Perspectivity and congruence in partial abelian semigroups
Alexander Wilce · Czech digital mathematics library · 1998
To any subset M of a partial abelian semigroup L, we associate a relation of perspectivity ~M, where a ~M b if and only if there exists some element c G L such that a® c and c® b both exist and belong to M. If ~M is a (faithful) congruence, we say that M is algebraic.The theory of pairs (L, M), where L is a partial abelian semigroup and M is a fixed algebraic subset provides a natural generalization of the theory of manuals.