Dependences Between Domain Constructions in Heterogeneous Relation Algebras.

We show the following dependences between relational domain constructions in the framework of heterogeneous relation algebras. If all power sets and subsets exist and objects are comparable, then all sums exist. If all sums exist and atoms are rectangular, then all products exist. If all atoms are rectangular, then all subsets exist if and only if all quotients exist. We give models with rectangular atoms which rule out further dependences between these constructions.