ u (t, s, x) = x + φ u (t, s, x). Φ u (t, s, ) is a C 1 -diffeomorphism and φ u (t, s, ) C 1.
6 Diffeomorphism groups generated by time-dependent vector fields
7 Diffeomorphism groups generated by time-dependent vector fields For a locally convex space E C 1 (Rd, R d ), we set X E... Space of pointwise time-dependent E-vector fields u : I R d R d with u(, x) is measurable for all x R d, u(t, ) E for all t I, 1 p(u(t, )) dt < for all continuous seminorms p.
8 Diffeomorphism groups generated by time-dependent vector fields For a locally convex space E C 1 (Rd, R d ), we set X E... Space of pointwise time-dependent E-vector fields u : I R d R d with u(, x) is measurable for all x R d, u(t, ) E for all t I, 1 p(u(t, )) dt < for all continuous seminorms p. Definition (Trouvé group) G E := { Φ u (1,, ) : u X E }.
9 Diffeomorphism groups generated by time-dependent vector fields For a locally convex space E C 1 (Rd, R d ), we set X E... Space of pointwise time-dependent E-vector fields u : I R d R d with u(, x) is measurable for all x R d, u(t, ) E for all t I, 1 p(u(t, )) dt < for all continuous seminorms p. Definition (Trouvé group) G E := { Φ u (1,, ) : u X E }. Lemma G E is a group with respect to composition.
10 Diffeomorphism groups generated by time-dependent vector fields For a locally convex space E C 1 (Rd, R d ), we set X E... Space of pointwise time-dependent E-vector fields u : I R d R d with u(, x) is measurable for all x R d, u(t, ) E for all t I, 1 p(u(t, )) dt < for all continuous seminorms p. Definition (Trouvé group) G E := { Φ u (1,, ) : u X E }. Lemma G E is a group with respect to composition. Definition (ODE-closedness) E is called ODE-closed iff G E Id +E.
11 Hölder Diffeomorphism groups
12 Hölder Diffeomorphism groups C n,β (R d, R d )... Space of n times continuously differentiable functions vanishing at infinity together with their derivatives up to order n and β-hölder continuous n-th derivative with global Hölder constant, i.e. for f C n,β, f (k) (x) as x for k n, and f n,β := max{ f (k) L (R d,l k (R d,r d )) : k n; sup x,y R d f (n) (x) f (n) (y) Ln(Rd,Rd ) x y β } <.
13 Hölder Diffeomorphism groups C n,β (R d, R d )... Space of n times continuously differentiable functions vanishing at infinity together with their derivatives up to order n and β-hölder continuous n-th derivative with global Hölder constant, i.e. for f C n,β, f (k) (x) as x for k n, and f n,β := max{ f (k) L (R d,l k (R d,r d )) : k n; f (n) (x) f (n) (y) Ln(Rd sup,rd ) x,y R d x y β } <. The set of orientation preserving diffeomorphisms that differ from the identity by a C n,β -function is denoted as Diff C n,β := { Φ Id +C n,β : det Φ (x) > x R d}. It is a Banach manifold modelled on C n,β with global chart Φ Φ Id. (Diff C n,β ) shall denote the connected component of the identity.
14 Hölder Diffeomorphism groups
15 Hölder Diffeomorphism groups Theorem 1 Diff C n,β is a group with respect to composition. Right translations are smooth, but left translations are in general discontinuous.
16 Hölder Diffeomorphism groups Theorem 1 Diff C n,β is a group with respect to composition. Right translations are smooth, but left translations are in general discontinuous. Theorem 2 G n,β = (Diff C n,β ). In particular C n,β is ODE-closed.
17 Proof(sketch): Diff C n,β is a group
18 Proof(sketch): Diff C n,β is a group Composition closedness: Classical proofs for composition closedness of C n,β (R d, R d ) can be modified for (Id +φ, Id +ψ) (Id +φ) (Id +ψ). Right translation (in chart representation) is affine and continuous and therefore smooth.
19 Proof(sketch): Diff C n,β is a group Composition closedness: Classical proofs for composition closedness of C n,β (R d, R d ) can be modified for (Id +φ, Id +ψ) (Id +φ) (Id +ψ). Right translation (in chart representation) is affine and continuous and therefore smooth. Inversion closedness: Let Φ = Id +φ, Φ 1 = Ψ = Id +ψ. Then ψ Φ = φ C n,β. Apply Faà di Bruno s formula, i.e. (ψ Φ) (n) (x) = sym n l=1 γ Γ(l,n) c γ ψ (l) (Φ(x)) (Φ (γ 1) (x),, Φ (γ l ) (x)), where Γ(l, n) := {γ N l > : γ = n}, c γ := n! l!γ!, and sym denotes the symmetrization of multilinear mappings.
20 Proof(sketch): Diff C n,β is a group
21 Proof(sketch): Diff C n,β is a group This yields ψ (n) (Φ(x))(Φ (x),, Φ (x)) ψ (n) (Φ(y))(Φ (y),, Φ (y)) = (φ (n) (x) φ (n) (y)) ( n 1 sym c γ ψ (l) (Φ(x)) (Φ (γ1) (x),, Φ (γ l ) (x)) n 1 l=1 γ Γ(l,n) l=1 γ Γ(l,n) ) c γ ψ (l) (Φ(y)) (Φ (γ1) (y),, Φ (γ l ) (y)), which (after some manipulations) yields ψ C n,β.
22 Proof(sketch): G n,β (Diff C n,β )
23 Proof(sketch): G n,β (Diff C n,β ) Let u be a pointwise time-dependent C n,β -vector field.
24 Proof(sketch): G n,β (Diff C n,β ) Let u be a pointwise time-dependent C n,β -vector field. Recall: u : I R d R d such that u(, x) is measurable for all x R d, u(t, ) C n,β for all t I, 1 u(t, ) n,β dt <.
25 Proof(sketch): G n,β (Diff C n,β ) Let u be a pointwise time-dependent C n,β -vector field. Recall: u : I R d R d such that u(, x) is measurable for all x R d, u(t, ) C n,β for all t I, 1 u(t, ) n,β dt <. Show: t φ u (t, ) C(I, C n,β ), where t Φ u (t, x) = x + φ u (t, x) = x + u(s, Φ u (s, x)) ds.
26 Proof(sketch): G n,β (Diff C n,β )
27 Proof(sketch): G n,β (Diff C n,β ) It is well known that φ u (t, ) C n and there are constants C 1, C 2 such that for all t I φ u (t, ) C n C 1 e C 1 2 u(s, ) C n ds.
28 Proof(sketch): G n,β (Diff C n,β ) It is well known that φ u (t, ) C n and there are constants C 1, C 2 such that for all t I φ u (t, ) C n C 1 e C 1 2 u(s, ) C n ds. And n x φ u (t, x) fulfills n x φ u (t, x) = t n x (u(s, Φ u (s, x)) ds. (1)
29 Proof(sketch): G n,β (Diff C n,β ) It is well known that φ u (t, ) C n and there are constants C 1, C 2 such that for all t I φ u (t, ) C n C 1 e C 1 2 u(s, ) C n ds. And n x φ u (t, x) fulfills Consider n x φ u (t, x) = t n x (u(s, Φ u (s, x)) ds. (1) V n x,y (t) := n x φ u (t, x) n x φ u (t, y) x y β.
30 Proof(sketch): G n,β (Diff C n,β ) It is well known that φ u (t, ) C n and there are constants C 1, C 2 such that for all t I φ u (t, ) C n C 1 e C 1 2 u(s, ) C n ds. And n x φ u (t, x) fulfills Consider n x φ u (t, x) = t n x (u(s, Φ u (s, x)) ds. (1) V n x,y (t) := n x φ u (t, x) n x φ u (t, y) x y β. Apply Faà di Bruno s formula to the integrand in (1); yields linear ODE for V n x,y.
31 Proof(sketch): G n,β (Diff C n,β ) It is well known that φ u (t, ) C n and there are constants C 1, C 2 such that for all t I φ u (t, ) C n C 1 e C 1 2 u(s, ) C n ds. And n x φ u (t, x) fulfills Consider n x φ u (t, x) = t n x (u(s, Φ u (s, x)) ds. (1) V n x,y (t) := n x φ u (t, x) n x φ u (t, y) x y β. Apply Faà di Bruno s formula to the integrand in (1); yields linear ODE for V n x,y. Gronwall s inequality together with integrability of t u(t, ) n,β yields uniform boundedness w.r.t. x, y, t of V n x,y (t).
32 Proof(sketch): G n,β (Diff C n,β ) It is well known that φ u (t, ) C n and there are constants C 1, C 2 such that for all t I φ u (t, ) C n C 1 e C 1 2 u(s, ) C n ds. And n x φ u (t, x) fulfills Consider n x φ u (t, x) = t n x (u(s, Φ u (s, x)) ds. (1) V n x,y (t) := n x φ u (t, x) n x φ u (t, y) x y β. Apply Faà di Bruno s formula to the integrand in (1); yields linear ODE for V n x,y. Gronwall s inequality together with integrability of t u(t, ) n,β yields uniform boundedness w.r.t. x, y, t of V n x,y (t). Integrability also gives continuity into C n,β.
33 Proof(sketch): G n,β (Diff C n,β )
34 Proof(sketch): G n,β (Diff C n,β ) For Φ = Id +φ (Diff C n,β ) there exists a polygon in (Diff C n,β with vertices Id = Φ, Φ 1 = Id +φ 1,, Φ k = Id +φ k = Φ. )
35 Proof(sketch): G n,β (Diff C n,β ) For Φ = Id +φ (Diff C n,β ) there exists a polygon in (Diff C n,β with vertices Id = Φ, Φ 1 = Id +φ 1,, Φ k = Id +φ k = Φ. ) Φ 1 = Φ u1 (1, ) with u 1 (t, x) = φ 1 ((1 t) Id +tφ 1 ) 1 (x); and u 1 is pointwise time-dependent Hölder vector field Φ 1 G n,β.
36 Proof(sketch): G n,β (Diff C n,β ) For Φ = Id +φ (Diff C n,β ) there exists a polygon in (Diff C n,β with vertices Id = Φ, Φ 1 = Id +φ 1,, Φ k = Id +φ k = Φ. ) Φ 1 = Φ u1 (1, ) with u 1 (t, x) = φ 1 ((1 t) Id +tφ 1 ) 1 (x); and u 1 is pointwise time-dependent Hölder vector field Φ 1 G n,β. Same argument gives Φ 2 Φ 1 1 G n,β. Since G n,β is a group, Φ 2 G n,β. Iterate argument another k 2 times; yields Φ k = Φ G n,β.
37 Further results
38 Further results Definition (Intermediate Hölder spaces) C n,β (R d, R d ) := α (,β) C n,α (R d, R d ), C n,β+ (R d, R d ) := α (β,1) C n,α (R d, R d ), endowed with natural projective and inductive locally convex topologies.
39 Further results Definition (Intermediate Hölder spaces) C n,β (R d, R d ) := α (,β) C n,α (R d, R d ), C n,β+ (R d, R d ) := α (β,1) C n,α (R d, R d ), endowed with natural projective and inductive locally convex topologies. Theorem Diff C n,β is a topological group, Diff C n,β+, group operations map smooth curves to continuous curves, G n,β± = (Diff C n,β± ).
40 Thanks for your attention!
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