Tuesday, 18 June 2013

at.algebraic topology - Whitehead Products without Base Points?

Let $(X, x_0)$ be a pointed space. Then we can define the homotopy groups $pi_i(X, x_0)$ for $i geq 1$. They are abelian groups for $i geq 2$. It is well-known that the fundamental group $pi_1(X, x_0)$ acts on each of the higher groups $pi_i(X, x_0)$, and that this action generalizes to the Whitehead Products which are maps



$$ pi_p(X, x_0) times pi_q(X, x_0) to pi_{p+q -1}(X, x_0).$$



The details are given in the wikipedia article I linked to above. Together the Whitehead products turn the graded group $pi_*(X, x_0)$ (for $* > 0$) into a graded (quasi-) Lie algebra over $mathbb{Z}$, where the grading is shifted so that $pi_i(X, x_0)$ is in degree $(i-1)$. Well, it is a little funny since the bottom group is not necessarily abelian.




This is all well and good, but what if we don't want to pick base points? Is there a similar algebraic gadget in that situation?




If we don't pick base points, then it seems natural to consider the fundamental groupoid $Pi_1(X)$. Then the different homotopy groups of $X$ at different base points can be assembled into local systems on $X$. That is for each $i geq 2$ we have a functor,



$$pi_i: Pi_1 X to AB$$



where $AB$ is the category of abelian groups. This already incorporates the action of $pi_1$ on the higher homotopy groups but does it in a way which doesn't depend on the choice of base point.




Question: Can we enhance these local systems with a structure which generalizes the Whitehead product, and if so what precisely is this extra structure?


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