1. Introduction and Preliminaries
Kadison and Kastler in [1] initiated the study of uniform perturbations of operator algebras. They considered a fixed C*-algebra
and equipped the set of all C*-subalgebras of
with a metric arising from Hausdorff distance between the unit balls of these subalgebras. We first recall the following definition of the metric d defined on the set of all C*-subalgebras of a C*-algebra
(see [1] ).
Definition 1.1. Let
and
be C*-subalgebras of a C*-algebra
. The Kadison-Kastler metric
between
and
is defined by
where
and
denote the unit ball of
and
respectively.
Kadison and Kastler conjectured in [1] that sufficiently close von Neumann algebras (or C*-algebras) are necessarily unitarily conjugate. The first positive answer to Kadison-Kastler’s conjecture was given by Christensen [2] when either
or
is a von Neumann algebra of type I. Many results related to this conjecture have been obtained during the past 40 years ( [3] [4] [5] [6] ). One-sided version of Kadison-Kastler’s conjecture was introduced and studied by Christensen in [4] as well. Christensen showed in [4] that a nuclear C*-algebra that is nearly contained in an injective von Neumann algebra is unitarily conjugate to this von Neumann algebra. Christensen, Sinclair, Smith and White showed in [5] that the property of having a positive answer to Kadison’s similarity problem transfers to close C*-algebras. Very recently, Kadison-Kastler’s conjecture has been proved for the class of separable nuclear C*-algebras in the remarkable paper [6] .
The problem we are going to consider is as follows: Suppose
are C*-subalgebras of a C*-algebra
. If
, is
and
share similar properties?
In this short note, we show that the sets of matricial field algebras (MF algebras) and quasidiagonal C*-algebras of a given C*-algebra are closed under the perturbation of uniform norm.
2. Main Results
In this section, we consider some topological properties of the set of all MF algebras and quasidiagonal C*-subalgebras under the perturbation of uniform norm. For basics of C*-algebras, we refer to [7] and [8] . We first recall the definition of MF algebras ( [9] ).
Suppose
is a sequence of complex matrix algebras. We can introduce the full C*-direct product
of
as follows:
(1)
Furthermore, we can introduce a norm closed two sided ideal in
as follows,
(2)
Let
be the quotient map from
to
. It is known that
is a unital C*-algebra. If we denote
by
, then
(3)
Now we are ready to recall an equivalent definition of MF algebras which is given by Blackadar and Kirchberg ( [9] ).
Definition 2.1. (Theorem 3.2.2, [9] ) Let
be a separable C*-algebra. If
can be embedded as a C*-subalgebra of
for a sequence
of integers, then
is called an MF algebra.
Lemma 2.2. ( [10] Lemma 2.12) Suppose that
is a separable C*-algebra. Assume for every finite family of elements
in
and every
, there is an MF algebra
such that
, (in the sense of Definition 2.3 in [10] ). Then
is also an MF algebra.
Proposition 2.3. Let
be a C*-algebra and
be the subset of all separable MF algebras contained in
. Then
is closed under the metric d.
Proof. Let
. Then there exist
such that
. For any
,
, there is an
such that
. Then there exist
such that
(4)
for all i. It follows from Lemma 2.2 that
is also a MF algebra.+
We will recall some results about quasidiagonal C*-algebras for the reader’s convenience. We refer the reader to [11] for a comprehensive treatment of this important class of C*-algebras.
Definition 2.4. A subset
is called a quasidiagonal set of operators if for each finite set
, finite set
and
, there exists a finite rank projection
such that
and
for all
and
.
Definition 2.5. A C*-algebra
is called quasidiagonal (QD) if there exists a faithful representation
such that
is a quasidiagonal set of operators.
The following result is Lemma 7.1.3 in [11] which is useful to determine whether a C*-algebra is quasidiagonal or not.
Lemma 2.6. A C*-algebra
is quasidiagonal if and only if for each finite set
and
, there exists a completely positive map
such that
(5)
and
(6)
for all
.
Proposition 2.7. Let
be a separable C*-algebra. Let
be the set of all quasidiagonal C*-subalgebras of
. Then
is closed under the metric d.
Proof. Let
and choose
such that
. Given finite subset
of the unit ball of
and
. There is a
such that
. Choose
in the unit ball of
such that
for
. Since
is QD, it follows from Lemma 2.6 that there is a c.c.p. map
such that
(7)
and
(8)
for all
. Now use Arveson’s extension theorem ( [11] ) to extend
to a c.c.p. map
from
to
. Let
be the restriction of
to
. Then for each
, we have
(9)
and
(10)
Use Lemma 2.6 again we have that
is quasidiagonal.+
3. Conclusion
In this paper, we use some characterizations of MF algebras and quasidiagonal C*-algebras to show that these two sets of C*-subalgebras of a given C*-algebras are closed with respect to the topology induced by the Kadison-Kastler metric.
Founding
Partially supported by NSFC (11871303 and 11671133) and NSF of Shandong Province (ZR2019MA039).