Crystalline Cohomology - Coefficients

Coefficients

If X is a variety over an algebraically closed field of characteristic p > 0, then the l-adic cohomology groups for l any prime number other than p give satisfactory cohomology groups of X, with coefficients in the ring Zl of l-adic integers. It is not possible in general to find similar cohomology groups with coefficients in the p-adic numbers (or the rationals, or the integers).

The classic reason (due to Serre) is that if X is a supersingular elliptic curve, then its ring of endomorphisms generates a quaternion algebra over Q that is non-split at p and infinity. If X has a cohomology group over the p-adic integers with the expected dimension 2, the ring of endomorphisms would have a 2-dimensional representation; and this is not possible as it is non-split at p. (A quite subtle point is that if X is a supersingular elliptic curve over the prime field, with p elements, then its crystalline cohomology is a free rank 2 module over the p-adic integers. The argument given does not apply in this case, because some of the endomorphisms of supersingular elliptic curves are only defined over a quadratic extension of the field of order p.)

Grothendieck's crystalline cohomology theory gets around this obstruction because it takes values in the ring of Witt vectors over the ground field. So if the ground field is the algebraic closure of the field of order p, its values are modules over the p-adic completion of the maximal unramified extension of the p-adic integers, a much larger ring containing n-th roots of unity for all n not divisible by p, rather than over the p-adic integers.

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