1. Introduction
Duffin and Schaeffer in [1] while working in nonharmonic Fourier series developed an abstract framework for the idea of time-frequency atomic decomposition by Gabor [2] and defined frames for Hilbert spaces. In 1986, Daubechies, Grossmann and Meyer [3] found new applications to wavelets and Gabor transforms in which frames played an important role.
Let
be an infinite dimensional separable complex Hilbert space with inner product
. A system
is a frame (Hilbert) for
if there exist positive constants A and B such that
(1.1)
The positive constants A and B are called the lower and upper bounds of the frame
, respectively. They are not unique. The inequality (1.1) is called the frame inequality of the frame.
Gröchenig in [4] generalized Hilbert frames to Banach spaces. Before the concept of Banach frames was formalized, it appeared in the foundational work of Feichtinger and Gröchenig [5] [6] related to the atomic decompositions. Atomic decompositions appeared in the field of applied mathematics providing many applications [7] . An atomic decomposition allows a representation of every vector of the space via a series expansion in terms of a fixed sequence of vectors which we call atoms. On the other hand Banach frames for a Banach space ensure reconstruction via a bounded linear operator or the synthesis operator.
Definition 1.1. [4] . Let
be a Banach space,
the conjugate space of
and let
be an asso-
ciated Banach space of scalar valued sequences. A pair
is called a Ba-
nach frame for
with respect to an associated sequence space
if
1)
, for each
.
2) There exist positive constants
such that
![]()
3)
is a bounded linear operator operator such that
.
In the later half of twentieth century, Coifman and Weiss in [8] introduced the notion of atomic decomposition for function spaces. Later, Feichtinger and Gröchenig [5] [6] extended this idea to Banach spaces. This concept was further generalized by Gröchenig [4] , who introduced the notion of Banach frames for Banach spaces. Casazza, Han and Larson [9] also carried out a study of atomic decompositions and Banach frames. For recent development in frames for Banach spaces one may refer to [10] -[17] . Recently, various generalizations of frames in Banach spaces have been introduced and studied. Han and Larson [18] defined a Schauder frame for a Banach space
to be an inner direct summand (i.e. a compression) of a Schauder basis of
. The reconstruction property in Banach spaces was introduced and studied by Casazza and Christensen in [19] and further studied in [20] -[23] . The basic theory of frames can be found in [24] -[26] .
Definition 1.2. [19] . Let
be a separable Banach space. A sequence
has the reconstruction property for
with respect to a sequence
if
(1.2)
In short, we will say that the pair
has the reconstruction property for
. More precisely, we say that
is a reconstruction system or the reconstruction property for
.
The reconstruction property is an important tool in several areas of mathematics and engineering. The reconstruction property is also used to study the geometry of Banach spaces. In fact, it is related to the bounded approximated property as observed in [9] [27] .
Recently, Kaushik et al. in [20] introduced Banach Λ-frame for operator spaces while working in the reconstruction property in Banach spaces. In this paper we give necessary and sufficient conditions for the existence of Banach Λ-frames for operator spaces. A Paley-Wiener type stability theorem for Λ-Banach frames is dis- cussed.
2. Banach Λ-Frames
The reconstruction property in Banach spaces is a source of other redundant systems! For example, if
has the reconstruction property for
with respect to
. Then, we can find a reconstruction operator
such that
is a Banach frame for
. The Banach frame
is called the associated Banach frame for the underlying space. Similarly we can find a reconstruction operator associated with the system
. It is natural to ask whether we can find Banach frames for a large class of spaces associated with a given reconstruction system. In this direction the Banach Λ-frames for the operator spaces introduced in [20] . First recall that the family of all bounded linear operator from a Banach space
into a Banach space
is denoted by
. If
, then we write
. An operator
is said to be coercive if there exists
such that
for all
.
Definition 2.1. [20] . Let
and
be Banach spaces and let
be a sequence space associated with
. A sequence
is a Banach Λ-frame for
if there exist positive constants
such that
(2.1)
If upper inequality in (2.1) is satisfied, then
is called a Λ-Bessel sequence for
with Bessel bound B0. The operator
given by
,
is called the pre-frame operator and the analysis operator
is given by
![]()
The positive constants
,
are called the lower and upper frame bounds of the Banach Λ-frame, respectively. If the removal of any
from the Banach Λ-frame renders the collection
to be a Banach Λ-frame for the underlying space, then
is said to be an exact Banach Λ-frame.
Remark 2.2. If
, then
. Therefore,
becomes a Banach frame for
with respect to the associated Banach space
.
Suppose that
has the reconstruction property for
with respect to
where
. Let
be a Banach space and let
![]()
be its associated Banach space of sequences with the norm given by
![]()
Then,
is a Banach Λ-frame for the operator space
with respect to
. There may be other sequence spaces with respect to which
form a Banach frame for the underlying space. The following theorem provides existence of the Banach Λ-frame for the operator spaces (see [20] ). We give the proof for the completeness.
Theorem 2.3. [20] . Suppose that
has the reconstruction property for
with respect to
. Then,
is a Banach Λ-frame for the operator space
with respect to
.
Proof. Let
be arbitrary. For each
, define
by
![]()
Then
![]()
Thus,
, for all
. Therefore, by using the Banach-Steinhaus Theorem, we have
.
Fix
. Then,
(2.2)
Also for all
, we have
(2.3)
Therefore, by using (2.3) we obtain
.
This gives
(2.4)
By using (2.2) and (2.4) with
, we have
![]()
Hence
is a Banach Λ-frame for the operator space
with respect to
. This completes the proof. □
The following theorem gives necessary and sufficient conditions for
to be a Λ-Banach frame for
with respect to an associated Banach space of scalar valued sequences
.
Theorem 2.4. A sequence
is a Banach Λ-frame for
with respect to
which is generated by
if and only if
is isomorphic to a closed subspace of
.
Proof. Assume that
is Banach Λ-frame for
with respect to
. Then, there exist positive constants A, B such that
(2.5)
By using lower frame inequality in (2.5), the analysis operator T of
is coercive. Thus T is injective and has close range. From the Inverse Mapping Theorem,
is isomorphic to the range
, which is a subspace of
. For the reverse part, assume that M is a closed subspace of
and U is an isomorphic from
onto M. Let
be the sequence coordinate operators on
, then
for all
.
Choose
,
. Then, for all
we have
![]()
Therefore
![]()
Hence
is Banach Λ-frame for
with respect to
. □
Theorem 2.5. A sequence
is a Banach Λ-frame for
if and only if
is isomor- phic to a complemented subspace of
which is generated by
.
Proof. Assume first that
is Banach Λ-frame for
and let T is the analysis operator and S is the synthesis operator for the Banach Λ-frame
. Then,
is the identity operator on
. Choose
. Then,
and
. Therefore, P is the projection from
to the range of T. Thus,
is an isomorphism and
is complemented subspace of
.
For the reverse part, if
is an isomorphism, where
is the complemented subspace of
. Then, by Theorem 2.4, the sequence
is a Banach Λ-frame for
. □
2.1. Construction of Banach Λ-Frames from Operators on ![]()
Let
ba a Banach Λ-frame for
and let
. Let
be such that
,
. Then,
is a Λ-Bessel sequence for
, but in general, not
a Banach Λ-frame for
.
The following theorem provides necessary and sufficient conditions for the construction of a Banach Λ-frame from a bounded linear operator on
.
Theorem 2.6. Let
ba a Banach Λ-frame for
and let
be such that
, where
. Then,
is a Banach Λ-frame for
if and only if
![]()
where
is a positive constant and
is such that
,
.
Proof. Assume first that
is a Banach Λ-frame for
with bounds
,
. Let
and
be the pre-frame operator and analysis operator associated with
, respectively. Choose
. Then,
is such that
,
. Let
be the pre-frame operator associated
with Banach Λ-frame
. Choose
. Then, for all
we have
![]()
For the reverse part, we compute
![]()
Hence
is a Banach Λ-frame for
with bounds
and
. □
The following theorem gives the better Λ-Bessel bound for the sum of two Banach Λ-frames.
Theorem 2.7. Let
and
be Banach Λ-frames for
with respect to
and let
be an invertible operator such that
,
. Then,
is a Λ-Bessel sequence with bound
![]()
where
,
are the analysis operators associated with
and
, respectively and
is the identity operator on
.
Proof. For all
, we have
![]()
Similarly, we can show that
![]()
Hence
is a Λ-Bessel sequence with required Bessel bound. □
Remark 2.8. The Λ-Bessel sequence
in Theorem 2.7, in general, not a Banach Λ-frame for
. If the analysis operator associated with the Λ-Bessel sequence is coercive, then a Λ-Bessel sequence turns out to be a Banach Λ-frame for the underlying space. This is summarized in the following lemma.
Lemma 2.9. Let
be Λ-Bessel sequence for
. Then
is a Banach Λ-frame for
if and only if its analysis operator is coercive.
The following theorem gives a relation between the bounds of a Banach Λ-frame
and Bessel bound for a Λ-Bessel sequence
such that
becomes a Banach Λ-frame for
.
Theorem 2.10. Let
be a Banach Λ-frame for
with bounds A, B and let
be a Λ-Bessel sequence for
with bound
, then
is a Banach Λ-frame for
.
Proof. Suppose that T and R are analysis operators associated with
and
for
. For any
, we have
![]()
Thus,
is a Λ-Bessel sequence for
.
Now
![]()
Hence
is a Banach Λ-frame for
. □
Given a Banach Λ-frame for
, we now give an estimate of the Bessel bound for
such that
becomes a Banach Λ-frame for
. This is given in the following proposition.
Proposition 2.11. Assume that
is a Banach Λ-frame for
with respect to
. Let
be a sequence such that
is a Λ-Bessel sequence for
with respect to
with Bessel bound
, where S is the pre-frame operator associated with
. Then,
is a Banach Λ-frame for
.
Proof. We compute
![]()
Hence
is a Banach Λ-frame for
. □
2.2. Perturbation of Λ-Banach Frames
Perturbation theory is a very important tool in various areas of applied mathematics [7] [19] [28] . In frame theory, it began with the fundamental perturbation result of Paley and Wiener. The basic of Paley and Wiener is that a system that is sufficient close to an orthonormal system (basis) in a Hilbert space also forms an orthonormal system (basis). Since then, a number of variations and generalization of this perturbation to the setting of Banach space and then to perturbation of the atomic decompositions, frames (Hilbert)and Banach frames, the reconstruction property in Banach spaces [19] [20] . The following theorem gives a Paley-Wiener type perturbation (in Banach space setting) for Λ-Banach frames.
Theorem 2.12. Let
be a Banach Λ-frame for
with bounds A, B and let
. As-
sume that
are non-negative real number such that
and
(2.6)
where T and R are the analysis operators associated with
and
, respectively. Then,
is a
Banach Λ-frame for
with bounds
and
.
Proof. For any
, we have
![]()
Since
(2.7)
By using (2.6) and (2.7), we have
(2.8)
Now
(2.9)
By using (2.6) and (2.9), we have
(2.10)
Therefore, by using (2.8) and (2.10) we conclude that
is a Banach Λ-frame for
with desired frame bounds. □
Remark 2.13. For other types of perturbation results one may refer to [11] , which can be generalized to Banach Λ-frame for
.