We consider fragments of first-order logic and as models we allow finite and infinite words simultaneously. The only binary relations apart from equality are order comparison < and the successor predicate +1. We give characterizations of the fragments Sigma2 = Sigma2[<,+1] and FO2 = FO2[<,+1] in terms of algebraic and topological properties. To this end we introduce the factor topology over infinite words. It turns out that a language L is in the intersection of FO2 and Sigma2 if and only if L is the interior of an FO2 language. Symmetrically, a language is in the intersection of FO2 and Pi2 if and only if it is the topological closure of an FO2 language. The fragment Delta2, which by definition is the intersection Sigma2 and Pi2 contains exactly the clopen languages in FO2. In particular, over infinite words Delta2 is a strict subclass of FO2. Our characterizations yield decidability of the membership problem for all these fragments over finite and infinite words; and as a corollary we also obtain decidability for infinite words. Moreover, we give a new decidable algebraic characterization of dot-depth 3/2 over finite words.

Decidability of dot-depth 3/2 over finite words was first shown by Glaßer and Schmitz in STACS 2000, and decidability of the membership problem for FO2 over infinite words was shown 1998 by Wilke in his habilitation thesis whereas decidability of Sigma2 over infinite words was not known before.