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Topology of cell complexes and manifolds, classification of manifolds (e.g. smoothing, surgery), low dimensional topology (e.g. knot theory, invariants of 4-manifolds), embedding theory, combinatorial and PL topology, geometric group theory, infinite dimensional topology (e.g. Hilbert cube manifolds, theory of retracts).

7 votes
Accepted

A homeomorphism with a prescribed action on the fundamental group - decidable or not?

This is undecidable. For a construction, see the first page of Nabutovsky and Weinberger, "Algorithmic aspects of homeomorphism problems", Rothenberg Festschrift 1998.
Fedya's user avatar
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6 votes
Accepted

Triangulation of a simplex

You are looking for the edgewise subdivision: Edelsbrunner, H.; Grayson, D. R., Edgewise subdivision of a simplex, Discrete Comput. Geom. 24, No. 4, 707-719 (2000). ZBL0968.51016. The basic idea is to …
Fedya's user avatar
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6 votes
Accepted

Unknotting algorithm in higher dimensions?

The version of the problem in dimensions higher than 4 is undecidable, by work of Nabutovsky and Weinberger: https://link.springer.com/content/pdf/10.1007/BF02566428.pdf This is related to the undecid …
Fedya's user avatar
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4 votes

Local complexity of triangulations

Here's a stab at a proof that works for non-smoothable manifolds, which seems to be related to Florian Frick's sketch. I'm going to call manifolds with at most $L$ simplices incident to a vertex "of …
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3 votes

A variant of $\ell^2$-cochains

I'm not sure if this should be a comment rather than an answer, since I'm not attempting to answer the hard questions (2) and (3). But it's too long to be a comment. That said: First of all, I just …
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