Decidability of equations and first-order theory in Seifert 3-manifold groups
Authors
Robert D. Gray
Alex Levine
Abstract
In [arXiv:1405.6274, Question 5.2 & Question 5.3] Aschenbrenner, Friedl and Wilton ask: (1) Is the equation problem solvable for the fundamental group of any $3$-manifold? and (2) Is the first-order theory of the fundamental group of any $3$-manifold decidable? In this paper we answer both of these questions by proving that Hilbert's tenth problem over the integers can be encoded in equations over any non-virtually abelian fundamental group of any Seifert fibered 3-manifold whose orbifold has non-negative Euler characteristic. We use this to show that the equation problem (and hence also the first-order theory) is undecidable in this infinite family of $3$-manifold groups and then apply it to classify the Seifert 3-manifold groups with decidable equation problems and decidable first-order theories, in the case that the orbifold has non-negative Euler characteristic. In contrast, we show that for this class of Seifert 3-manifold groups the single equation problem is decidable. For every Seifert 3-manifold group $G$ where the orbifold has negative Euler characteristic we show that either $G$ has decidable equation problem or $G$ has a finite index subgroup of index $2$ that has decidable equation problem. These negative Euler characteristic results follow from work of Liang on central extensions of hyperbolic groups. We also discuss why Liang's results do not suffice to deal with all the negative Euler characteristic cases. We show how to construct several other infinite families of $3$-manifold groups with undecidable equation problem (and hence also undecidable first-order theory) including examples that are not Seifert manifold groups and examples that are not virtually nilpotent. In addition, we observe that there are numerous other infinite families for which the first-order theory is undecidable such as fundamental groups of manifolds modeled on 3-dimensional Sol geometry.
