Solutions sets to systems of equations in hyperbolic groups are EDT0L in PSPACE

Publication Type:
Conference Proceeding
Leibniz International Proceedings in Informatics, LIPIcs, 2019, 132
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© Laura Ciobanu and Murray Elder; licensed under Creative Commons License CC-BY We show that the full set of solutions to systems of equations and inequations in a hyperbolic group, with or without torsion, as shortlex geodesic words, is an EDT0L language whose specification can be computed in NSPACE(n2 log n) for the torsion-free case and NSPACE(n4 log n) for the torsion case. Our work combines deep geometric results by Rips, Sela, Dahmani and Guirardel on decidability of existential theories of hyperbolic groups, work of computer scientists including Plandowski, Jeż, Diekert and others on PSPACE algorithms to solve equations in free monoids and groups using compression, and an intricate language-theoretic analysis. The present work gives an essentially optimal formal language description for all solutions in all hyperbolic groups, and an explicit and surprising low space complexity to compute them.
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