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The contraharmonic mean of HSD matrices. (English) Zbl 0641.15009
The contraharmonic mean of two positive semidefinite Hermitian matrices A and B is defined by the relation $C(A,B)=A+B-2(A:B),$ where $A:B=A(A+B)\sp{-1}B$ is the so-called parallel addition introduced by {\it W. N. Anderson} jun. and {\it R. J. Duffin} [J. Math. Anal. Appl. 26, 576-594 (1969; Zbl 0177.049)]. The dual of the contraharmonic mean of A and B is given by $C'(A,B)=C(A\sp{-1},B\sp{-1})\sp{-1}.$ It is shown that $$ C'(A,B)=A:B+2(A:B)C(A,B)\sp{-1}(A:B)=(A(A:B)\sp{- 1}A):(B(A:B)\sp{-1}B). $$ With the aid of the contraharmonic mean and its dual the authors study fixed point problems, the monotonicity behaviour of C(A,B), an infinite family of means for positive semidefinite Hermitian matrices that generalize C(A,B), inverse mean problems, and connections between C(A,B) and least square problems.
Reviewer: A.R.Kräuter

15A45Miscellaneous inequalities involving matrices
15A24Matrix equations and identities
15A27Commutativity of matrices
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