Friday, 23 July 2010

motives - Kunneth formula for motivic cohomology

I now remember a nice argument, why there's no Kunneth formula for Chow groups of XtimesX unless X has a Tate motive. Let X be smooth projective of dimension d.
We start with a decomposition of a diagonal:
[Delta]=sumi,jalphaijbetadijinoplusiCHi(X)otimesCHdi(X)


We can assume alphaij are linearly independent.
In this case we can show that alphaij form a basis of Chow groups and
betadij is the dual basis.



Indeed, as a correspondence [Delta] acts as identity on Chow groups,
so for any class c, c=[Delta]c=sumi,jalphaijdeg(betadijcupc),


and the claim follows if we substitute c=alphaij.



Now CHi(X)=Hom(mathbbZ(i)[2i],M(X)) and we can consider the set of alphaij as a morphism of motives oplusi,jmathbbZ(i)[2i]toM(X).


A simple computation shows that it is an isomorphism with the inverse given by betaij.



And of course, on the other hand, if X has a Tate motive, then Kunneth formula for Chow groups follows (it doesn't answer the question, since I only consider smooth projective varieties).

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