I was going through some of my old notes today and came across investigations I had done several years ago on implicit operations in Universal Algebra. These are definable partial operations on algebras that are preserved by all homomorphisms. Here are two examples: (1) Pseudocomplements in distributive lattices. Given a <= b <= c in a distributive lattice, there is at most one b' such that b /\ b' = a and b \/ b' = c. Because lattice homomorphisms preserve these inequalities and equations, the uniqueness of pseudocomplements implies that, when they exist, they are also preserved by homomorphisms. (2) Multiplicative inverses in monoids. Similarly, given an element m in a monoid (M, *, 1), there is at most one element m' such that m * m' = 1 and m' * m = 1. It follows that inverses, when they exist, are also preserved by monoid homomorphisms. Now, the investigation of these partial operations gets one quickly into non-surjective epimorphisms, dominions in the sense of Isbell, algebraic elements in the sense of Bacsich, implicit partial operations in the sense of Hebert, and other topics. Some of the references that I know about are listed below. My question is this: Does a definitive treatment of this phenomenon in "algebraic" categories exist? Are there still some mysteries/open problems? REFERENCES PD Bacsich, "Defining algebraic elements", JSL 38:1 (Mar 1973), 93-101. PD Bacsich, "An epi-reflector for universal theories", Canad. Math. Bull. 16:2 (1973), 167-171. PD Bacsich, "Model theory of epimorphisms", Canad. Math. Bull. 17:4 (1974), 471-477. JR Isbell, "Epimorphisms and dominions", Proc. of the Conference on Categorical Algebra, La Jolla, Lange and Springer, Berlin 1966, 232-246. JM Howie and JR Isbell, "Epimorphisms and dominions, II", J. Algebra, 6 (1967), 7-21. JR Isbell, "Epimorphisms and Dominions, III", Amer. J Math. 90:4 (Oct 1968), 1025-1030. M Hebert, "Sur les operations partielles implicites et leur relation avec la surjectivite des epimorphismes", Can. J. Math. 45:3 (1993), 554-575. M Hebert, "On generation and implicit partial operations in locally presentable categories", Appl. Cat. Struct. 6:4 (Dec 1998), 473-488. -- Todd Wilson A smile is not an individual Computer Science Department product; it is a co-product. California State University, Fresno -- Thich Nhat Hanh