Recursion theory has been dominated by one example. The notion of a dominical category was introduced (see [2] and [3]) at least in part in the hope of subverting this dominance. The structures of number theory, freed from this original context, led to much of modern algebra with a consequent enrichment of number theory itself: it seemed not unreasonable to attempt to contribute, however modestly, to a similar development of recursion theory. Thus (in the loosest sort of analogy) recursion categories are intended to stand in relation to the class of partial recursive functions as rings do to the rational integers.

But if such a generalization is to serve its intended purpose, it ought to allow examples substantially different in spirit from the prototype. Recursion categories are, of course, the proper generalization of classical recursion theory. As a novice in this formidable field I am incapable of encompassing its full extent and must rely on the advice of others (I thank in particular R. DiPaola) in concluding that most of these earlier generalizations are not of this character. Indeed, the examples of recursion categories so far adduced (cf., in addition to the references above, [1] and [4]) are themselves closely tied to the classical one.

We attempt here to open the door to radically disparate examples by proving an existence theorem allowing the construction of recursion categories in a wide, and so far unexplored, variety of contexts very distant from that of the natural or ordinal numbers and their subsets, the locus of the traditional theory.

In order to do this, it has been necessary to introduce a number of unfamiliar notions—unfamiliar, that is, even in the context of the earlier discussion of dominical categories. This novelty is perhaps mitigated by the fact that, however unfamiliar, they cannot be said to be unusual, as the discussion later will show. We mention in particular those of a “formally free semigroup”, which retains the indexation function of a free semigroup, while giving up the characteristic universal property and that of “uniform generation”, which generalizes finite generation in the fashion assorted to our argument.