Geodesic
Parallel addition and parallel subtraction of operators
Izvestiya. Mathematics , Tome 10 (1976) no. 2, pp. 351-370.

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The parallel sum $A:B$ of two invertible nonnegative operators $A$ and $B$ in a Hilbert space $\mathfrak H$ is the operator $(A^{-1}+B^{-1})^{-1}=A(A+B)^{-1}B$. This definition was extended to noninvertible operators by Anderson and Duffin for the case $\dim\mathfrak H\infty$ and by Fillmore and Williams for the general case. The investigation of parallel addition is continued in this paper; in particular, associativity is proved. Criteria are established for solvability of the equation $A:X=S$ with an unknown operator $X$ when $A$ and $S$ are given. In the case of solvability, the existence of a minimal solution $S\div A$, called the parallel difference, is proved. Parallel subtraction in a finite-dimensional space is considered in the last section. Bibliography: 11 titles. 
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È. L. Pekarev; Yu. L. Shmul'yan. Parallel addition and parallel subtraction of operators. Izvestiya. Mathematics , Tome 10 (1976) no. 2, pp. 351-370. http://geodesic.mathdoc.fr/item/IM2_1976_10_2_a7/

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