Extremal Density Matrices for Qudit States
  An algebraic procedure to find extremal density matrices for any Hamiltonian of a qudit system is established. The extremal density matrices for pure states provide a complete description of the system, that is, the energy spectra of the Hamiltonian and their corresponding projectors. For extremal density matrices representing mixed states, one gets mean values of the energy in between the maximum and minimum energies associated to the pure case. These extremal densities give also the corresponding mixture of eigenstates that yields the corresponding mean value of the energy. We enhance that the method can be extended to any hermitian operator. 
Armando Figueroa, Julio A. L\'opez-Sald\'ivar, Octavio Casta\~nos,   Ram\'on L\'opez-Pe\~na
math-ph math.MP quant-ph