- Arkadiusz Jadczyk, Spinors, Twistors, Clifford Algebras and Quantum Deformations, 1993, 129

- Lech Jakóbczyk, “On representations of canonical commutation relations in indefinite metric quantum field theory”, Reports on Mathematical Physics, 22, no. 1, 1985, 71

- Gerald Hofmann, “On the GNS representation of generalized free fields with indefinite metric”, Reports on Mathematical Physics, 38, no. 1, 1996, 67

- J. -P. Antoine, S. �ta, “Unbounded GNS representations of a *-algebra in a Krein space”, Lett Math Phys, 18, no. 4, 1989, 267
