Abstract
Given a K3 surface X over a field of characteristic p, Artin conjectured that
if X is supersingular (meaning infinite height) then its Picard rank is 22.
Along with work of Nygaard-Ogus, this conjecture implies the Tate conjecture
for K3 surfaces over finite fields with p 5. We prove Artin's conjecture
under the additional assumption that X has a polarization of degree 2d with p >
2d+4. Assuming semistable reduction for surfaces in characteristic p, we can
improve the main result to K3 surfaces which admit a polarization of degree
prime-to-p when p 5.
The argument uses Borcherds' construction of automorphic forms on O(2,n) to
construct ample divisors on the moduli space. We also establish
finite-characteristic versions of the positivity of the Hodge bundle and the
Kulikov-Pinkham-Persson classification of K3 degenerations. In the appendix by
A. Snowden, a compatibility statement is proven between Clifford constructions
and integral p-adic comparison functors.
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