# Supersingular prime (for an elliptic curve)

Elkies (1987) showed that any elliptic curve over the rational numbers has infinitely many supersingular primes. However, the set of supersingular primes has asymptotic density zero. Lang & Trotter (1976) conjectured that the number of supersingular primes less than a bound X is within a constant multiple of $\frac{\sqrt{X}}{\ln X}$, using heuristics involving the distribution of Frobenius eigenvalues. As of 2012, this conjecture is open.
More generally, if K is any global field—i.e., a finite extension either of Q or of Fp(t)—and A is an abelian variety defined over K, then a supersingular prime $\mathfrak{p}$ for A is a finite place of K such that the reduction of A modulo $\mathfrak{p}$ is a supersingular abelian variety.