Two-Variable Wasserstein Means of Positive Definite Operators

We investigate the two-variable Wasserstein mean of positive definite operators, as a unique positive solution of the nonlinear equation obtained from the gradient of the objective function of the least squares problem. A various properties of two-variable Wasserstein mean including the symmetry and the refinement of the self-duality are shown. Furthermore, interesting inequalities such as the Ando-Hiai inequality and bounds for the difference between the two-variable arithmetic and Wasserstein mean are provided. Finally, we explore the relationship between the tolerance relation and two-variable Wasserstein mean of positive definite Hermitian matrices.