Dear all, I remember that there was some time ago a discussion on this list about limits and colimits in the category Rel of binary relations. Unfortunately, I can not remember or trace the final answer. But, if I remember right there are, besides initial and terminal objects, in general no limits or colimits in Rel. So my questions are: 1. Is there a characterization of monomorphisms and epimorphisms in Rel? 2. Is it true that there are, in general, no products and equalizer (sums and coequalizer) in Rel? 3. Are there some general results about what limits/colimits exist or don't exist? 4. Is the presumable non-existence related to the fact that the formation of converse relations establishes an isomorphism between Rel and its opposite Rel^op? Any reply or reference is well-come. Best regards Uwe Wolter [For admin and other information see: http://www.mta.ca/~cat-dist/ ]