Dirac, Paul Adrien Maurice Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 114, 1927, pàg. 243–265. Bibcode: 1927RSPSA.114..243D. DOI: 10.1098/rspa.1927.0039 [Consulta: free].
Dirac, Paul Adrien Maurice Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 114, 1927, pàg. 243–265. Bibcode: 1927RSPSA.114..243D. DOI: 10.1098/rspa.1927.0039 [Consulta: free].