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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).

16 votes
1 answer
651 views

Elements of a free group that can't be inverted by automorphisms

Let $F_n$ be a free group of rank $n$. Say that $w \in F_n$ is non-reversible if there does not exist any $f \in \text{Aut}(F_n)$ such that $f(w) = w^{-1}$. Original Question. Intuitively, I expect …
Andy Putman's user avatar
  • 44.8k
7 votes
0 answers
258 views

Homotopy type of space of embeddings of a disk

Let $M^n$ be a smooth $n$-dimensional manifold and let $\mathbb{D}^n$ be the open unit disk in $\mathbb{R}^n$. Consider the space $\text{Emb}(\mathbb{D}^n,M^n)$ of smooth embeddings of $\mathbb{D}^n$ …
Andy Putman's user avatar
  • 44.8k
17 votes
1 answer
666 views

Relationship between Smith's special homology groups and equivariant homology theory

EDIT: Tyler Lawson's answer was so nice that I was inspired to rewrite the notes discussed below to use Bredon homology in the definition of the Smith special homology groups. The original version is …
Andy Putman's user avatar
  • 44.8k
11 votes
2 answers
973 views

Subtle point in definition of BNS invariant

Let $G$ be a finitely generated group. Let $S(G)$ be the quotient of $\text{Hom}(G,\mathbb{R}) \setminus \{0\}$ by the equivalence relation that identifies two homomorphisms if they differ by scaling …
Andy Putman's user avatar
  • 44.8k
20 votes
2 answers
3k views

First Chern class of a flat line bundle

A referee asked me to include a reference or proof for the following classical fact. It's not hard to prove, but I'd prefer to just give a reference -- does anyone know one? Let $X$ be a nice space …
Andy Putman's user avatar
  • 44.8k
7 votes
2 answers
441 views

Fragmenting a homeomorphism of a compact manifold

Let $M$ be a compact manifold and let $f : M \rightarrow M$ be a homeomorphism which is isotopic to the identity. We will say that $f$ can be fragmented if it satisfies the following property. Let $ …
Andy Putman's user avatar
  • 44.8k
12 votes
0 answers
784 views

Commutator subgroup of a surface group

Let $\Sigma_{g,n}$ denote a compact orientable genus $g$ surface with $n$ boundary components. Assume that $g \geq 1$ and fix a basepoint $p \in \Sigma_{g,n}$. Define $S \subset [\pi_1(\Sigma_{g,n}, …
Andy Putman's user avatar
  • 44.8k
14 votes
3 answers
979 views

Homotopy type of set of self homotopy-equivalences of a surface

Let $\Sigma$ be an oriented topological surface. For simplicity, assume that the genus of $\Sigma$ is at least $2$. There are a number of classical results on the homotopy types of various groups of …
Andy Putman's user avatar
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62 votes
9 answers
9k views

Fundamental groups of noncompact surfaces

I got fantastic answers to my previous question (about modern references for the fact that surfaces can be triangulated), so I thought I'd ask a related question. A basic fact about surface topology …
Andy Putman's user avatar
  • 44.8k
53 votes
7 answers
10k views

Triangulating surfaces

I've had a few undergraduate students ask me for references for the classical fact (due to Rado) that closed topological surfaces can be triangulated. I know two sources for this, namely Ahlfors's bo …
Andy Putman's user avatar
  • 44.8k