数理解析研究所講究録
第 1893 巻 2014 年 57-66

57
MATRIX MEANS OF FINITE ORDERS

DINH TRUNG HOA, TOAN M. HO, AND HIROYUKI OSAKAA

ABSTRACT. Using the same idea of the definition of means for positive operators by Kubo and Ando, for each natural number , wecan define means of two positive definite matrices of order andtry to study the canonical map: we can describe an one to one
$n$

$n$

corresponding from the class of matrix connections of order tothe class of positive -monotone functions on
and the rangeof this corresponding covers the class of interpolation functions oforder $2n$ . In particular, the space of symmetric connections is isomorphic to the space of symmetric positive -monotone functions.
Moreover, we show that, for each , the class of -connections
extremely contains that of $(n+2)$ -connections.
$n$$(0, \infty)$

$n$

$n$

$n$

$n$

1. INTRODUCTION

Averaging operations are useful in science. The arithmetic, geometric and harmonic means are the three $best-$ known ones. When we try
to generalize these operations on positive definite matrices, there are
no difficulty for the arithmetic and harmonic means while the geometric one needs some more efforts: since
is positive definite
if $AB=BA$ and even not symmetric in general for positive definite
matrices $A,$ $B$ . Trying to generalize these notions, Fumio Kub andTsuyoshi Ando [11] introduced the connections of positive operatorson an infinite dimensional Hilbert space $H$ via three axioms. Furthermore, they showed that there is an affine order-isomorphism from theclass of means to the class of positive operator monotone functions
on $(0, \infty)$ with $f(1)=1$ . This theory has found a number of applications in operator theory. In particular, Petz [16] connected thetheory of monotone metrics with the theory of connections and means
by Kubo and Ando. He proved that an operator monotone function:
satisfying the functional equation$A^{\frac{1}{2}}B^{\frac{1}{2}}$

$0$

$f$

$(0, \infty)arrow \mathbb{R}$

$f$

(1)

$f(t)=tf(t^{-1}) , t>0$

related to a Morozova-Chentsov function which gives a monotone metric on the manifold ofdensity matrices.
$n\cross n$$1A$

Research partially supported by the JSPS grant for Scientific Research No.

20540220.
Date: 25-02-2014.
2000 Mathematics Subject Classification. $46L30,15A45.$

Key words and phrases. interpolation functions, matrix monotone functions,mean of positive matrices.

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D. T. HOA, M. T. HO, AND H. OSAKA

Restricting the definition of operator connections from [11] on theset of positive semidefinite matrices of order , in this paper, we geta concept of matrix connections of order (or -connections). The
following natural question is one of the motivations of our study: Will
there exist an affine order-isomorphism from theof -connections
onto the class of positive matrix monotone functions (or of interpolation functions of order )?
A -monotone function on
is a function which preserves the
positive semi-definite matrices. Moreover, if
order on the set of all, then is called operator monotone. Noteis -monotone for allthat this is equivalent to that is a Pick function. Each -connection
induces a -monotone function (so an interpolation function of ordernon
via the identity $f(t)I_{n}=I_{n}\sigma(tI_{n})$ . This correspondingis an one to one map from the class of -connections to the class of pos(hence, to the set of interpolation
itive -monotone functions onfunctions of order ).
On the other hand, an interpolation function of order is a positive
function onsuch that for each -subsetofinterpolating at . Using
there exists a Pick function on
the integral representation of the Pick function, we know that $f(A)$ has
the integral representation on
for any positive matrix (Theorem 3.4). Applying this representation, we can get a ‘local’ integral
formula for a connection of order corresponding to a -monotonefunction on. Furthermore, this ‘local’ formula also establishes,for each interpolation function of order $2n$ , a connection of ordercorresponding to the given interpolation function . Therefore, it
shows that the map from the -connections to the -monotone functions is one to one with the range containing the interpolation functionsof order $2n$ . Moreover, we also show that the class of 1-connections is
isomorphic to the class of interpolation functions of order 2 and asmuch as properties we know in the space of -connections also hold inthe spaceof interpolation functions of order $2n$ (Proposition 4.1
and Proposition 3.8). This gives a hope that the class of -connections
is isomorphic to the class
Using the definition of symmetric connections, we can also give a corresponding concept for interpolation functions and -monotone functions. It is shown that the space of -connections is strictly subsetof the space of positive -monotone functions on(Corollary
3.9). However, restricting on the symmetric functions, the space ofsymmetric -monotone functions is the same as that of symmetric nconnections (Theorem 3.10).
In this short note, we state the results obtaining when we study
the connections of finite order positive definite matrices and sketch theideas of some proofs. For details of the proofs, please see [10].
$n$

$n$

$n$

$cla\mathcal{S}\mathcal{S}$

$n$

$n$

$[0, \infty)$$n$$n\cross n$$f$$n\in \mathbb{N}$

$n$

$f$

$f$

$n$

$n$

$\sigma$$)$$(0, \infty)$

$f$

$n$

$(0, \infty)$

$n$

$n$

$n$

$(0, \infty)$

$f$

$h$

$(0, \infty)$$S=\{\lambda_{i}\}_{i=1}^{n}$$n$$(0, \infty)$

$S$

$f$

$A$

$[0, \infty]$

$n$

$n$

$(0, \infty)$$f$$\sigma$

$f$

$n$

$n$

$n$

$n$

$C_{2n}$$n$$C_{2n}.$

$n$

$n$

$n$

$n$

$(0, \infty)$

59
MATRIX MEANS OF FINITE ORDERS

2. MEANS OF POSITIVE LINEAR OPERATORS IN THE SENSE OF$KUBO-ANDO [11 ]$Taking an axiomatic approach, Fumio Kubo and Tsuyoshi Ando
introduced the generalization of above notions namely connections andmeans of positive operators as follows.
forDefinition 2.1 ([11]).
connection is a binary operation
positive semi-definite operators $A,$ $B$ on an infinite dimensional Hilbert
spaces $H$ , which satisfies the following:(I) Monotonicity: $A\leq C$ and $B\leq D\Rightarrow A\sigma B\leq C\sigma D.$(II) Ransformer inequality: $C(A\sigma B)C\leq(CAC)\sigma(CBC)$ .
(III) Continuous from above:
A mean is a connection with normalization condition, that is $I\sigma I=I$
(I is the identity).
$A$$A\sigma B$$\sigma$$A_{n}\downarrow A,$$B_{n}\downarrow B\Rightarrow A_{n}\sigma B_{n}\downarrow A\sigma B.$

is a scalar for any positive number
We can show thatbyHence one can define a function $f(x)$ on$I\sigma(xI)$$x.$$(0, \infty)$$f(x)I=I\sigma(xI)$ .

, and, $f(A)=I\sigma A$ for
Then is monotone and positive on
any positive definite operator , so, is operator-monotone since ismonotone. Furthermore, with help of (2), we have the relation$(0, \infty)$

$f$

$A$

$f$

$\sigma$$A\sigma B=A^{\frac{1}{2}}f[A^{\frac{-1}{2}BA^{\frac{-1}{2}}}]A^{\frac{1}{2}}.$On the other hand, the Lowner theory on operator-monotone functions gives an integral representation of operator-monotone functions
as follows.
Theorem 2.2 ([11]). The map which associates each positive Radon
to a positive operator-monotone function definedmeasure $m$ on
by
on
$f$

$[0, \infty]$$(0, \infty)$(2)$f(x)= \int_{[0,\infty]}\frac{x(1+t)}{x+t}dm(t)$for $x>0$

is an isomorphism from the class of positive Radon measures ononto the class of positive operator-monotone functions.

3. MEANS AND CONNECTIONS OF ORDER$[0, \infty]$$n$

3.1. Interpolation functions. Throughout the paper, let us denotethe algebra of square maof the real line
the subset
the cone of positive
andtrices of order with coefficients in. In this section we study some properties
semi-definite matrices inof interpolation functions of order and their local integral represen-$\mathbb{R}_{+}$$(0, \infty)$$\mathbb{R},$$\mathbb{C}$$n$$M_{n}$$n$tations.

$M_{n}$$M_{n}^{+}$

60
D. T. HOA, M. T. HO, AND H. OSAKA

Definition 3.1 ([1]). function
is called an interpolation$A\inM_{n}$
any
with $A>0$ and $T^{*}T\leq 1$
function of order if for$A$$f:\mathbb{R}_{+}arrow \mathbb{R}_{+}$

$T,$

$n$

.
We denote by
the class of all interpolation functions of order on. Similarly, we can define an interpolation classfor an interval$T^{*}AT\leq A \Rightarrow T^{*}f(A)T\leq f(A)$$C_{n}$$n$$C_{n}(I)$$\mathbb{R}_{+}$(see [15, Definition 1]). From the(orabove definition it is straightforward to check thatfor any interval ).

$I\subset \mathbb{R}$$(I: open,$ closed, $half- open)$$C_{n}\circ C_{n}\subset C_{n}$$I$$C_{n}\circ C_{n}(I)\subset C_{n}(I)$Let $P’$ be a set of all positive Pick functions on
the form$\mathbb{R}_{+}$, i.e., functions of$h(s)= \int_{[0,\infty]}\frac{(1+t)s}{1+ts}d\rho(t) , s>0,$where$\rho$

is some positive Radon measure on$[0, \infty].$denote by
the set of all strictly positive
Remark 3.2. For-monotone functions. We have the following properties can be foundin [9], [14], [1], [2], [3] or [4]:$n\in \mathbb{N}$$P_{n}’$

$n$

(i)

(ii)
(iii)
(iv)

$P’= \bigcap_{n=1}^{\infty}P_{n}’,$$C_{n+1}\subseteq C_{n}$$P’= \bigcap_{n=1}^{\infty}C_{n}$;;$P_{n+1}’\subseteq C_{2n+1}\subseteq C_{2n}\subseteq P_{n}’,$$P_{n}’\subsetneq C_{n}$$C_{2n}\subsetneq P_{n}’[15].$

The following useful characterization of a function in
Donoghue (see [6], [7]), and to Ameur (see [1]).
Theorem 3.3. ([4, Corollary 2.4])
if and only if for every -setsuch thatfor $i=1,$$n$$C_{n}$$h$$f(\lambda_{i})=h(\lambda_{i})$$A$function$\{\lambda_{i}\}_{i=1}^{n}\subset \mathbb{R}_{+}$is due to$C_{n}$$f:\mathbb{R}_{+}arrow \mathbb{R}_{+}$

there exists a

belongs to$P’$-function$n.$$\ldots,$andWe have a cal integral representation of every function inthat the representation will be used in study of matrix connections in
the next sections.
$10$$C_{n}$

be a positive definite matrix
and $f\in C_{n}$ . Then there exists a positive Radon measuresuch that

Theorem 3.4. ([2, Theorem 7.1]) Letin$M_{n}$

on$[0, \infty]$$\rho_{\sigma(A)}$

(3)

where

$A$

$f(A)= \int_{[0,\infty]}A(1+s)(A+s)^{-1}d\rho_{\sigma(A)}(s)$$\sigma(A)$,is the set of eigenvalues of $A.$

3.2. Interpolation functions and Means of positive matrices.
Similarly to Definition 2.1 of connections of positive operators on infinite dimensional Hilbert space, we can give the definition of positive$(A, B)\mapsto A\sigma B$ isbinary operation on
matrices of order :called a matrix connection of order (or -connection) if it satisfies
three axioms an in Definition 2.1. Note that the order relation $A\leq B$
$n$

$A$

$M_{n}^{+},$$\sigma$

$n$

$n$

61
MATRIX MEANS OF FINITE ORDERS

always means the positivity of $B-A$ . That $A_{1}\geq A_{2}\geq\ldots$ andconverges strongly to is denoted by
In [11], there is an affine ordcr-isomorphism from the set of connections onto the set of operator monotone functions. In this section, we
describe the similar relation between the connections of order and. Note that every positive semi-definite matrix can be obtained as a limit of a decreasing sequence of positive definite matrices,from now on, we can always assume that connections are defined on
positive definite matrices.
is a scalar (see the proof
For any -connection , the matrixbyof [11, Theorem 3.2] , and so we can define a function on$A_{n}$$A$$A_{n}\downarrow A.$$n$$P_{n}’\supsetneq C_{2n}$$n$$I_{n}\sigma(tI_{n})$$\sigma$$(0, \infty)$

$f$

$)$

$f(t)I_{n}=I_{n}\sigma(tI_{n})$,. As in the proof of [11, Theorem 3.2],
whereis the identity inusing the property (I) of the definition of connection, is a -monotone.
function on
Now we can state the main theorem.
$M_{n}$$I_{n}$$f$$n$$(0, \infty)$

Theorem 3.5. For any natural number there is an injective map
associatingfrom the set of matrix connections of order tosuch thateach connection to the functionfor$t>0$ . Furthermore, the range of this map contains$\Sigma$$n$$P_{n}’\supset C_{2n}$$n$$f_{\sigma}(t)I_{n}=I_{n}\sigma(tI_{n})$$f_{\sigma}$$\sigma$$C_{2n}.$Sketch the proof. Step 1. Let be a function belonging to . Define
by:a binary operation on positivc definite matrices in$C_{n}$$f$$M_{n}$$\sigma$(4)$A\sigma B=A^{\frac{1}{2}}f[A^{\frac{-1}{2}BA^{\frac{-1}{2}}}]A^{\frac{1}{2}} \forall A, B>0.$

It is straightforward to check thatsatisfies the axiom (III) of thedefinition of -connection.
Step 2. To check the axioms (I) and (II), we first show that for$f\in C_{n}$ , there exists a Radon measuresuchon$\sigma(A^{-T}BA)$that$\sigma$$n$$\rho=\rho$

$-1$

$-1$

$[0, \infty]$$f[A^{\frac{-1}{2}BA^{\frac{-1}{2}]=}} \int_{[0,\infty]^{A^{\frac{-1}{2}BA^{\frac{-1}{2}}}}}A^{\frac{-1}{2}BA^{\frac{-1}{2}}},$

is the set of eigenvalues ofwheretuting this equality into (4), we obtian$\sigma(A^{\frac{-1}{2}BA^{\frac{-1}{2}}})$$A^{\frac{-1}{2}BA^{\frac{-1}{2}}}$$A \sigma B=\int_{[0,\infty]}\frac{1+s}{s}\{(sA):B\}d\rho(s)$Substi-.

step 3. Now for any $f\in C_{2n}$ , there exists a Radon measuresuch that$\sigma(A^{\frac{-1}{2}BA^{\frac{-1}{2}}})\cup\sigma(c\frac{-1}{2}Dc^{\frac{-1}{2}})$$A \sigma B=\int_{[0,\infty]}\frac{1+s}{s}\{(sA):B\}d\rho(s)$,$C \sigma D=\int_{[0,\infty]}\frac{1+s}{s}\{(sC):D\}d\rho(s)$.

$\rho$on

62
D. T. HOA, M. T. HO, AND H. OSAKA

Since $\{(sA):B\}\leq\{(sC):D\}$ for $A\leq C$ and $B\leq D$ , the conditions(I), (II) hold.

$\square$Remark 3.6. Sinceorder to a function in, the map associating each connection ofas above is not surjective.

$P_{n}’\subsetneq C_{n}$$n$$C_{n}$

3.3. Decreasing inclusion of the connections of order . Via the, it is straightforward to check that
intousual embedding of
the classes of connections of order is decreasing. It is natural to ask
the following question: Is there a matrix meanof the order onsuch thatis not of order $n+1$ ?
The following observation gives partially affirmative data to theabove question.
$n$$M_{n+1}$$M_{n}$$n$$n$$\sigma_{n}$$M_{n}$$\sigma_{n}$Proposition 3.7.
(1) For any $n\geq 2$ there is a matrix meanof order which is
not of order $n+2.$(2) There is a matrix meanof order 1 which is not of order 2.
$n$$\sigma_{n}$$\sigma_{1}$Proof. Denote by

the image of the class of connections of orderis isomorphic
via the map in Theorem 3.5 for each . Therefore,
is decreasing) andto the class of -connections (so the sequence. From now on, we can identify the space of -connections
with(1): On account of Remark 3.2 and Theorem 3.5, we obtain thefollowing inclusion:$n$$\Sigma_{n}$$n$$\Sigma_{n}$$\{\Sigma_{n}\}$$n$$\Sigma_{n}\subseteq P_{n}’$$n$$\Sigma_{n}.$$\Sigma_{n+2}\subseteq P_{n+2}’\subseteq C_{2(n+1)+1}\subseteq C_{2(n+1)}\subseteq\Sigma_{n+1}$$\subseteq P_{n+1}’\subseteq C_{2n+1}\subseteq C_{2n}\subseteq\Sigma_{n}.$, we imply that
And since(2): Using Remark 3.2 again and the characterization of matrix connections of order one (Subsection 3.5 below), we get$P_{n+2}’\subsetneq P_{n+1}’$$\Sigma_{n+2}\subsetneq\Sigma_{n}.$$\Sigma_{2}\subseteq P_{2}’\subseteq C_{3}\subseteq C_{2}=\Sigma_{1}.$By [4, Proposition 3.14]$P_{2}’\neq C_{3}$, we then have the statement.

$\square$3.4. Symmetric connections. As the same in [11], we can recallbesome notations and properties of connections as follows. Letofa -connection. The transpose , the adjoint and the dual
are defined by$\sigma$$\sigma’$$n$$\sigma^{*}$$\sigma^{\perp}$$\sigma$$A\sigma’B=B\sigma A, A\sigma^{*}B=(A^{-1}\sigma B^{-1})^{-1}, \sigma^{\perp}=\sigma^{\prime*}$

A connection is called symmetric if it equals to its transpose. Dethe set of -monotone representing functions of symnoted byis the image of the set of all symmetric
metric -connections, i.e.,-connections via the canonical map in Theorem 3.5.
(1)
are symmetric.
and$n$$\Sigma_{n}^{sym}$$n$$\Sigma_{n}^{sym}$$n$$\sigma+\sigma’$$\sigma(:)\sigma’$

63

MATRIX MEANS OF FINITE ORDERS(2), where $A\omega_{l}B=A$ and $A\omega_{r}B=B.$(3) The -monotone representing function of the -connectionis $f(x)g[h(x)/f(x)]$ , where $f,$
are the representing functions ofin Theorem 3.5, respectively, and $A\sigma(\tau)\rho B=$$(A\sigma B)\tau(A\rho B)$ .
(4) is symmetric if and only if its -monotone representing function is symmetric, that is, $f(x)=xf(x^{-1})$ .
Each -connection corresponds to a positive -monotone function
belonging toby Theorem 3.5. Therefore, combining with the observation above, we get the following.
$\omega_{l}(\sigma)\omega_{r}=\sigma;\omega_{r}(\sigma)\omega_{l}=\sigma’$

$n$

$n$

$g,$

$\sigma,$$\tau,$$\sigma(\tau)\rho$$h$$\rho$$n$$\sigma$

$f$

$n$

$n$

$\Sigma_{n}$Proposition 3.8. Let $f(x),$ $g(x),$ $h(x)$ belong tostatements hold true:(i) $k(x)=xf(x^{-1}),$ $f^{*}(x)=f(x^{-1})^{-1},$$af(x)+bg(x)$ all belong to;(ii) $f(x)+k(x),$all belong to$\Sigma_{n}$. Then the following$\frac{x}{f(x)},$$f(x)g[h(x)/f(x)],$$\Sigma_{n}$$\frac{f(x)k(x)}{f(x)+k(x)}$$\Sigma_{n}^{sym}.$

We know that (from Theorem 3.5):
Corollary 3.9.
$C_{2n}\subset\Sigma_{n}\subsetneq P_{n}’.$But if restricting our attention to the class of the symmetric, we get
the following equality.

Theorem 3.10.
$\Sigma_{n}^{sym}=P_{n}^{\prime sym},$

where$P_{n}^{\prime sym}$is the set of all ymmetric functions in$\mathcal{S}$$P_{n}’.$

However,
Example 3.11. Lettobut does not belong to$q(x)$
Let
be the symmetrization of$p(x)=x+ \frac{1}{2}x^{2}+\frac{1}{6}x^{3}$$P_{2}’(0, \alpha)$

be a polynomial which belongs
for some $\alpha>0$ (see [12]).

$P_{3}’(0, \alpha)$

$p$

by

$q(x)=p(x)+xp(x^{-1})$ .

Thenis symmetric. However, we can show that$q$

$P_{2}’(0, \alpha)$.

$q$does not belong to

3.5. Matrix means of order one. We recall the results in [4] for thesets
as follows.
is the set of all positive functions on.
consists of all quasi-concave functions $( i.e., f(s)\leq f(t)\max\{1, \frac{s}{t}\}$for all $t>0$ ).
$C_{1},$$\bullet$$\bullet$$C_{2}$$(0, \infty)$$C_{1}$$C_{2}$$s,$

64
D. T. HOA, M. T. HO, AND H. OSAKA

For any connection of order 1, then the corresponding function
belongs to . Indeed, for any numbers $0<t\leq s$ , we have$\sigma$$f$$C_{2}$$f(t) \max\{1, \frac{s}{t}\}=(1\sigma t)\frac{s}{t}=\frac{S}{t}\sigma s$$\geq 1\sigma s=f(s)$, and,$f(s) \max\{1, \frac{t}{s}\}=(1\sigma s)$$\geq 1\sigma t=f(t)$.

Thus, we can characterize connections of order 1 completely:a Every connection of order one can be determined uniquelyby:$)$$\sigma$$x \sigma y=xf(\frac{y}{x}) \forall x, y>0,$

is an interpolation function in
can be represented uniquely by:b Every function inwhere$C_{2}.$

$f$

$f$

$)$

$C_{2}$$f(x)=1\sigma x \forall x>0,$

where is a connection of order 1.
Fr $om$ this result, we can easily get a functions in from the corresponding connections and vise versa. For example, the functions inwhich correspond to arithmetic mean, harmonic mean and the geometand ; and any (positive) linear combination ofric mean arethese functions also belongs toin [4, Example
If we take the functionof order 1 which is not of order 2 as
3.13], we have a connection$\sigma$$C_{2}$$C_{2}$$x^{\frac{1}{2}}$$\frac{1+x}{2},$$\frac{2}{1+x}$$C_{2}.$$f(x)=2 \frac{x}{1+x}+(\frac{x}{1+x}I^{2}\in C_{2}\backslash C_{3}$$\sigma_{f}$follows:$x \sigma_{f}y=xf(\frac{y}{x})$$=2 \frac{xy}{x+y}+\frac{xy^{2}}{(x+y)^{2}}$for$x,$ $y\in \mathbb{R}^{+}.$

4. TOWARD THE CONJECTURE

andWe know thata conjecture that, for any positive integer$C_{2n}\subseteq\Sigma_{n}\subseteq P_{n}’$$C_{2n}=\Sigma_{n}$and$C_{2}=\Sigma_{1}$$\mathcal{C}_{2n}=\sum_{n}$. Therefore, we may give$n,$$\Sigma_{n}^{sym}=C_{2n}^{sym}$or not, but they have
Even we still do not know whether
propertiesof the spacesome similar properties. In particular, thebyrepresented in Proposition 3.8 also hold true when we replace. That is,$C_{2n}=\Sigma_{n}$$\Sigma_{n}$$\Sigma_{n}$$C_{2n}$

Proposition 4.1. The statements in Proposition 3.8 hold if we replaceby$\Sigma_{n}$$C_{2n}.$65

MATRIX MEANS OF FINITE ORDERS

Note that Proposition 4.1 still holds true in the space$C_{n}.$

5. ACKNOWLEDGEMENT
The authors would like to thanks Prof. Takashi Sano who is theorgarnizer of RIMS worshop” Operator monotone functions and related
topics”, November 6- 8, Kyot , Japan for giving us the opportunity
to present a talk about these results.
$0$

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D. T. HOA, M. T. HO, AND H. OSAKA

66

CENTER OF RESEARCH AND DEVELOPMENT, DUY TAN UNIVERSITY, $K7/25$
QUANG TRUNG, DA NANG, VIET NAM-mail address: trunghoa.math@gmail.com$E$

INSTITUTE OF MATHEMATICS, 18 HOANG QUOC VIET, HANOI, VIETNAM-mail address: hmtoan@math.ac.vn$E$

DEPARTMENT OF MATHEMATICAL SCIENCES, RITSUMEIKAN UNIVERSITY, KUSATSU,SHIGA 525-8577, JAPAN-mail address: osaka@se.ritsumei.ac.jp$E$