1. R. Parr, in Density Matrices and Density Functional, R. Erdahl and V.H. Smith, Jr. (eds.), D. Reidel Publishing Company, Holland (1987), p. 663.
2. E. A. Hylleraas, Zeit. Physik 5, 209 (1930).
3. P. A. M. Dirac, in A series of papers in the Proc. Camb. Phil. Soc. culminating in 26, 376 (1930).
4. N. M. Hugenholtz, Physica 23, 481 (1957); L. van Hove, Physica 25, 849 (1956).
5. K. Husimi, Proc. Phys. Mat. Soc. Japan 22, 264 (1940).