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The study of differentiable manifolds and differentiable maps. One fundamental problem is that of classifying manifolds up to diffeomorphism. Differential topology is what Poincaré understood as topology or “analysis situs”.

9 votes

Are the associative grassmannian and the quaternionic projective plane diffeomorphic?

To expand on Oscar's answer: the principal $SO(4)$ bundle over $G_2/SO(4)$ gives us $$ \qquad \qquad \qquad \ldots \to \pi_2(G_2) \to \pi_2(G_2/SO(4)) $$ $$ \to \pi_1(SO(4)) \to \pi_1(G_2) \to \pi_1(G …
Allen Knutson's user avatar
6 votes

Is it true that all sphere bundles are some double of disk bundle?

The connected double cover of $S^1$ (boundary of the Möbius strip) is an $S^0$ bundle that is not the double of the unique $0$-disc bundle over $S^1$.
Allen Knutson's user avatar
5 votes

On the generalized Gauss-Bonnet theorem

Mathai and Quillen have a theorem that computes the Euler characteristic as an integral of a form defined using a section of and a connection on the tangent bundle. If one scales the section by a fact …
Allen Knutson's user avatar