Discrete Convexity and Hermitian Matrices

The question (Horn problem) about the spectrum of the sum of two real symmetric (or complex Hermitian) matrices with given spectra is considered. This problem was solved by A. Klyachko. We suggest a different formulation of the solution to the Horn problem with a significantly more elementary proof. Our solution is that the existence of the required triple of matrices $(A,B,C)$ for given spectra $(\alpha,\beta,\gamma)$ is equivalent to the existence of a so-called discrete concave function on the triangular grid $\Delta(n)$ with boundary increments $\alpha$,$\beta$, and $\gamma$. In addition, we propose a hypothetical explanation for the relation between Hermitian matrices and discrete concave functions. Namely, for a pair $(A,B)$ of Hermitian matrices, we construct a certain function $\phi (A,B;\cdot)$ on the grid $\Delta(n)$. Our conjecture is that this function is discrete concave, which is confirmed in several special cases.