Kepka, T.; Němec, P. (1996), "Simple balanced groupoids"(PDF), Acta Universitatis Palackianae Olomucensis. Facultas Rerum Naturalium. Mathematica, 35 (1): 53–60.
Ježek, Jaroslav; Kepka, Tomáš (1981), "Free entropic groupoids"(PDF), Commentationes Mathematicae Universitatis Carolinae, 22 (2): 223–233, MR0620359.
doi.org
Hausmann, B. A.; Ore, Øystein (October 1937), "Theory of quasi-groups", American Journal of Mathematics, 59 (4): 983–1004, doi:10.2307/2371362, JSTOR2371362.
jstor.org
Hausmann, B. A.; Ore, Øystein (October 1937), "Theory of quasi-groups", American Journal of Mathematics, 59 (4): 983–1004, doi:10.2307/2371362, JSTOR2371362.