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Continuum theory, point-set topology, spaces with algebraic structure, foundations, dimension theory, local and global properties.
11
votes
Accepted
Approximate classifying space by boundaryless manifolds?
Yes, this is possible. The construction below is fairly standard and I'm not sure where I learned it.
The aim is to show that for each compact Lie group $G$, there is a closed $n$-connected manifold $ …
4
votes
$4$-manifold with simply connected boundary
This is a substantially more interesting question than the votes indicate; while the initial phrasing is more naive than the real situation permits, a slight generalization is indeed true. The answer …
7
votes
Accepted
Finite CW complex with finite non-abelian fundamental group and higher homologies zero
Theorem. Let $G$ be a group. There exists a finite 3-complex $X_G$ with $\pi_1 X_G = G$ and $H_i X_G = 0$ for $i > 1$ if, and only if, $G$ is finitely presentable and has second group homology $H_2(G) …
8
votes
Accepted
Nonvanishing section of infinite-dimensional tautological bundle
Let $X$ be any paracompact space. Then Hilbert vector bundles over $X$ are classified by homotopy classes of maps $[X, BU(\mathcal H)]$. But when $\mathcal H$ is infinite-dimensional, the group $U(\ma …
16
votes
Accepted
Obstruction of spin-c structure and the generalized Wu manifods
Define the Wu manifold $W(n) = SU(n)/SO(n)$, the inclusion $SO \to SU$ given by thinking of $\Bbb C^n = \Bbb R^n \otimes \Bbb C$ (that is, including real matrices into complex matrices). Note that $W( …