weakly periodic cohomology theory
Special and general types
A multiplicative cohomology theory is weakly periodic if the natural map
is an isomorphism for all .
e.g. Lurie, remark 1.4
Compare with the notion of a periodic cohomology theory.
Relation to formal groups
One reason why weakly periodic cohomology theories are of interest is that their cohomology ring over the space defines a formal group.
To get a formal group from a weakly periodic, even multiplicative cohomology theory , we look at the induced map on from a morphism
and take the kernel
to be the ideal that we complete along to define the formal scheme (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 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 of the formal group
is a free -module, we can pick a generator and this gives an isomorphism
if then “forgets the -coordinate”.
Revised on October 31, 2014 14:42:27
by Urs Schreiber