Karlhede, A. (1980), «A review of the geometrical equivalence of metrics in general relativity», General Relativity and Gravitation, 12: 693, Bibcode:1980GReGr..12..693K, doi:10.1007/BF00771861
Karlhede, A. (1980), «A review of the geometrical equivalence of metrics in general relativity», General Relativity and Gravitation, 12: 693, Bibcode:1980GReGr..12..693K, doi:10.1007/BF00771861