It has been shown that the ground state energy E₀ of a quantum system of N identical fermi particles satisfies inequality E₀ ≥ (1 / (N − n)) Σᵢⁿ⁻ⁿ εᵢ , (1) where n = 1, 2, …, N − 1 and εᵢ are the eigenvalues of “n + 1-body Hamiltonian”. The summation is performed over the first N − n states.