Refinement Monoids, Vaught Monoids, and Boolean Algebras. Hans Dobbertin (1983) Mathematische Annalen
Remarks on tolerance algebras Karel Drbohlav (1981) Acta Universitatis Carolinae. Mathematica et Physica
Representation of undirected graphs by anticommutative conservative groupoids Bohdan Zelinka (1994) Mathematica Bohemica The paper studies tolerances and congruences on anticommutative conservative groupoids. These groupoids can be assigned in a one-to-one way to undirected graphs.