On Maps Preserving Unitarily Invariant Norms of the Spectral Geometric Mean ()
1. Introduction
In order to study the geometry on the cones
of the positive definite
matrices, people consider different means of positive definite matrices. Szokol, Tsai and Zhang [1] investigated the structure of geodesic affine maps in
and considered the following three types of geodesics:
• The weighted arithmetic mean
• The weighted geometric mean
• The weighted log-Euclidean mean
The authors in [1] considered the maps preserving
(
) under the p-norm for some
. They showed that those maps are the restriction of algebra *-automorphisms and *-antiautomorphisms on
. Furthermore, Gaál and Nagy ( [2], Theorem 1) obtained the same results as in [1] concerning the bijective transforms of
which preserve any unitary invariant norm of some quasi-arithmetric means of elements (it includes the weighted arithmetic mean and the weighted log-Euclidean mean) for all
. For the log-Euclidean mean, the Gaál and Nagy ( [2], Theorem 2) obtained a general results concerning the p-norm in a C*-algebra
equipped with a faithful tracial state, where
. All the results showed that any correspondence preserver is the restriction of a Jordan *-isomorphism of
multiplied by a central positive invertible element [3] [4]. Molnár and Szolol ( [5], Theorem 2) considered those bijective maps between the positive semidefinite cones of standard operator algebras which preserve some given symmetric norm of a rather general Kubo-Ando operator mean of the elements.
Suppose that
is a C*-algebra, then we denote by
the set of all positive invertible elements in
, which is called the positive definite cone of
. The spectral geometric mean is the operation on
defined by
where
It requires some algebraic manipulations to verify that
(which is not a Kubo-Ando type mean) is also symmetric (see [6] ). We refer to [7] for more details about spectral geometric mean. Concerning a map preserving the spectral geometric mean, we have only for the full operator algebra over a Hilbert space.
Theorem 1.1. (see ( [7], Theorem 3)) Let
be a complex Hilbert space and let
be a continuous bijective map such that
If
and
has a continuous bijective extension to
, then there is a unitary or antiunitary operator U on
such that
for all
.
For the C*-algebra norm, Proposition 7 in [7] showed that:
Theorem 1.2. Let
be C*-algebras. Let
be a surjective map. Then
if and only if there is a Jordan *-isomorphism
which extends
.
In this present note, we consider maps on positive definite cones of C*-algebras or von Neumann algebras preserving unitarily invariant norms of the spectral geometric means.
Let
be a C*-algebra and N a norm on
. We say that N is unitarily invariant if
for all unitaries
and
. We say a norm N on
has the property P if
,
and
implies
. Let
be a von Neumann algebra with a faithful tracial state
and let
. It can be verified that the function
defines a unitary invariant norm on
(see ( [8], Section 3)). Moreover, Molnár ( [9], Lemma 4) proved that the above p-norm with
has the property (P).
2. Main Results
Let
be a C*-algebra with a unitarily invariant mean N. Let
. It follows from [7] that
is unitarily congruent to
and therefore
.
Lemma 2.1. Let
be a C*-algebra with a unitarily invariant mean N having property P. Let
. Then
iff for any
,
.
Proof. (
) Let
and suppose
. For any
, we have
and therefore
Then by the proof of Proposition 3 in [10], we have
and this implies
.
(
) Suppose for any
,
. Our aim is to show
. Let P be the spectral projection of
corresponding to
. Then
and
. Hence
But we also have
(since for any
,
,
and let
, we get
) and therefore
Since N has property P, we have
and therefore
, i.e.,
. It follows that
.
We also need the following result in [11].
Lemma 2.2. Let
be a von Neumann algebra and N is a unitarily invariant norm on
with property (P). Let
be fixed. For
, we have
if and only if
for all
.
Theorem 2.3. Suppose
and
are von Neumann algebras with unitarily invariant norms N and M respectively and both having property (P). Let
be a bijective map. If
then there is a Jordan *-isomorphism
and an element
such that
for all
.
Proof. We first show that
is positive homogeneous. Indeed, for any
,
, we have
and therefore
This implies that
by Lemma 2.1 and
is positive homogeneous. Since for any
, we have
is an order isomorphism. It follows from [10] that there exist an element
and a Jordan *-isomorphism
such that
Note that we have
and
for all
. Also we have
.
Remark 2.3. Suppose
are von Neumann algebras with unitarily invarinat norms N and M having property (P). When there is a Jordan *-isomorphism
and a central element
such that
and
for all
, it is easy to check that
Corollary 2.4. Suppose
and
are von Neumann algebras. Assume that
is a factor with a unique tracial state Tr and
is a tracial state of
. Let
and
the p-norm corresponding to Tr and
. Suppose
is a bijective map such that
Then
is also a factor and there is a bijective linear map
which is an algebra *-isomorphism or *-antiisomorphism such that
for all
.
Proof. The proof is similar to that of Corollary 5 in [10] and we omit it.
Theorem 2.5. Suppose
and
are von Neumann algebras with unitarily invariant norms N and M both having property (P). Let
be a bijective map. If
holds for all
, then there exist a Jordan *-isomorphism
and an element
such that
for all
.
Proof. We first show that
is positive homogeneous. Indeed, for any
,
, we have
Therefore
and this implies that
. Hence
is positive homogeneous.
For any
, we have
(note the first equivalence follows from Lemma 2.2) and therefore
is an order isomorphism. It follows from [10] that there exist an element
and a Jordan *-isomorphism
such that
for all
.
3. Conclusion
Mean is an important concept in mathematics. There are many interesting results studying preserver transformations relating operator means. In this paper, we show that maps on positive definite cones of von Neumann algebras preserving unitarily invariant norms of the spectral geometric means can be characterized by Jordan *-isomorphisms.
Acknowledgements
The authors would like to thank the anonymous referee for constructive criticisms and valuable comments.
Founding
Partially supported by NFS of China (11871303, 11971463, 11671133) and NSF of Shandong Province (ZR2019MA039 and ZR2020MA008).