Remarks about the wave-function representability of the first-order reduced density matrices
Per-Olov L�wdin · International Journal of Quantum Chemistry · 1986
It is pointed out that, if {Γy} is the set of Nth-order density matrices associated with a given first-order reduced density matrix γ = γ(x1|x1), then γ is wave-function representable, if and only if there is at least one Γγ for which one has μ = Tr(Γγ − Γ) = 0.