Evil primes and superspecial moduli

Abstract

For a quartic non-biquadratic CM field K, we say that a rational prime p is evil for K if at least one of the principally polarized abelian varieties with CM by K reduces modulo a prime ideal p|p to a product of supersingular elliptic curves with the product polarization. We showed that for fixed K, such primes are bounded by a quantity related to the discriminant of K. We show that evil primes are ubiquitous in the sense that for any rational prime p, there are an infinite number of such CM fields K for which p is evil. (Assuming a standard conjecture, the result holds for a finite set of primes simultaneously.) The proof consists of two parts: (1) showing the surjectivity of the principally polarized abelian varieties with CM by K, for K satisfying some conditions, onto the superspecial points of the reduction modulo p of the Hilbert modular variety associated to the intermediate real quadratic field of K, and (2) showing the surjectivity of the superspecial points of the reduction modulo p of the Hilbert modular variety associated to a real quadratic field with large enough discriminant onto the superspecial points on the reduction modulo p of the Siegel moduli space parameterizing abelian surfaces with principal polarization.