group cohomology, nonabelian group cohomology, Lie group cohomology
cohomology with constant coefficients / with a local system of coefficients
differential cohomology
A multiplicative cohomology theory $A$ is weakly periodic if the natural map
is an isomorphism for all $n \in \mathbb{Z}$.
e.g. Lurie, remark 1.4
Compare with the notion of a periodic cohomology theory.
One reason why weakly periodic cohomology theories are of interest is that their cohomology ring over the space $\mathbb{C}P^\infty$ defines a formal group.
To get a formal group from a weakly periodic, even multiplicative cohomology theory $A^\bullet$, we look at the induced map on $A^\bullet$ from a morphism
and take the kernel
to be the ideal that we complete along to define the formal scheme $Spf A^0(\mathbb{C}P^\infty)$ (see there for details).
Notice that the map from the point is unique only up to homotopy, so accordingly there are lots of chocies here, which however all lead to the same result.
The fact that $A$ is weakly periodic allows to reconstruct the cohomology theory essentially from this formal scheme.
To get a formal group law from this we proceed as follows: if the Lie algebra $Lie(Spf A^0(\mathbb{C}P^\infty))$ of the formal group
is a free $A^0({*})$-module, we can pick a generator $t$ and this gives an isomorphism
if $A^0(\mathbb{C}P^\infty) A^0({*})[ [t] ]$ then $i_0^*$ “forgets the $t$-coordinate”.
Last revised on October 31, 2014 at 14:42:27. See the history of this page for a list of all contributions to it.