Orthonormal bases of regular wavelets in spaces of homogenous type

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1 Orthonormal bases of regular wavelets in spaces of homogenous type P. Auscher 1 1 Université Paris-Sud 19/11/15

2 joint work with Tuomas Hytönen (ACHA, 2012)

3 quasi-metric geometrically doubling space X set, d quasi-distance on X X: 0 d(x, y) = d(y, x) < d(x, y) = 0 iff x = y d(x, y) A 0 (d(x, z) + d(z, y)) with A 0 1 (the quasi-metric constant). Open balls B(x, r) = {y ; d(x, y) < r} form a basis of the topology. If A 0 > 1, they may not be open sets, nor Borel sets, but B(x, r) B(x, 2A 0 r) and B(x, r/a 0 ) Int B(x, r). (X,d) called geometrically doubling (GD) with doubling constant N if every open ball B(x, 2r) can be covered by at most N balls B(x i, r). We always assume (GD).

4 doubling measure (X, d) is quasi-metric and µ a Borel measure. µ is doubling if x X, r > 0, 0 < µ(b(x, 2r)) Dµ(B(x, r)) <. [If B(x, r) not Borel set, replace it by its closure. We assume B(x, r) Borel set to simplify] (X, d, µ) called space of homogeneous type in the sense of Coifman-Weiss. The best D is the doubling constant of µ. Assume {f continuous with bbd support} dense in L p (µ), 1 p <. OK if µ is a true Borel measure. If µ defined on a larger σ-algebra, one needs µ to be regular [or restrict to Borel functions] If (X, d) admits a doubling measure, it is geometrically doubling.

5 Wavelet frames Frames: no orthogonality and f, ϕ k α 2 f 2. Theory of Han-Sawyer (1995) on Ahlfors-David sets: µ(b(x, r)) r [One can change d to a topologically equivalent quasi-distance with this property: this changes the balls]. Theory of Han-Müller-Yang (2008) with reverse doubling : µ(b(x, 2r)) (1 + ε)µ(b(x, r)). Such spaces have no holes. Petrushev-Kerkyacharian (2014): doubling metric spaces equipped with a diffusive self-adjoint operator built from a Dirichlet form [Analog of the ϕ-transform of Frazier-Jawerth] This excludes point masses situations. No Littlewood-Paley type analysis available on arbitrary SHT until now.

6 Wavelet frames Frames: no orthogonality and f, ϕ k α 2 f 2. Theory of Han-Sawyer (1995) on Ahlfors-David sets: µ(b(x, r)) r [One can change d to a topologically equivalent quasi-distance with this property: this changes the balls]. Theory of Han-Müller-Yang (2008) with reverse doubling : µ(b(x, 2r)) (1 + ε)µ(b(x, r)). Such spaces have no holes. Petrushev-Kerkyacharian (2014): doubling metric spaces equipped with a diffusive self-adjoint operator built from a Dirichlet form [Analog of the ϕ-transform of Frazier-Jawerth] This excludes point masses situations. No Littlewood-Paley type analysis available on arbitrary SHT until now.

7 Wavelet bases OK with no regularity: Haar bases using the dyadic cubes of M. Christ. Regular wavelet bases on R n, open sets, on certain Lie groups, discrete groups: symmetries and group representations. Regular wavelet bases on spaces of homogeneous types (even under AD or RD)?

8 orthogonal and regular wavelets Theorem Let (X, d, µ) SHT with quasi-triangle constant A 0. Let a := (1 + 2 log 2 A 0 ) 1 or a := 1 if d is Lipschitz-continuous. Let δ > 0 small. There exist C <, η > 0, γ > 0 and an orthonormal basis ψ k, k Z (and k k 0 if X is bounded) of L 2 (µ) (or the orthogonal space to constants if X is bounded), and points y k X, with ψ k (x) C exp ( γ(δ k d(y k, ) x))a := CG µ(b(y k, k (x), δk )) ( d(x, y) ) η(g ψ k (x) ψk (y) C k δ k (x) + G k (y)), ψ k (x) dµ(x) = 0. X

9 Nested sets Assume (X, d) is (GD). Let 0 < δ < One can find nested meshes: these are numerable sets X k, k Z, k k 0, δ k -separated and 2A 0 δ k -dense, with the nested property X k X k+1. k is the generation number and δ k the scale at that generation. Y k := X k+1 \ X k. This will be the label set for wavelets called the wavelet centers

10 Wavelet centers The distance to the set Y k will play an important role: it measures the holes in X at scale δ k. Set G k (x) = exp ( γ(δ k d(y k, ) x))a µ(b(y k, δk )) k,y k Yk G k (x)gk (y) Cµ(B(x, d(x, y))) 1. (1) Does not work for summation with yα k X k if X has holes so that growth of volume of balls is too slow. Holes imply failure of µ(b(x, δ k )) 1 µ(b(x, r)) 1. δ k r Holes imply big (relatively to scale) distance of x, y to Y k, forcing convergence of (1).

11 The idea 1,5 1 0,5 1-0,5 0 0,5 1 1,5 2 2,5 3-0,5 The linear spline function on R: s(x) = 1 [0,1) 1 [0,1) (x) = [0,1) (x u)du = [u,u+1) (x)du Random dyadic intervals of sidelength 1, in the sense of Nazarov, Treil and Volberg : translate the standard intervals [k, k + 1) by a random number u [0, 1). Splines are expected values of random indicators: s(x) = E u (1 [u,u+1) (x)) = P u (x [u, u + 1)). Need random dyadic systems on spaces of homogeneous type (Hytönen-Martikainen, Hytönen-Kairema)

12 Dyadic structures Assume (GD). A system of dyadic cubes follows from a partial order (l, ) (k, α) (descendant of) on some meshes at different scales, constructed with constraints on distances (M. Christ). Any (k, α) has 1 parent (k 1, ) and boundedly many children (k + 1, ). From meshes X k = {x k α}, get preliminary dyadic cubes ˆQ k α from the order: One has ˆQ α k = {x l : (l, ) (k, α)}. B(x k α, c 0 δ k ) ˆQk α B(x k α, c 1 δ k ), No need for nestedness of meshes at this stage.

13 Random dyadic structures I Randomness is adapted to design splines with hierarchical properties. Identify x k α and (k, α). x k α has at most M children. x k α has at most L neighbours: two mesh points of kth generation are neighbours if they have children within distance c 3 δ k for some c 3 > 0. Assign to x k α two labels: l 1 (x k α) {0, 1,..., L}, l 2 (x k α) {1,..., M}, in such a way that two neighbours have different label 1 and two children of x k α have different label 2.

14 Random dyadic structures II Probability space: Ω = ( {0, 1,..., L} {1,..., M}) Z, ω = (ω k ), ω k = (l k, m k ) {0, 1,..., L} {1,..., M}. Natural uniform probability on each level. The new dyadic points zα k = zα(ω) k = zα(ω k k ): { x zα k k+1 if l 1 (x k := α) = l k, and child of xα k with l 2 (x k+1 ) = m k, xα k if l 1 (xα) k l k, or child of xα k with l 2 (x k+1 ) = m k. New points are c 4 δ k -separated and c 5 δ k -dense. Let x k+1 be a fixed child of xα. k Then P ω (zα(ω) k = x k+1 ) 1 (L + 1)M.

15 Random dyadic structures III From new points zα(ω), k get a partial order ω in such a way that truth or falsity of (k + 1, ) ω (k, α) depends only on ω k : If x k+1 is close to some new dyadic point zα(ω k k ), then set (k, α) to be the new parent of (k + 1, ). If no such close point exists, use parent for. Proposition ˆQ k α(ω) = {z l (ω) : (l, ) ω (k, α)}. B(x k α, c 6 δ k ) ˆQk α (ω) B(x k α, c 7 δ k ). Small boundaries in probability for fixed generation k: ( P ω x ) ɛ Qα(ω) k Cɛ η α ɛ Q k α(ω) := {y ˆQk α (ω) : d(y, c Qk α (ω)) < ɛδ k }

16 Splines ) sα(x) k := P ω (x ˆQk α (ω). Bounded support 1 B(x k α, 1 8 A 3 0 δk ) (x) sk α(x) 1 B(x k α,8a 5 0 δk ) (x); Interpolation and reproducing properties sα(x k k ) = δ α, sα(x) k = 1, sα(x) k = α pα k sk+1 (x) where {pα k } is a finitely nonzero set of nonnegative coefficients with pk α = 1; and Hölder-continuity ( d(x, y) ) η. sα(x) k sα(y) k C δ k

17 s k α(x) = P ω (x = = = :(k+1,) ω(k,α) ) ˆQ k+1 (ω) } P ω ({(k + 1, ) ω (k, α) { x ˆQk+1 }) (ω) ( ) P ω ((k + 1, ) ω (k, α) )P ω x ˆQk+1 (ω) ) P ω ((k + 1, ) ω (k, α) s k+1 (x) =: pα k sk+1 (x), where the key third step used the independence of the two events; namely, the event (k + 1, ) ω (k, α) depends only on ω k, while being in the cube ˆQk+1 (ω) depends on ω l for l k + 1.

18 Spline Multiresolution Analysis Introduce now doubling measure µ. Set V k be the closed linear span of {sα} k α in L 2 (µ). Then V k V k+1, and { V k = L 2 {0}, if X is unbounded, (µ), V k = V k0 = {constants}, if X is bounded, k Z k Z where k 0 is some integer. Moreover, the functions s k α/ µ k α form a Riesz basis of V k : for all sequences of numbers λ α, α with µ k α := µ(b(x k α, δ k )). ( λ α sα k 1/2, λ α 2 µ L α) k 2 (µ) α

19 Wavelets W k = orthogonal complement of V k in V k+1. Follow an algorithm of Y. Meyer to get the wavelets: this is where we use the nestedness to have a label set Y k. Project the linearly independent {s k+1 : x k+1 X k+1 \ X k } orthogonally onto W k. Use orthogonalisation method (inverse square roots of Gram matrices) and that the inverse square roots of band-matrices have exponential decay by adapting results of Demko to 1-separated sets for a quasi-distance (get exp( γd(x, y) a ) for specified a > 0 of statement). Remark: in metric case, Hytönen-Tapiola propose a different random construction to get arbitrary regularity η < 1, but not η = 1. Questions: - can one get η = 1? No - can one get bounded support and orthogonality? unknown

20 Main result again Theorem Let (X, d, µ) SHT with quasi-triangle constant A 0. Let a := (1 + 2 log 2 A 0 ) 1 or a := 1 if d is Lipschitz-continuous. Let δ > 0 small. There exist C <, η > 0, γ > 0 and an orthonormal basis ψ k, k Z (and k k 0 if X is bounded), localised at y k Yk, of L 2 (µ) (or the space to constants if X is bounded) with ψ k (x) C exp ( γ(δ k d(y k, ) x))a := CG µ(b(y k, k (x), δk )) ( d(x, y) ) η(g ψ k (x) ψk (y) C k δ k (x) + G k (y)), ψ k (x) dµ(x) = 0. X

21 Thank you