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Lie Groups are Groups that are additionally smooth manifolds such that the multiplication and the inverse maps are smooth.

73 votes

Homotopy groups of Lie groups

The elementary proof that $\pi_1$ is abelian applies more generally to H-spaces (spaces $X$ with a continuous multiplication map $X \times X \to X$ having a 2-sided identity element) without any assum …
Allen Hatcher's user avatar
18 votes

Topological structure of SO(n) as a product

Another line of reasoning: If $SO(n)$ were homeomorphic to the product $S^{n-1}\times SO(n-1)$ then $S^{n-1}$ would be a retract (not deformation retract!) of $SO(n)$, hence $S^{n-1}$ would be an H-sp …
Allen Hatcher's user avatar
8 votes
Accepted

Existence of an isotopy in Riemannian manifold

For the first question, which concerns just the smooth category without reference to metrics, the answer depends on the dimension of $M$. Before explaining this a small clarification is needed. By "a …
Allen Hatcher's user avatar