The Isomorphism Problem for Familiar Classes of Computable Structures I

Theories of classification distinguish classes with some good structure theorem from those for which none is possible. Some classes (Dense linear orders, for instance) are non-classifiable in general, but are classifiable when we consider only countable members. This paper explores such a notion for classes of computable structures by working out several examples. One motivation is to see whether some classes whose set of countable members is very complex become classifiable when we consider only computable members. We follow recent work by Goncharov and Knight in using the degree of the isomorphism problem for a class to distinguish classifiable classes from nonclassifiable. For some classes (undirected graphs, fields of fixed characteristic, and real closed fields) we show that the isomorphism problem is ∑ 1 1 complete (the maximum possible), and for others it is of relatively low complexity. For instance, for algebraically closed fields, archimedean real closed fields, or vector spaces, we show that the isomorphism problem is ∏ 0 3 complete.