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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

Constructing 4-manifolds with fundamental group with a given presentation.

Taking the boundary of tubular neighborhood in codimension two doesn't preserve fundamental group -- the codimension must be at least three. For a simple example, if you take the boundary of a tubula …
Jim Belk's user avatar
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7 votes
Accepted

Extending a map from $S^n\to M^n$ to a nice map from $B^{n+1}\to M^n$

The answer is no, essentially since higher homotopy groups of spheres are nontrivial. For example, in the $n=3$ case let $M=B^3$ and let $\sigma\colon S^3 \to M$ be the Hopf map from $S^3$ to the bou …
Jim Belk's user avatar
  • 8,483
9 votes
Accepted

Growth rates of surface groups

Just to make the method as concrete as possible, I'll compute the growth rate for the fundamental group $G$ of a surface of genus two. The Cayley graph of $G$ is the 1-skeleton of a tiling of the hyp …
Jim Belk's user avatar
  • 8,483
18 votes

Folner sets and balls

This is not exactly an answer to the question, but is instead essentially a comment that was way too long for the comment space. The OP mentioned that he doesn't have a good sense for the shapes of F …
Jim Belk's user avatar
  • 8,483
4 votes
Accepted

Starting letters of equivalent infinite geodesic paths of hyperbolic Coxeter groups

This is false. The archetypical family of hyperbolic Coxeter groups are the hyperbolic triangle groups $$ T(l,m,n) = \langle a,b,c \mid a^2=b^2=c^2=(ab)^l=(bc)^m=(ca)^n=1\rangle $$ where $l,m,n\geq 2 …
Jim Belk's user avatar
  • 8,483
8 votes
Accepted

Asymmetry of outer space - injectivity radius

Given a geodesic metric space $X$ and a point $p\in X$, the injectivity radius $\mathrm{injrad}(p)$ is the maximum value of $r$ such that every point in the open ball $B(p,r)$ is connected to $p$ by a …
Jim Belk's user avatar
  • 8,483
7 votes
2 answers
342 views

Convex subcomplexes of CAT(0) cubical complexes

Is the following statement true? If so, can anyone provide a reference? Let $X$ be a CAT(0) cubical complex, and let $Y$ be a connected subcomplex of $X$. Then the following are equivalent: …
Jim Belk's user avatar
  • 8,483