Positivity Constraints on Density Matrices
S. M. Deen, P. K. Kabir, G. Karl · Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields · 1971
The requirements of positivity on Hermitian matrices are stated as conditions on the elementary symmetric functions ${c}_{j}$ of the eigenvalues. The exact restrictions are found for matrices of arbitrary dimensionality; weaker conditions on the ${c}_{j}$ are also given which hold in every case.