Recall the category of noncommutative space covers in the sense of Kontsevich-Rosenberg. The objects are called here simply space covers; an object is given by a ring , a -coring over and a structure map which is a map of corings and epimorphism of -bimodules. (see also: noncommutative space as a cover).
Let be a Hopf algebra. We are going to generalize the cleft -extensions of rings to cleft -extensions of space covers.
Let be a space cover with structure map and a Hopf algebra. Then a -cleft extension of consists of the following data:
a right -comodule algebra which is an -cleft extension of
an -coring in the category of right -comodules equipped with a monomorphism of -bimodules such that the image is precisely the subbimodule of -coinvariants in ; we identify with its image and require that is generated as an -bimodule with .
a structure map such that is a space cover, and is a map in the monoidal category of right -comodules
extension of -bimodules is cleft compatibly with cleavage of in the following sense. Let be the cleavage of . Then it induces an measuring (in smash case an action) as well known, i.e. . Now there should be left and right cleavage on in the sense that there are two “extensions” of this measuring to a semimeasuring of (we will take these semimeasurings as a part of the structure). This means that for all , and is a map such that
In fact, regarding that is a map of -bimodules, the first two properties follow from the other three. Now will be called the left and the right semimeasuring of on .
There is a construction which produces some examples from local data. Here I sketch this construction for the simpler case of a smash product rather than a general case of cleft product.
Namely, we start with , Hopf action of on with semimeasuring extensions as above, and a transition cocycle in the following sense:
Notice that this equation on the left involves comultiplication in the Hopf algebra, while on the right hand side the comultiplication of the coring.
hold as equalities in .
One should have a theorem of the form that that suffice to construct etc. such that the data form a cleft -extension of and that every cleft -extension, up to a natural isomorphism comes from a cocycle of that form.
Then there is an idea how to compare different cocycles and finally the Čech cohomology related to such cocycles.
Now the main examples should come from covers by localizations and globalized torsors over them (there is a different theory of those). We will here call instead of previous as there is for another object in this example. If is a family of localizations of a right -comodule algebra , then we say that the family is coaction compatible if in each localization there is a (automatically unique) extension of the coaction to the localization as an algebra map, and then it is automatically a comodule algebra. This means that , where is the localization map (the component of the unit of the adjunction between the localization functor and its adjoint, taken at the object ). Then for coaction). Define , , , , and . The action is then estricting to as an extension of action on understood as an element in (along canonical map from ), and similarly for the other leg. Thus we have, for ,
as an equality in ,
where is the projection of to . One denotes the restriction . Notice that is here extended from to (one could also extend to in another leg). The cocycle condition is for every we have in . The convolution invertibility is for every , in . Though one should check if this fits with the coring version (it looks like there is -condition there as well, what is strange, but one can not isolate.
Finally at least in a smash product case one expects some sort of coming from the data. For example, by that isomorphism of -comodules one should have but ; here is used and not version 2, probably because it is right smash product and left actions. This looks a bit nonsymmetric.
Last revised on April 19, 2012 at 00:01:31. See the history of this page for a list of all contributions to it.