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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).

37 votes
0 answers
2k views

What is the three-dimensional hyperbolic volume of a four-manifold?

Every smooth closed orientable 4-manifold may be constructed via a handle decomposition. Before asking a couple of questions, I recall some well-known facts about handle-decompositions of 4-manifolds. …
Bruno Martelli's user avatar
28 votes
4 answers
2k views

Can all n-manifolds be obtained by gluing finitely many blocks?

Fix a dimension $n\geqslant 2$. Let $S= \{M_1,\ldots, M_k\}$ be a finite set of smooth compact $n$-manifold with boundary. Let us say that a smooth closed $n$-manifold is generated by $S$ if it may …
Bruno Martelli's user avatar
24 votes
2 answers
2k views

Explicit homeomorphism between Thurston's compactification of Teichmuller space and the clos...

Thurston's celebrated compactification of Teichmuller space was first described in his famous Bulletin paper. Teichmuller space is famously homeomorphic to an open disc of some dimension (this can be …
Bruno Martelli's user avatar
21 votes
6 answers
3k views

Is there a good notion of morphism between orbifolds?

Following Thurston, an orbifold is a topological space which looks locally like a finite quotient of $\mathbb R^n$ by a finite group of $O(n)$: this is expressed using charts as for differentiable m …
Bruno Martelli's user avatar
19 votes
0 answers
847 views

Which manifolds decompose into pants?

In this nice paper Mikhalkin uses certain (more geometrical than algebraic) aspects of tropical geometry to prove that every complex projective hypersurface in $\mathbb C \mathbb P ^n$ decomposes as a …
Bruno Martelli's user avatar
17 votes
1 answer
2k views

Proofs of Rohlin's theorem (an oriented 4-manifold with zero signature bounds a 5-manifold)

A celebrated theorem of Rohlin states the following An oriented closed 4-manifold $M^4$ bounds an oriented 5-manifold if and only if the signature of $M^4$ is zero. Simple homological arguments …
Bruno Martelli's user avatar
16 votes
2 answers
606 views

Does a small-area sphere in a 3-manifold bound a small ball?

Let $M$ be a closed riemannian 3-manifold. I think that the following fact should be true and should have a relatively simple proof, but I cannot figure it out. For every $\varepsilon>0$ there is …
Bruno Martelli's user avatar
15 votes
3 answers
832 views

Maximal euler characteristic of surfaces bounding two fixed curves

Let $\gamma_0$ and $\gamma_1$ be two simple closed curves in a closed surface $S$. What is the maximum Euler characteristic of a compact properly embedded surface $\Sigma \subset S\times [0,1]$ s …
Bruno Martelli's user avatar
12 votes
3 answers
1k views

Is the cut locus of a generic point in a hyperbolic manifold a generic polyhedron?

Let $p\in M$ be a point in a closed riemannian manifold $M$. Recall that the cut locus of $p$ is the subset of $M$ consisting of all points that are connected to $p$ by at least 2 distance-minimizing …
Bruno Martelli's user avatar
11 votes
0 answers
354 views

Fox re-imbedding theorem in dimension four

Fox re-imbedding theorem states the following: A compact 3-manifold $M$ with boundary that embeds in the three-sphere $S^3$, can be re-imbedded in $S^3$ so that its complement is a union of handle …
Bruno Martelli's user avatar
10 votes
1 answer
566 views

Is the spectrum of closed geodesics in a closed hyperbolic 3-manifold asymptotically homogen...

It seems to me that the results in this important paper of Kahn-Markovich imply the following fact. Let $M^3$ be any closed hyperbolic 3-manifold. For every $\epsilon > 0$ there is a natural number $R …
Bruno Martelli's user avatar
9 votes
0 answers
135 views

Is there a closed aspherical manifold with infinitely many symmetries and without essential ...

The precise question is the following: Is there a closed aspherical manifold $M$ of dimension $n\geq 3$ such that Out($\pi_1(M)$) is infinite and $\pi_1(M)$ does not contain $\mathbb Z \times \mathbb …
Bruno Martelli's user avatar