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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).

1 vote
0 answers
129 views

When is a nested sequence of closed sets a colimit?

Let $X$ denote a topological space and $X_0\subset X_1\subset \ldots\subset X$ a nested sequence of closed subsets of $X$ such that $$ \bigcup_i X_i =X$$ It is easy to see that in the general case $X …
ThorbenK's user avatar
  • 1,174
0 votes
1 answer
166 views

Mappings of reducible 3 manifolds with boundary

In section 3 of his paper "Mappings of reducible 3 manifolds" McCullough, proves that every self-homeomorphism of a reducible 3 manifold can up to isotopy be written as a composition of particularly n …
ThorbenK's user avatar
  • 1,174
0 votes

Bounded cohomology motivation

Even though this is an old question, I want to add a different answer by saying that to me personally bounded cohomology in itself is not very interesting. It tends to be infinite dimensional or zero. …
ThorbenK's user avatar
  • 1,174
11 votes
2 answers
668 views

Can we embed a closed manifold into a homotopy equivalent CW complex?

Suppose $X$ is a CW complex and $M$ is a closed manifold and suppose further that there exists a homotopy equivalence $X \simeq M$. Does there exists an embedding of $M$ into $X$ (i.e. an injective (p …
ThorbenK's user avatar
  • 1,174
2 votes
1 answer
118 views

Hyperbolization with word-hyperbolic fundamental group

In Davis-Januszkiewica´s paper Hyperbolization of polyhedra it is shown that for every manifold $M$ there exists a map $N \to M$ of non-zero degree such that $N$ is aspherical (plus some more properti …
ThorbenK's user avatar
  • 1,174
6 votes

Can we embed a closed manifold into a homotopy equivalent CW complex?

While thinking about it with a friend, we came up with the following two dimensional counter example: Take the standard knot diagram of the trefoil knot (as a self-intersecting curve in $\mathbb{R}^2$ …
ThorbenK's user avatar
  • 1,174
8 votes
0 answers
219 views

Representing the fundamental class of an aspherical manifold in the bar complex

Suppose $M$ is a compact orientable aspherical manifold and $G$ its fundamental group. Is there a nice description of representatives of the fundamental class of $M$ and its dual in the (homogenous) b …
ThorbenK's user avatar
  • 1,174
5 votes
1 answer
231 views

Amenable link groups

The unknot and the Hopf link are (as far as I know) the only links whose complements have abelian fundamental groups. Are there more examples whose complement have amenable fundamental group?
ThorbenK's user avatar
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5 votes
1 answer
316 views

0-surgery on a fibered hyperbolic ribbon knot

Does there exist a fibering hyperbolic ribbon knot such that the 0 surgery is exceptional? If so does there exist such an example where the result of 0-surgery is Seifert fibered? I tried looking at k …
ThorbenK's user avatar
  • 1,174
7 votes
0 answers
317 views

Different definitions of Stiefel-Whitney classes

It is quite easy to show that different definitions of the Stiefel-Whitney classes agree by showing that they satisfy the well-known axioms. Nevertheless I have been asking myself wether one can prove …
ThorbenK's user avatar
  • 1,174
9 votes
1 answer
434 views

Action of diffeomorphism group on non-vanishing vector fields

Let $M$ denote a closed manifold. Let $\Gamma(TM\setminus 0) $ denote the space of non-vanishing sections of $TM$. Note that the diffeomorphism group $\text{Diff} (M)$ acts on $\Gamma(TM\setminus 0 …
ThorbenK's user avatar
  • 1,174
10 votes
3 answers
656 views

Doubles of 2-handlebodies

Let $X$ denote a $4$-manifold with boundary obtained by adding $k_1$ $1$-handles to $B^4$ and $k_2$ many $2$-handles to the resulting manifold i.e. $X$ is an arbitrary $4$-dimensional $2$-handlebody. …
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4 votes
Accepted

Doubles of 2-handlebodies

Let $X$ denote a $2$-handlebody. I claim that the inclusion $\partial X \to X$ induces a surjection on fundamental groups. Indeed, let $Y\subset X$ denote the underlying $1$-handlebody (i.e. the union …
ThorbenK's user avatar
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12 votes
1 answer
368 views

Fundamental group of the complement of a codimension two submanifold

Let $M$ denote an arbitrary closed, connected, n-dimensional manifold for $n\geq 4$. Does there always exist a closed (not necessarily connected!) codimension two submanifold $S \subset M$ such that $ …
ThorbenK's user avatar
  • 1,174