Sign up ×
MathOverflow is a question and answer site for professional mathematicians. It's 100% free, no registration required.

We know that the homotopy class of the maps $S_d \to X$ is $\pi_d(X)$. What is the homotopy class of the maps $(S_1)^d \to X$? Here $X$ can be an $n$-dimensional sphere $S_n$, or a classifying space $BG$ of some simple groups $G=Z_2,U(1),SU(n)$ etc.

share|cite|improve this question
The cases of $BG$ where $G=\mathbb{Z}/2$ and $U(1)$ are easy, since these are $K(\mathbb{Z}/2,1)$ and $K(\mathbb{Z},2)$ respectively, so you get $H^1(T^d,\mathbb{Z}/2)$ and $H^2(T^d,\mathbb{Z})$ (when looking at pointed homotopy classes). – Mark Grant Feb 8 '13 at 7:09
For $d=2$, there is an action of $\pi_2(X)$ on $[S^1\times S^1,X]$ whose quotient set is in bijection with $\{a,b\in\pi_1(X);a+b=b+a\}\subset[S^1\vee S^1,X]$ via the restriction along $S^1\vee S^1\subset S^1\times S^1$. – Fernando Muro Feb 8 '13 at 7:42
@Dylan, tori split stably as wedges of spheres. – Fernando Muro Feb 8 '13 at 8:56
@Xiao-Gang Wen: A general method for computing torus homotopy groups is given in Section 3.1 of the recent preprint by Lupton and Smith. It's worth pointing out though, that $\tau_d(X)$ is not the same thing as $[T^d,X]$, which is what you originally asked about. – Mark Grant Feb 22 '13 at 17:03

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.