Here the possibility of superselection rules is ignored. It may be the case that a system cannot be prepared in specific states. For instance, superposition of states with different spin is generally believed impossible. Likewise, states being superpositions of states with different charge are considered impossible. Minor complications due to those issues are treated in Bogoliubov, Logunov & Todorov (1975)Bogoliubov, N. N.; Logunov, A.A.; Todorov, I. T. (1975). Introduction to axiomatic quantum field theory. Mathematical Physics Monograph Series. Vol. 18. Translated to English by Stephan A. Fulling and Ludmila G. Popova. New York: Benjamin. ASINB000IM4HLS.
Wigner 1931, pp. 251–254 (in German), Wigner 1959, pp. 233–236 (English translation). Wigner, E. P. (1931). Gruppentheorie und ihre Anwendung auf die Quanten mechanik der Atomspektren (in German). Braunschweig, Germany: Friedrich Vieweg und Sohn. pp. 251–254. ASINB000K1MPEI. Wigner, E. P. (1959). Group Theory and its Application to the Quantum Mechanics of Atomic Spectra. translation from German by J. J. Griffin. New York: Academic Press. pp. 233–236. ISBN978-0-1275-0550-3.
Moretti, Valter (October 2016). "Mathematical Foundations of Quantum Mechanics: An Advanced Short Course". International Journal of Geometric Methods in Modern Physics. 13 (Supp. 1): 1630011–1630843. arXiv:1508.06951. Bibcode:2016IJGMM..1330011M. doi:10.1142/S0219887816300117.
Moretti, Valter (October 2016). "Mathematical Foundations of Quantum Mechanics: An Advanced Short Course". International Journal of Geometric Methods in Modern Physics. 13 (Supp. 1): 1630011–1630843. arXiv:1508.06951. Bibcode:2016IJGMM..1330011M. doi:10.1142/S0219887816300117.
Moretti, Valter (October 2016). "Mathematical Foundations of Quantum Mechanics: An Advanced Short Course". International Journal of Geometric Methods in Modern Physics. 13 (Supp. 1): 1630011–1630843. arXiv:1508.06951. Bibcode:2016IJGMM..1330011M. doi:10.1142/S0219887816300117.