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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).

28 votes
1 answer
1k views

Is there a general theory of fiber theorems?

Here are three vague theorems rolled up in one. Let $X$ and $Y$ be sufficiently nice topological spaces and $f:X \to Y$ a sufficiently nice surjection. If for each $y \in Y$, the fiber $f^{-1}(y) …
Vidit Nanda's user avatar
  • 15.5k
22 votes
Accepted

fixed point property for maps of compacts

Lovely question! Sadly, the answer is "no" in the sense that the fixed point property is not homotopy-invariant even in the category of finite polyhedra. In fact, it is also not invariant under the op …
Vidit Nanda's user avatar
  • 15.5k
15 votes
3 answers
2k views

Combinatorial analogues of curvature

There appear to be many "combinatorial" definitions of curvature as applied to finite simplicial (or regular CW) complexes. For instance, we have the ideas of Cheeger, Muller and Schrader, of Forman a …
Vidit Nanda's user avatar
  • 15.5k
14 votes

Good covers of manifolds

I am not aware of a general result regarding the existence of good covers (and would guess that the general answer is negative). However, if you are willing to make certain sacrifices in terms of addi …
Vidit Nanda's user avatar
  • 15.5k
13 votes
3 answers
832 views

What fraction of n-point sets in the unit ball have diameter smaller than 1?

This question is inspired by a recent talk by Matt Kahle on random geometric complexes. Some simple notation: let $\mathcal{B} \subset \mathbb{R}^d$ be the unit ball in $d$-dimensional Euclidean spa …
Vidit Nanda's user avatar
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12 votes

Can we define Whitney stratification algebraically?

There is a purely algebraic characterisation of Condition (B) due to Le and Teissier, see Proposition 1.3.8 of the paper Lê Dũng Tráng; Teissier, Bernard, Limites d’espaces tangents en géométrie analy …
Vidit Nanda's user avatar
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8 votes
Accepted

Is every triangulation of a Euclidean ball by convex tetrahedra shellable?

I haven't had time to check through the details of the construction, but the example B_3_9_18 found in the proof of Theorem 2 here by Frank Lutz appears to be embeddable in 3-space. Frank specializes …
Vidit Nanda's user avatar
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8 votes

Is there a discrete Cerf theory?

I realize that I am several months late to the Cerf theory party, but this paper of Chari and Joswig might be of interest to the original poster and certainly deserves a mention in the context of this …
Vidit Nanda's user avatar
  • 15.5k
8 votes

Homological computations

What is the fundamental domain of the action? In case you can easily create a cubical or simplicial decomposition of this region, there is tons of software out there to help you with the grunt-work. …
Vidit Nanda's user avatar
  • 15.5k
8 votes
3 answers
481 views

Does the metric space of compact metric spaces satisfy the binary intersection property?

A metric space $Y$ has the binary intersection property provided that whenever a collection of closed balls in $Y$ intersects pairwise, then there is a common intersection point. Does the metric s …
Vidit Nanda's user avatar
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7 votes
Accepted

Who first considered constructibility of simplicial complexes?

If you want the first use of the term "constructible" in this context, then your reference to Mel Hochster's work is right-on. But if you want the actual notion, then things get slightly hazy. I think …
Vidit Nanda's user avatar
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7 votes

Good books on Geometric Theory of Dynamical Systems

Pick up (almost) anything by Ethan Akin. I particularly recommend "The General Topology of Dynamical Systems" available on Amazon. Although it is somewhat older than what you indicate you are looking …
7 votes
1 answer
817 views

Which Abelian Group sequences arise as the Homology of Embedded CW Complexes?

Background Let $\mathcal{A} = \lbrace A_0, \ldots, A_M \rbrace$ be an arbitrary sequence of finitely generated Abelian groups. It is well-known that a finite CW complex $X_\mathcal{A}$ may be construc …
Vidit Nanda's user avatar
  • 15.5k
6 votes

Voronoi cells and the dual complexes in Riemannian manifolds

At least partial answers to your first two questions can be found in the brief article called Delaunay triangulations and Voronoi diagrams for Riemannian manifolds by Leibon and Letscher available her …
Vidit Nanda's user avatar
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4 votes
Accepted

Discrete Morse function from smooth one

This is a rapidly developing area, and there are many short-cuts if all you want to do is compute the homology of sub-level sets of $f$. To answer your main question, as Liviu has already mentioned: t …
Vidit Nanda's user avatar
  • 15.5k

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