Classes closed under isomorphisms, retractions, and products.
Alg. Universalis 30 (1993), 140-148.
Let A be a structure (a set equipped with operations and relations). A
substructure B < A is a retract of A if there is a homomorphism
f: A -> B which fixes every element of B; f is a retraction. In
this paper we discuss the syntax of classes of structures closed under
the formation of isomorphic images, retracts, and direct products; that
is, classes K such that K = IRP(K) where I, R, and P are the corresponding
class operators.
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