Separating Notions in Effective Topology

Abstract

. We compare several natural notions of effective presentability of a topological space up to homeomorphism. We note that every left-c.e. (lower-semicomputable) Stone space is homeomorphic to a computable one. In contrast, we produce an example of a locally compact, left-c.e. space that is not homeomorphic to any computable Polish space. We apply a similar technique to produce examples of computable topological spaces not homeomorphic to any right-c.e. (upper-semicomputable) Polish space, and indeed to any arith-metical or even analytical Polish space. We then apply our techniques to totally disconnected locally compact (tdlc) groups. We prove that every effectively locally compact tdlc group is topologically isomorphic to a computable tdlc group; all notions will be clarified. The result is perhaps unexpected since the hypothesis of the theorem may seem rather weak.