Abstract
In this paper, an equivalent quasinorm for the Lipschitz space of noncommutative martingales is presented. As an application, we obtain the duality theorem between the noncommutative martingale Hardy space
1 Introduction
In the past two decades, due to the excellent work of Pisier and Xu on noncommutative martingale inequalities [1], the study of noncommutative martingale theory has attracted more and more attention. Especially in recent years, some meaningful research results on the noncommutative martingale theory have emerged continuously, and it has become a research hotspot in the field of noncommutative analysis.
The Lipschitz space was first introduced in the classical martingale theory by Herz and plays an important role in it. For instance, the Lipschitz space is the generalization of the BMO space and the dual space of the Hardy space
with equivalent norms. As its application, we have the duality equalities for
This answers positively a question asked in [2]. The other main result of this paper concerns the equivalent quasinorms for
which give a new characterization of
This paper is organized as follows. Some definitions and notations are given in Section 2. An equivalent quasinorm for noncommutative martingale Lipschitz space is shown in Section 3. Equivalent quasinorms for
2 Preliminaries
Let
where
Let us recall the general setup for noncommutative martingales. Let
If additionally,
Let
Let
and
These will be called the column and row conditioned square functions, respectively. Let
For more information of noncommutative martingales, see the seminal article of Pisier and Xu [3] and the sequels to it.
The main object of this paper is the noncommutative Lipschitz spaces
Definition 2.1
Let
with
where
Similarly, we define
The classical martingale space
Definition 2.2
We define
where
It is clear that
Definition 2.3
[2] Let
For
Similarly, define
Remark that for
where
Then
We recall the definition of the space
We usually write
Definition 2.4
[7] Let
equipped with the norm
Then
equipped with the norm
3 An equivalent quasinorm for the Lipschitz space of noncommutative martingales
In this section, we prove the noncommutative equivalent quasinorms for Lipschitz spaces
Theorem 3.1
For
Similarly,
The following Lemma is the key ingredient of our proof.
Lemma 3.2
For
where
Proof
By the definition of
Let P be a projection with respect to
Thus by (3.2), we get that
When
Note that
Now we prove (3.1) holds. Let
Thus using (3.2), we get that
It is easy to see that
Let
It follows that
Note that
The proof is complete.□
We will also need the following well-known lemma from [8].
Lemma 3.3
Let f be a function in
Proof of Theorem 3.1
Let
Noting that
Thus, we have that
Now, let
Let
where
By the continuous function calculus, we have
Applying Lemma 3.3 with
where we have used the fact that the operator function
The proof is complete.□
Using the dual result in Theorem 3.2 in [2], we will describe the dual space of
Corollary 3.4
Let
and
with equivalent norms.
4 Equivalent quasinorms for
h
1
c
(
ℳ
)
and
h
p
c
(
ℳ
)
(
2
<
p
<
∞
)
In this section, we first describe an equivalent quasinorm for
Theorem 4.1
We have that
with equivalent norms.
For the proof we need the following lemmas.
Lemma 4.2
Let
Proof
Let
Note that the set
Indeed, for any
Then we have that
Thus, the set
Lemma 4.3
Let
where
Proof
For any finite sequences
where
where
Let
Thus,
Therefore, by the definition of
□
Proof of Theorem 4.1
First let
Thus, we have that
Using (4.2), (4.3), and the fact
Thus, we deduce that
Therefore, we get that
where for every
It follows that
We turn to the converse inequality. Let
We will show
Indeed, let
where
Taking the infimum as in (4.6), we obtain
Similarly, we have that
Let
Theorem 4.4
Let
with equivalent norms. More precisely,
Similarly,
with equivalent norms.
Proof
Step 1: Let
Let
Therefore, we obtain that
Assume
It follows that
Now let
Thus, we get that
Therefore, the inequality
Step 2: Let
Thus, we have that
Now let
Thus, we have that
The proof of the theorem is complete.□
The following is an immediate consequence of Theorem 4.4 and Theorem 3.3 in [2] (or Theorem 3.1 in [7]).
Corollary 4.5
Let
and
with equivalent norms.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (11871195, 11671308, and 11471251).
References
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© 2020 Congbian Ma and Yanbo Ren, published by De Gruyter
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