1. von Neumann J (1932) Mathematische Grundlage der Quantemechanik. Springer, Berlin
2. Dirac PAM (1958) The principles of quantum mechanics, 4th ed, Clarendon Press, Oxford
3. Löwdin P-O (1965) Phys Rev 139A:357
4. Löwdin P-O (1990) J Math Chem 4:401
5. Löwdin P-O (1983) Int J Quantum Chem Symp QC 16:485