Let E be an elliptic curve over Q. Let p be a prime of good reduction for E. Then, for a prime p = ℓ, the Frobenius automorphism associated to p (unique up to conjugation) acts on the ℓ-adic Tate module of E. The characteristic polynomial of the Frobenius automorphism has rational integer coefficients and is independent of ℓ. Its splitting field is called the Frobenius field of E at p. Let E 1 and E 2 be two elliptic curves defined over Q that are non-isogenous over Q and also without complex multiplication over Q. In analogy with the well-known Lang-Trotter conjecture for a single elliptic curve, it is natural to consider the asymptotic behaviour of the function that counts the number of primes p ≤ x such that the Frobenius fields of E 1 and E 2 at p coincide. In this short note, using Heath-Brown's square sieve, we provide both conditional (upon the Generalized Riemann Hypothesis) and unconditional upper bounds.
To V. Kumar Murty: On the occasion of his sixtieth birthdayConjecture 5. 1 [5, Conjecture 1] (Pair of elliptic curves and equal Frobenius fields) Let E 1 and E 2 be two elliptic curves over the rationals, and both without complex multiplication over Q. Then, E 1 is not isogenous to E 2 over Q if and only ifRemark 3. Suppose E 1 and E 2 are isogenous over Q, hence over some number field L. By extending L if necessary, we can assume that L is Galois over Q. Then, for any prime p ∤ N 1 N 2 and that splits in L, a p (E 1 ) = a p (E 2 ). Thus, from the Chebotarev density theorem, it follows that S(E 1 , E 2 ; x) grows at least as much as