VECTOR VALUED MULTIVARIATE SPECTRAL MULTIPLIERS, L ITTLEWOOD-PALEY FUNCTIONS, AND SOBOLEV SPACES IN THE HERMITE SETTING Alejandro Sanabria Department de Mathematical Analysis University of La Laguna Complex Analysis and Approximation National University of Ireland, Maynooth 17-19th June 2013
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VECTOR VALUED MULTIVARIATE SPECTRAL MULTIPLIERS,LITTLEWOOD-PALEY FUNCTIONS, AND SOBOLEV SPACES
IN THE HERMITE SETTING
Alejandro SanabriaDepartment de Mathematical Analysis
University of La Laguna
Complex Analysis and ApproximationNational University of Ireland, Maynooth
17-19th June 2013
1 IntroductionIntroductionHeat and Poisson semigroupsHermite Littlewood-Paley functionsUMD-Banach spacesγ-radonifying operators.
Sjögren (1982), Urbina (1990), Fabes, Gutiérrez and Scotto (1994): harmonic analy-sis operators associated with the Ornstein-Uhlenbech operator in Rn.
In the last decade this topic has been studied by a host of authors: García-Cuerva,Mauceri, Meda, Sjögren, and Torrea (2000), Pérez, and Soria (2000), Harboure,Torrea, and Viviani (2003).
Hermite function setting:
Thangavelu (1993) , K. Stempak and J.L. Torrea (2003): harmonic analysis opera-tors associated with the Hermite operator (Hermite function expansions setting).
C. Segovia and R. Wheeden (J. Math. Mech.; 19 (1969-70); 247-262) introducedthe notion of fractional derivative:
∂σt f (t,x) =
e−π(m−σ)i
Γ(m−σ)
∫∞
0∂
mt f (t + s,x)sm−σ−1ds, x ∈ Rn and t ∈ (0,∞).
where σ > 0 and m ∈ N\0 such that m−1≤ σ < m.The generalized Littlewood-Paley function associated with the Poisson semigroupfor the Hermite operator is defined by
gHP,σ(f )(x) =
(∫∞
0|tσ
∂σt PH
t (f )(x)|2 dtt
) 12
, σ > 0,
For every 1 < p < ∞ there exists C > 0 such that
1C‖f‖Lp(Rn) ≤ ‖gH
P,σ(f )‖Lp(Rn) ≤ C‖f‖Lp(Rn), f ∈ Lp(Rn), σ > 0.
J.L. Torrea and C. Zhang, Fractional vector-valued Littlewood-Paley-Stein theoryfor semigroups (arXiv: 1105.6022.v3) considered generalized Littlewood-Paley g-functions associated with diffusion semigroups.
Suppose B is a Banach space and σ > 0. A first and natural definition of theLittlewood-Paley function on Lp(Rn,B), 1< p <∞, is the following one. If Lp(Rn,B),1 < p < ∞, we define
GHP,σ,B(f )(x) =
(∫∞
0‖tσ
∂σt PH
t (f )(x)‖2B
dtt
) 12
, σ > 0,
M. Martínez, J.L. Torrea, Q. Xu, C. Zhang, amongst others.
TheoremLet B is a Banach space. The following assertions are equivalent:
i) B is isomorphic to a Hilbert space
ii) For some (equivalently, for any) 1 < p < ∞ and σ > 0, we have that
Motivated by the ideas in:C. Kaiser, Wavelet transforms for functions with values in Lebesgue spaces, Procee-dings of SPIE Optics and Photonics, Conf. on Mathematical Methods. Wavelets XI5914, 2005.C. Kaiser and L. Weis, Wavelet transform for functions with values in UMD spaces,Studia Math., 186 (2), 101-126, 2008.
Take a different point of view of vector valued Littlewood-Paley functions looking forthe validity of the equivalence
1C‖f‖Lp(Rn,B) ≤ ‖GH
P,σ,B(f )‖Lp(Rn,B) ≤ C‖f‖Lp(Rn,B), f ∈ Lp(Rn,B),
for other Banach spaces that are not Hilbert spaces.
The Hilbert transform of f ∈ Lp(R), 1≤ p < ∞ is defined by
H(f )(x) = limε→0
1π
∫|x−y |>ε
f (y)
y− xdy , a.e. x ∈ R.
TheoremThe operator H is a bounded operator from Lp(R) into itself, for every 1 < p < ∞, andfrom L1(R) into L1,∞(R)
If 1< p <∞ and B is a Banach space, the Hilbert transform is defined on Lp(R)⊗Bin a natural way as the operator H⊗ IdB.
DefinitionA Banach space B is said to be a UMD space when the Hilbert transform H can be ex-tended to the Bochner-Lebesgue space Lp(R,B) as a bounded operator from Lp(R,B)into itself, for some 1 < p < ∞.
B is a UMD Banach space if and only if the Hilbert transform can be extended toL1(R,B) as a bounded operator from L1(R,B) into L1,∞(R,B).
There exist a lot of other characterizations for the UMD Banach spaces (Burkholder,Bourgain, Hytönen, Torrea, Xu, Harboure, Viviani, Guerre-Delabrière,...).
DefinitionA bounded operator T ∈ L(H ,B) is called γ-radonifying if it satisfies the equivalentconditions of the theorem. For such an operator we define
‖T‖γ =
(∫B‖x‖2dµ(x)
) 12
=
E
∥∥∥∥∥ ∞
∑n=1
gnThn
∥∥∥∥∥2 1
2
Theorem(γ(H ,B),‖ · ‖γ) is a real Banach space, which is separable provided that B isseparable.
If T ∈ L(H ,B) is γ-radonifying, then T is compact.
If B is a real Hilbert space, then T ∈ L(H ,B) is γ-radonifying if and only if T isHilbert-Schmidt.
Suppose thatH = L2(W ,µ) where (W ,B,µ) is a σ-finite measure space with a countably genera-ted σ-algebra B and thatf : W → B is a strongly measurable function such that f is weakly H , that is, for everyS ∈ B∗, the dual space of B, the function S f ∈H .
Then there exists Tf ∈ L(H ,B) satisfying that
〈S,Tf (ϕ)〉B∗,B =∫
W〈S, f (w)〉B∗,Bϕ(w)dµ(w), ϕ ∈H .
where 〈·, ·〉B∗,B denotes the (B∗,B) duality.
We say that f ∈ γ(W ,µ;B) when Tf ∈ γ(H ;B) and then we define
J. Betancor, A. Castro, J. Curbelo, J. Fariña, and L. Rodríguez-Mesa. J. Funct.Anal., 263 (12): 3804-3856, 2012.The univariate fractional g-function operator associated with the Hermite-Poissonsemigroup is defined by
GHP,σ;B(f )(t,x) =
∫R
tσ∂
σt PH
t (x ,y)f (y)dy , σ > 0, x ∈ R and t > 0,
for every f ∈ Lp(Rn,B).
TheoremLet H = L2
((0,∞), dt
t
), B be a UMD Banach space and σ > 0. Then, the operator
GHP,σ,B is bounded
from Lp(Rn,B) into Lp(Rn,γ(H ,B)), for every 1 < p < ∞,
from L1(Rn,B) into L1,∞(Rn,γ(H ,B)), and
from H1(Rn,B) into L1(Rn,γ(H ,B)).
Moreover, for every 1 < p < ∞, ‖f‖Lp(Rn,B) ∼ ‖GHP,σ,B(f )‖Lp(Rn,γ(H ,B)), f ∈ Lp(Rn,B).
Note that, since γ(H ,C) = H , if f ∈ Lp(Rn), 1 < p < ∞, then
If εj∞j=1 is a sequence of independent symmetric ±1-valued random variables
(usually called Rademacher variables) on some probability space, we denote byEε the corresponding expectation operator.
Suppose that εj∞j=1 and ηj∞
j=1 are two independent sequences of Rademachervariables. We say that a Banach space B has (Pisier’s) property (α) when thereexists C > 0 such that
EεEη
∥∥∥ N
∑i,j=1
αi,jεiηjxi,j
∥∥∥B≤ EεEη
∥∥∥ N
∑i,j=1
εiηjxi,j
∥∥∥B,
for every αi,j ∈ +1,−1, xi,j ∈ B, i, j = 1, . . . ,N, and N ∈ N\0.UMD and (α) properties of Banach spaces are crucial in order to prove Banachvalued Fourier multipliers of Mikhlin type.
Multiparametric case Multiparametric Littlewood-Paley functions
Since B has the property (α) we have that γ(H n+1,B)' γ(H 1,γ(H n,B)) (γ-Fubiniproperty - Van Neerven, Weis). By using the induction hypothesis and taking intoaccount that γ(H l ,B), l ∈ N\0, is UMD with the property (α) we obtain
‖GHP,β;B(f )‖Lp(Rn+1,γ(H n+1,B))
=
(∫Rn+1‖GH
P,β;B(f )(·,x)‖pγ(H n+1,B)dx
) 1p
=
(∫Rn+1
∥∥∥∥∫R tβ11 ∂
β1t1 PH
t1 (x1,y1)GHP,β;B(f )(t, x)dy1
∥∥∥∥p
γ(H 1,γ(H n,B))dx
) 1p
≤ C
(∫R
∫Rn‖GH
P,β;B(f (x1, y))(t, x)‖pγ(H n,B)dxdx1
) 1p
≤ C‖f‖Lp(Rn+1)
The operator GHP,β;B can be extended to Lp(Rn+1,B) as a bounded operator from
Lp(Rn+1,B) into Lp(Rn+1,γ(H n+1,B)). This extension operator is denoted by GHP,β;B.
Suppose now that m is a bounded Borel measurable function from (0,∞)n into C.The Hermite multivariate multiplier Tm associated with m is defined by
Tmf = ∑k=(k1,...,kn)∈Nn
m(λk1 , · · · ,λkn )ck (f )hk , f ∈ L2(Rn),
Tm is a bounded operator from L2(Rn) into itself.
Thangavelu (Theorem 4.2.1, Lectures on Hermite and Laguerre operators, Prince-ton Univ. Press, Princeton, N.J., 1993) established Mikhlin-Hörmander type condi-tion on m under that Tm is bounded from Lp(Rn) into itself, for every 1 < p < ∞.
We define the operator Tm⊗ IdB on L2(Rn)⊗B in the usual way.
We are motivated by the results of Meda (Proc. Amer. Math. Soc., 110 (3), (1990),639-647) and Wróbel (Monatsh. Math., 168 (1) (2012), 125-149).
The operator Liβ⊗ IdB is defined in the usual way on L2(Rn)⊗B.
Liβ⊗ IdB can be extended to Lp(Rn,B) as a bounded operator from Lp(Rn,B) intoitself, for every 1 < p < ∞, provided that B is a UMD Banach space. (Betancor,Castro, Curbelo, Fariña, and Rodríguez-Mesa, Complex Anal. Op. Th., 2011).
TheoremLet B be a UMD Banach space with the property (α) and 1 < p < ∞. Suppose that m isa bounded Borel measurable function on (0,∞)n, such that for some γ ∈ Nn,∫
Rnsup
t∈(0,∞)n|Mγ(t,u)|‖Li u
2 ‖Lp(Rn,B)→Lp(Rn,B)du < ∞.
Then, the multiplier operator Tm is bounded from Lp(Rn,B) into itself.
We need to use intermediate UMD spaces in the following theorem to get a suitableestimate for the operator norm ‖Liγ‖Lp(Rn,B)→Lp(Rn,B), γ ∈ Nn and 1 < p < ∞.
We consider a class of UMD Banach spaces, called intermediate UMD spaces,that includes all known examples of UMD spaces.
We say that B is an intermediate UMD space when B is isomorphic to a closedsubquotient of a complex interpolation space [X ,Q ]θ, where θ ∈ (0,1), X is aUMD Banach space and Q is a Hilbert space.
This class of UMD spaces has been used recently by Berkson and Gillespie, Hytö-nen, Hytönen and Lacey, and Taggart.
It is, as far as we know, an open problem if every UMD space is an intermediateUMD space. This question was posed by Rubio de Francia.
TheoremSuppose that B is isomorphic to a closed subquotient of [X ,Q ]θ, where θ ∈ (0,1), X isa UMD Banach space and Q is a Hilbert space and that B has the property (α). If m isa bounded holomorphic function in Γψ for some ψ > π
4 , then the multiplier operator Tmcan be extended to Lp(Rn,B) as a bounded operator from Lp(Rn,B) into itself, provided
Let β > 0. The −β-power H−β of the Hermite operator is defined by
H−βf = ∑k∈Nn
ck (f )
(2|k |+ n)βhk , f ∈ Lp(Rn,B), 1 < p < ∞.
H−β is bounded, positive and one to one from Lp(Rn,B) into itself, for every 1 <p < ∞.The potential space
LpH,β(Rn,B) = f ∈ Lp(Rn,B) : f = H−βg, for some g ∈ Lp(Rn,B).
‖f‖LpH,β
(Rn,B) = ‖g‖Lp(Rn,B)
TheoremSuppose that B is isomorphic to a closed subquotient of [X ,Q ]θ where θ ∈ (0,1), X isa UMD Banach space, and Q is a Hilbert space, and that B has the property (α). Then,for every ` ∈ N\0,
The Hermite Riesz transform Rm,j is defined as follows
Rm,j f = Am11 · · ·A
mjj A
mj+1−(j+1) · · ·A
mn−nH−
|m|2 f , f ∈ FH .
where m = ∑nj=1 mj . FH denotes the linear space spanhkk∈Nn .
PropositionSuppose that B is isomorphic to a closed subquotient of [X ,Q ]θ, where θ ∈ (0,1), X isa UMD Banach space and Q is a Hilbert space and that B has the property (α) and 1 <
p <∞ being∣∣∣ 2
p −1∣∣∣< θ. If m = (m1, . . . ,mn)∈Nn and j = (j1, . . . , jn)∈Zn, being |ji |= i ,
i = 1, . . . ,n, the Hermite Riesz transform R jm defined by R j
mf = ∏ni=1 Ami
ji H−|m|2 f , f ∈
FH , where |m| = ∑ni=1 mi , can be extended to Lp(Rn,B) as a bounded operator from