Normal congruence relations on division groupoids Tomáš Kepka (1975) Acta Universitatis Carolinae. Mathematica et Physica
Normal functors and strong protomodularity. Bourn, Dominique (2000) Theory and Applications of Categories [electronic only]
Normalisation for the fundamental crossed complex of a simplicial set. Brown, R., Sivera, R. (2007) Journal of Homotopy and Related Structures
Notes on associative triples of elements in commutative groupoids Tomáš Kepka (1981) Acta Universitatis Carolinae. Mathematica et Physica
Notes on distributive groupoids Jaroslav Ježek, Tomáš Kepka (1983) Commentationes Mathematicae Universitatis Carolinae