POSITIVENESS FOR THE RIEMANNIAN GEODESIC BLOCK MATRIX

It has been shown that the geometric mean A#B of positive definite Hermitian matrices A and B is the maximal element X of Hermitian matrices such that $$\(\array{A&X\\X&B}\)$$ is positive semi-definite. As an extension of this result for the 2 × 2 block matrix, we consider in this article the block matrix [[A#wijB]] whose (i, j) block is given by the Riemannian geodesics of positive definite Hermitian matrices A and B, where wij ∈ ℝ for all 1 ≤ i, j ≤ m. Under certain assumption of the Loewner order for A and B, we establish the equivalent condition for the parameter matrix ω = [wij] such that the block matrix [[A#wijB]] is positive semi-definite.