1. G. Birkhoff, Lattice Theory (Providence, Rhode Island, 1967).
2. K. Nagy, State vector space with indefinite metric in quantum field theory (Akademiai Kiado, Budapest, 1966).
3. K. Yu. Dadashyan and S. S. Khoruzhij, Theoretical and Mathematical Physics 54, 35 (1983).
4. S. Hassi and K. Nordstrom, Linear Algebra Appl. 208/209, 401 (1994).
5. A. Maestripieri and F. M. Peria, Acta. Sci. Math. (Szeged) 72, 611 (2006).