On two geometric means and sum of adjoint orbits

Abstract

In this paper, we study the metric geometric mean introduced by Pusz and Woronowicz and the spectral geometric mean introduced by Fiedler and Pták, originally for positive definite matrices. The relation between $t$-metric geometric mean and $t$-spectral geometric mean is established via log majorization. The result is then extended in the context of symmetric space associated with a noncompact semisimple Lie group. For any Hermitian matrices $X$ and $Y$, So's matrix exponential formula asserts that there are unitary matrices $U$ and $V$ such that $$e^{X/2}{e}^Y{e}^{X/2}=e^{UX{U}^{\ast }+VY{V}^{\ast }}.$$ In other words, the Hermitian matrix $\log (e^{X/2}{e}^Y{e}^{X/2})$ lies in the sum of the unitary orbits of $X$ and $Y$. So's result is also extended to a formula for adjoint orbits associated with a noncompact semisimple Lie group.

14 pages, 2 figures, latest revision on Aug 28, 2021