R E S E A R C H
Open Access
Boundedness of localization operators on
Lorentz mixed-normed modulation spaces
Ay¸se Sandıkçı
*Dedicated to Professor Ravi P Agarwal
*Correspondence:
[email protected] Department of Mathematics, Faculty of Arts and Sciences, Ondokuz Mayıs University, Samsun, Turkey
Abstract
In this work we study certain boundedness properties for localization operators on Lorentz mixed-normed modulation spaces, when the operator symbols belong to appropriate modulation spaces, Wiener amalgam spaces, and Lorentz spaces with mixed norms.
Keywords: localization operator; Lorentz spaces; Lorentz mixed normed spaces; Lorentz mixed-normed modulation spaces; Wiener amalgam spaces
1 Introduction
In this paper we will work onRd with Lebesgue measuredx. We denote byS(Rd) the
space of complex-valued continuous functions onRdrapidly decreasing at infinity. For
any functionf :Rd→C, the translation and modulation operator are defined asT xf(t) =
f(t–x) andMwf(t) =eπiwtf(t) forx,w∈Rd, respectively. For ≤p≤ ∞, we write the
Lebesgue spaces (Lp(Rd), · p).
Letx,t=di=xitibe the usual scalar product onRd. The Fourier transformfˆ(orFf)
off ∈L(Rd) is defined to be
ˆ f(t) =
Rd
f(x)e–πix,tdx.
For a fixed nonzerog∈S(Rd) the short-time Fourier transform (STFT) of a function f∈S(Rd) with respect to the windowgis defined as
Vgf(x,w) =f,MwTxg=
Rdf(t)g(t–x)e
–πitwdt,
forx,w∈Rd. Then the localization operatorAϕ,ϕ
a with symbolaand windowsϕ,ϕis
defined to be
Aϕ,ϕ
a f(t) =
Rd
a(x,w)Vϕf(x,w)MwTxϕdx dw.
Ifa∈S(Rd) andϕ
,ϕ∈S(Rd), then the localization operator is a well-defined
continu-ous operator fromS(Rd) toS(Rd). Moreover, it is to be interpreted in a weak sense as
Aϕ,ϕ
a f,g
=aVϕf,Vϕg=a,Vϕf Vϕg
forf,g∈S(Rd), [, ].
Fix a nonzero windowg∈S(Rd) and ≤p,q≤ ∞. Then the modulation spaceMp,q(Rd)
consists of all tempered distributionsf ∈S(Rd) such that the short-time Fourier
trans-form Vgf is in the mixed-norm space Lp,q(Rd). The norm on Mp,q(Rd) is fMp,q =
VgfLp,q. Ifp=q, then we writeMp(Rd) instead ofMp,p(Rd). Modulation spaces are
Ba-nach spaces whose definitions are independent of the choice of the windowg(see [, ]).
L(p,q) spaces are function spaces that are closely related toLp spaces. We consider
complex-valued measurable functionsf defined on a measure space (X,μ). The measure
μis assumed to be nonnegative. We assume that the functionsf are finite valued a.e. and somey> ,μ(Ey) <∞, whereEy=Ey[f] ={x∈X| |f(x)|>y}. Then, fory> ,
λf(y) =μ(Ey) =μ
x∈X|f(x)>y
is the distribution function off. The rearrangement off is given by
f∗(t) =infy> |λf(y)≤t =sup
y> |λf(y) >t
fort> . The average function off is also defined by
f∗∗(x) = x
x
f∗(t)dt.
Note thatλf,f∗, andf∗∗are nonincreasing and right continuous functions on (,∞). If λf(y) is continuous and strictly decreasing thenf∗(t) is the inverse function ofλf(y). The
most important property off∗is that it has the same distribution function asf. It follows that
X
f(x)pdμ(x)
p =
∞
f∗(t)pdt
p
. (.)
The Lorentz space denoted byL(p,q)(X,μ) (shortlyL(p,q)) is defined to be vector space of all (equivalence classes) of measurable functionsf such thatf∗pq<∞, where
f∗pq=
(qp∞tqp–[f∗(t)]qdt)q, <p,q<∞, supt>tpf∗(t), <p≤q=∞.
By (.), it follows thatf∗pp=fp and soL(p,p) =Lp. Also,L(p,q)(X,μ) is a normed
space with the norm
fpq=
For any one of the casesp=q= ;p=q=∞or <p<∞and ≤q≤ ∞, the Lorentz spaceL(p,q)(X,μ) is a Banach space with respect to the norm · pq. It is also well known
that if <p<∞, ≤q≤ ∞we have
·∗
pq≤ ·pq≤
p
p– ·
∗ pq
(see [, ]).
LetXandY be two measure spaces withσ-finite measuresμandν, respectively, and
letf be a complex-valued measurable function on (X×Y,μ×ν), <P= (p,p) <∞, and
≤Q= (q,q)≤ ∞. The Lorentz mixed norm spaceL(P,Q) =L(P,Q)(X×Y) is defined
by
L(P,Q) =L(p,q)
L(p,q)
=f :fPQ=fL(p,q)(L(p,q))=fpqpq<∞ . Thus,L(P,Q) occurs by taking anL(p,q)-norm with respect to the first variable and an
L(p,q)-norm with respect to the second variable. TheL(P,Q) space is a Banach space
under the norm · PQ(see [, ]).
Fix a window functiong∈S(Rd)\{}, ≤P= (p
,p) <∞, and ≤Q= (q,q)≤ ∞.
We letM(P,Q)(Rd) denote the subspace of tempered distributionsS(Rd) consisting of
f ∈S(Rd) such that the Gabor transformV
gf off is in the Lorentz mixed norm space
L(P,Q)(Rd). We endow it with the normfM(P,Q)=VgfPQ, where · PQis the norm
of the Lorentz mixed norm space. It is well known thatM(P,Q)(Rd) is a Banach space
and different windows yield equivalent norms. If p=q =pandp=q=q, then the
spaceM(P,Q)(Rd) is the standard modulation spaceMp,q(Rd), and ifP=pandQ=q, in
this caseM(P,Q)(Rd) =M(p,q)(Rd) (see [, ]), where the spaceM(p,q)(Rd) is Lorentz
type modulation space (see []). Furthermore, the spaceM(p,q)(Rd) was generalized to
M(p,q,w)(Rd) by taking weighted Lorentz space rather than Lorentz space (see [, ]). In this paper, we will denote the Lorentz space byL(p,q), the Lorentz mixed norm space
byL(P,Q), the standard modulation space byMp,q, the Lorentz type modulation space by
M(p,q), and the Lorentz mixed-normed modulation space byM(P,Q).
Let ≤r,s≤ ∞. Fix a compactQ⊂Rdwith nonempty interior. Then the Wiener
amal-gam space W(Lr,Ls)(Rd) with local componentLr(Rd) and global componentLs(Rd) is
defined as the space of all measurable functionsf :Rd→Csuch thatfχK ∈Lr(Rd) for
each compact subsetK⊂Rd, for which the norm
fW(Lr,Ls)=Ffs=fχQ+xr
s
is finite, whereχKis the characteristic function ofKand
Ff(x) =fχQ+xr∈Ls
Rd.
It is known that ifr≥rands≤sthenW(Lr,Ls)(Rd)⊂W(Lr,Ls)(Rd). Ifr=sthen
W(Lr,Lr)(Rd) =Lr(Rd) (see [–]).
norms. Our results extend some results in [, ] to the Lorentz mixed-normed modula-tion spaces.
2 Boundedness of localization operators on Lorentz mixed normed modulation spaces
We start with the following lemma, which will be used later on.
Lemma . Let
P+
P = ,
Q+
Q ≥,f ∈L(P,Q)(R
d),h∈L(P,Q
)(Rd).Then f∗h∈
L∞(Rd)and
L(P,Q)
Rd∗LP,Q
Rd→L∞Rd (.)
with the norm inequality
f∗h∞≤ fPQhPQ, (.)
where P= (p,p),Q= (Q,Q),Q= (Q,Q).
Proof It is well known that there areL(p,q)∗L(p,q)→L∞convolution relations
be-tween Lorentz spaces and
f∗h∞≤ fpqhpq,
where
p +
p = ,
q +
q ≥, by Theorem . in []. Then (.) and (.) can easily be
verified by using iteration and the one variable proofs given in [].
Letg∈D(Rd) be a test function such that
x∈ZdTxg≡. LetX(Rd) be a translation
invariant Banach space of functions with the property thatD·X⊂X. In the spirit of [,
], the Wiener amalgam spaceW(X,L(P,Q)) with local componentXand global
compo-nentL(P,Q) is defined as the space of all functions or distributions for which the norm
fW(X,L(P,Q))=f·T(z,z)gXPQ
is finite, where ≤P<∞, ≤Q≤ ∞. Moreover, different choices ofg∈Dyield equiva-lent norms and give the same space.
The boundedness ofAϕ,ϕ
Mζa fora∈M
∞is established by our next theorem. The proof is
similar to Lemma . in [] but let us provide the details anyway, for completeness’ sake.
Theorem .
(i) Let <P<∞,≤Q<∞.Iff∈M(P,Q)(Rd)andg∈M(Rd),then
Vgf∈W(FL,L(P,Q))(Rd)with
VgfW(FL,L(P,Q))≤ fM(P,Q)gM.
(ii) Let
P+
P = ,
Q +
Q ≥.Iff∈M(P,Q)(R
d)andg∈M(P,Q
)(Rd),then
Vgf∈W(FL,L∞)(Rd)with
Proof (i) Letϕ∈S(Rd)\{}and set=Vϕϕ∈S(Rd). By using the equalityV
gf(x,w) =
(f ·Txg)∧(w), we write
Vgf ·T(z,z)FL=
Rd
(Vgf ·T(z,z))
∧(t)dt
=
Rd
VVgf(z,z,t,t)dtdt
=
Rd
Vϕg(–z–t,t)Vϕf(–t,z+t)dtdt
=
Rd
Vϕf(u,u)Vϕg(u–z,u–z)dudu
=|Vϕf| ∗ |Vϕg|∼(z,z), (.)
for f,g ∈ S(Rd), where (Vϕg)∼(z) = (Vϕg)(–z), z ∈Rd. Since f,g ∈S(Rd), then f ∈
M(P,Q)(Rd) andg∈M(Rd) by Proposition in []. SoVϕf ∈L(P,Q)(Rd) andVϕg∈
L(Rd). Then, by Proposition in [], we obtain
VgfW(FL,L(P,Q))=Vgf ·T(z,z)FLPQ =|Vϕf| ∗ |Vϕg|∼PQ
≤ VϕfPQVϕg
=fM(P,Q)gM. (.)
This completes the proof.
(ii) Using Lemma . and (.), we have
VgfW(FL,L∞)=|Vϕf| ∗ |Vϕg|∼∞≤ VϕfPQVϕgPQ=fM(P,Q)gM(P,Q).
Theorem . Let <P<∞, ≤Q<∞.If a∈M∞(Rd),ϕ
,ϕ∈M(Rd),then AϕMζ,ϕa is
bounded on M(P,Q)(Rd)for everyζ∈Rdwith
Aϕ,ϕ
MζaB(M(P,Q))≤ aM∞ϕMϕM.
Proof Letf ∈M(P,Q)(Rd) andg∈M(P,Q)(Rd), where
P+
P = ,
Q+
Q = . Then we
writeVϕf ∈W(FL,L(P,Q))(Rd) andVϕg∈W(FL,L(P,Q))(Rd) by above theorem. Moreover, sinceM(, )(Rd) =M(Rd), we haveW(FL,L) =M=M(, ) by []. Hence
using the Hölder inequalities for Wiener amalgam spaces [] and (.) we obtain
Vϕf ·VϕgM =Vϕf·VϕgW(FL,L)
≤ VϕfW(FL,L(P,Q))VϕgW(FL,L(P,Q))
Thus by using (.) we have
Aϕ,ϕ
Mζaf,g=Mζa,Vϕf·Vϕg≤ MζaM(∞,∞)Vϕf·VϕgM(,) ≤ aM∞ϕMϕMfM(P,Q)gM(P,Q).
Hence we get
Aϕ,ϕ
MζaB(M(P,Q))≤ aM∞ϕMϕM.
Theorem . Letϕ∈S(Rd)\{}be a window function.If <P,Q<∞,t ∈(,∞),s≤t ≤
r and a∈W(Lr,Ls),then
AϕM,ϕ
ζa:M(tP,tQ)
Rd→MtP,tQRd
is bounded for everyζ ∈Rd,where
P+
P = ,
Q+
Q = ,and
t +
t = ,and the operator
norm satisfies the estimate
AϕM,ϕ
ζa≤ aW(Lr,Ls).
Proof Let t < ∞, f ∈ M(tP,tQ)(Rd), and h ∈ M(tP,tQ)(Rd). Then we have V
ϕf ∈
L(tP,tQ)(Rd) andV
ϕh∈L(tP,tQ)(Rd). SinceVϕf ∈L(tP,tQ)(Rd), thenVϕf∗(tP)(tQ)<
∞. By using the equality (.) in [], we get
Vϕf∗(tP)(tQ)=Vϕf∗(tp)(tq)∗(tp
)(tq)=|Vϕf|
t∗ pq
t∗
(tp)(tq) =|Vϕf|t
∗ pq
tt∗
pq
t =|V
ϕf|t ∗ pq
∗ pq
t
=|Vϕf|t∗PQ
t. (.)
Hence we have|Vϕf|t∈L(P,Q)(Rd). Similarly,|Vϕh|t∈L(P,Q)(Rd). By the Hölder
in-equality for Lorentz spaces with mixed norm and (.) we have
Vϕf ·Vϕhtt=|Vϕf|t|Vϕh|t≤|Vϕf|tPQ|Vϕh|tPQ
=Vϕft(tP)(tQ)Vϕht(tP)(tQ). (.)
Since a∈W(Lr,Ls), then M
ζa∈W(Lr,Ls) for everyζ ∈Rd. Also since W(Lr,Ls)⊂
W(Lt,Lt) =Lt(Rd), then we have
at =Mζat ≤ MζaW(Lr,Ls)=aW(Lr,Ls). (.)
By using (.), (.), and applying again the Hölder inequality, we get
AϕM,ϕζaf,h=MζaVϕf,Vϕh
≤
Rd
Mζa(x,w)(Vϕf·Vϕh)(x,w)dx dw
≤ atVϕf(tP)(tQ)Vϕh(tP)(tQ)
≤ aW(Lr,Ls)fM(tP,tQ)hM(tP,tQ). (.)
If (tp), (tq) =∞, then (M((tP), (tQ))(Rd))∗=M(tP,tQ)(Rd) by Theorem in []. Thus
we have from (.) that
AϕM,ϕ
ζafM((tP),(tQ))= sup =h∈M(tP,tQ)
|AϕM,ϕ ζaf,h| hM(tP,tQ) ≤
aW(Lr,Ls)fM(tP,tQ).
HenceAϕM,ϕ
ζais bounded. Also we have
AϕM,ϕ
ζa= sup
=f∈M(tP,tQ)
AϕM,ϕζafM((tP),(tQ))
fM(tP,tQ) ≤
aW(Lr,Ls).
Theorem . Letϕ∈≤R,S<∞M(R,S)(Rd),where R= (r
,r),S= (s,s).If≤s≤r≤ ∞
and a∈W(Lr,Ls)then
AϕM,ϕζa:M(P,Q)Rd→M(P,Q)Rd
is bounded for everyζ∈Rd,with
AϕM,ϕ
ζa≤CaW(Lr,Ls)
for some C> .
Proof Sincea∈W(Lr,Ls), thenM
ζa∈W(Lr,Ls) for everyζ ∈Rd. Also sinces≤r, there
exists ≤t≤ ∞such thats≤t≤r. ThenW(Lr,Ls)(Rd)⊂Lt(Rd) and
Mζat=at≤ aW(Lr,Ls)=MζaW(Lr,Ls) (.)
for alla∈W(Lr,Ls)(Rd). LetB(M(P,Q)(Rd),M(P,Q)(Rd)) be the space of the bounded
linear operators from M(P,Q)(Rd) into M(P,Q)(Rd). Also let T be an operator from
L(Rd) intoB(M(P,Q)(Rd),M(P,Q)(Rd)) byT(a) =Aϕ,ϕ
Mζa. Take anyf ∈M(P,Q)(R
d) and
h∈M(P,Q)(Rd). Assume thata∈W(L,L)(Rd) =L(Rd). By the Hölder inequality we get
T(a)f,h=AϕM,ϕζaf,h=MζaVϕf,Vϕh
≤
Rd
Mζa(x,w)Vϕf(x,w)Vϕh(x,w)dx dw
=
Rd
a(x,w)f,MwTxϕh,MwTxϕdx dw
≤
Rd
a(x,w)fM(P,Q)MwTxϕM(P,Q)hM(P,Q)
× MwTxϕM(P,Q)dx dw
Hence by (.)
T(a)fM(P,Q)=AMϕ,ϕζafM(P,Q)= sup
=h∈M(P,Q)
|AϕM,ϕζaf,h| hM(P,Q)
≤ ϕM(P,Q)ϕM(P,Q)fM(P,Q)a.
Then
T(a)=AϕM,ϕ
ζa= sup
=f∈M(P,Q)
AϕM,ϕ
ζafM(P,Q) fM(P,Q) ≤
ϕM(P,Q)ϕM(P,Q)a. (.)
Thus the operator
T:LRd→BM(P,Q)Rd,M(P,Q)Rd (.)
is bounded. Now let a∈W(L∞,L∞)(Rd) =L∞(Rd). Take any f ∈ M(P,Q)(Rd) and h∈M(P,Q)(Rd). ThenVϕf ∈L(P,Q)(Rd),Vϕh∈L(P,Q)(Rd). Applying the Hölder
in-equality
T(a)f,h=AϕM,ϕζaf,h=MζaVϕf,Vϕh
≤
Rd
Mζa(x,w)Vϕf(x,w)Vϕh(x,w)dx dw
≤ a∞
Rd
Vϕf(x,w)Vϕh(x,w)dx dw
≤ a∞VϕfPQVϕhPQ. (.)
By using (.) we write
T(a)fM(P,Q)=AMϕ,ϕζafM(P,Q)= sup
=h∈M(P,Q)
|AϕM,ϕζaf,h| hM(P,Q) ≤
a∞fM(P,Q). (.)
Hence by (.)
T(a)=AϕM,ϕζa= sup
=f∈M(P,Q)
AϕM,ϕζafM(P,Q)
fM(P,Q) ≤
a∞.
That means the operator
T:L∞Rd→BM(P,Q)Rd,M(P,Q)Rd (.)
is bounded. Combining (.) and (.) we obtain
T:LtRd→BM(P,Q)Rd,M(P,Q)Rd
is bounded by interpolation theorem for ≤t≤ ∞. That means the localization operator
is bounded for ≤t≤ ∞. Hence there existsC> such that
T(a)=AϕM,ϕζa≤Cat. (.)
This implies that it is also true for ≤t≤ ∞. From (.) and (.) we write
T(a)=AϕM,ϕ
ζa≤Cat≤CaW(Lr,Ls).
Proposition . Letϕ∈≤R,S<∞M(R,S)(Rd),where R= (r
,r),S= (s,s).If <s≤
and a∈W(L,Ls)(Rd)then
AϕM,ϕζa:M(P,Q)Rd→M(P,Q)Rd
is bounded.
Proof Let <s≤ and let a∈W(L,Ls)(Rd). Then M
ζa∈W(L,Ls) for every ζ ∈
Rd. Since W(L,Ls)(Rd)⊂L(Rd), there exists a numberC> such that M ζa ≤
CMζaW(L,Ls). Hence by (.),
AϕM,ϕζa≤ ϕM(P,Q)ϕM(P,Q)Mζa
≤CϕM(P,Q)ϕM(P,Q)MζaW(L,Ls) =CϕM(P,Q)ϕM(P,Q)aW(L,Ls).
Then the localization operator from M(P,Q)(Rd) into M(P,Q)(Rd) is bounded for
<s≤.
Proposition . Letϕ∈≤R,S<∞M(R,S)(Rd),where R= (r,r),S= (s,s).If≤P,Q<
∞and a∈L(P,Q)(Rd)then the localization operator
AϕM,ϕ
ζa:M(P,Q)
Rd→M(P,Q)Rd
is bounded,where
P+
P = ,
Q+
Q = .
Proof Let a ∈ L(P,Q)(Rd). Then M
ζa ∈ L(P,Q)(Rd) for every ζ ∈ Rd with
MζaPQ =aPQ. Take anyf ∈M(P,Q)(Rd) andh∈M(P,Q)(Rd). Applying the Hölder
inequality we have by (.)
AϕM,ϕζaf,h≤
Rd
Mζa(x,w)Vϕf(x,w)h,MwTxϕdx dw
≤
Rd
a(x,w)Vϕf(x,w)hM(P,Q)MwTxϕM(P,Q)dx dw
=hM(P,Q)ϕM(P,Q)
Rd
a(x,w)Vϕf(x,w)dx dw
Similarly to (.), we get
AϕM,ϕ
ζa≤ ϕM(P,Q)aPQ.
Then the localization operatorAϕM,ϕζafromM(P,Q)(Rd) intoM(P,Q)(Rd) is bounded.
Corollary . It is known by Propositionin[]thatS(Rd)⊂M(R,S)(Rd)for≤R,S<
∞.ThenS(Rd)⊂
≤R,S<∞M(R,S)(Rd).So,Theorem.,Propositions.and.are still
true under the same hypotheses for them ifϕ∈S(Rd).
Corollary . It is known[]that if P=p and Q=q,then Lorentz mixed-normed
modu-lation space M(P,Q)(Rd)is the Lorentz type modulation space M(p,q)(Rd).Therefore our
theorems hold for a Lorentz type modulation space rather than for a Lorentz mixed-normed modulation space.
Competing interests
The author declares that she has no competing interests.
Received: 21 August 2014 Accepted: 24 October 2014 Published:31 Oct 2014
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Cite this article as:Sandıkçı:Boundedness of localization operators on Lorentz mixed-normed modulation spaces.