History of early days of reduced density matrix theory

How I get here?
The first significant appearance of reduced density matrix was due to P.A.M. Dirac for the Hartree-Fock theory. After, Husimi Kodi (modern Japanese may write as Fushimi Koji) defined p-th order reduced density matrices. Unfortunately his paper had been ignored for long time as his paper has been submitted just before the World War II. Per-Olov Löwdin and Mayer's paper are the first ones that using second-order reduced density matrix as basic variables. Note that Lowdin was a student of Dirac.