Conjecture on decidable properties of the space of marked groups

Let G\mathcal{G} be the space of marked groups and let ΛWP\Lambda_{WP} denote the word-problem representation. A property of GWP\mathcal{G}_{WP} is ΛWP\Lambda_{WP}-decidable when its characteristic decision problem is decidable relative to ΛWP\Lambda_{WP}; a subset is clopen when it is both open and closed.

Decidability–clopenness conjecture. Every ΛWP\Lambda_{WP}-decidable property of GWP\mathcal{G}_{WP} is clopen.

The conjecture asks whether the topological classification of natural properties captures their decidability status. The converse, that clopen properties are decidable, is known in the compact bounded-generator case but fails in G\mathcal{G}; the forward implication remains open.

Sources & referencesView supporting material

Primary source

Emmanuel Rauzy, “Computable analysis on the space of marked groups”, arXiv:2111.01179 (2025).

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