The ABC conjecture of Masser and Oesterlé states that if (a, b, c) are coprime integers with a+ b+ c = 0, then sup(|a|, |b|, |c|) < cǫ(rad(abc)) 1+ǫ for any ǫ > 0. In [2], Oesterlé observes that if the ABC conjecture holds for all (a, b, c) with 16|abc, then the full ABC conjecture holds. We extend that result to show that, for every integer N , the “congruence ABC conjecture” that ABC holds fo...

We prove that the equation A+B = Cp has no solutions in coprime positive integers when p ≥ 211. The main step is to show that, for all sufficiently large primes p, every Q-curve over an imaginary quadratic field K with a prime of bad reduction greater than 6 has a surjective mod p Galois representation. The bound on p depends on K and the degree of the isogeny between E and its Galois conjugate...

Let E be an elliptic curve defined over the rationals and in minimal Weierstrass form, and let P = (x1/z 2 1 , y1/z 3 1) be a rational point of infinite order on E, where x1, y1, z1 are coprime integers. We show that the integer sequence (zn)n>1 defined by nP = (xn/z 2 n, yn/z 3 n) for all n > 1 does not eventually coincide with (un2)n>1 for any choice of linear recurrence sequence (un)n>1 with...

Let F be a real quadratic field, and let R be an order in F . Suppose given a polarized abelian surface (A,λ) defined over a number field k with a symmetric action of R defined over k. This paper considers varying A within the k-isogeny class of A to reduce the degree of λ and the conductor of R. It is proved, in particular, that there is a k-isogenous principally polarized abelian surface with...

We answer a question asked by Hajdu and Tengely: The only arithmetic progression in coprime integers of the form (a, b, c, d) is (1, 1, 1, 1). For the proof, we first reduce the problem to that of determining the sets of rational points on three specific hyperelliptic curves of genus 4. A 2-cover descent computation shows that there are no rational points on two of these curves. We find generat...

The second part of this conjecture is analogous to Conjecture 2.2 (i) in [PV], which considers hypersurfaces in P. If C is a curve of genus 2 as above and P = (a : y : b) is a rational point on C (i.e., we have F (a, b) = y with a, b coprime integers), then we denote by H(P ) the height H(a : b) = max{|a|, |b|} of its x-coordinate. Conjecture 2. Let ε > 0. Then there is a constant Bε and a Zari...

Let C be an algebraic curve defined over a number field K, of positive genus and without K-rational points. We conjecture that there exists some extension field L over which C violates the Hasse principle, i.e., has points everywhere locally but not globally. We show that our conjecture holds for all but finitely many Shimura curves of the form XD 0 (N)/Q or X D 1 (N)/Q, where D > 1 and N are c...

In this note we consider two S-unit equations for which we will exhibit many solutions. Our first problem concerns solutions to the equation a + b = c where a, b, and c are coprime integers such that all prime factors of abc lie in a given set S of s primes. In [8] J.-H. Evertse showed that this S-unit equation has at most exp(4s+ 6) solutions. On the other hand, in [6] P. Erdős, C. Stewart, an...

Let Ω be a finite symmetric subset of GLn(Z[1/q0]), Γ := 〈Ω〉, and let πm be the group homomorphism induced by the quotient map Z[1/q0] → Z[1/q0]/mZ[1/q0]. Then the family of Cayley graphs {Cay(πm(Γ), πm(Ω))}m is a family of expanders as m ranges over fixed powers of square-free integers and powers of primes that are coprime to q0 if and only if the connected component of the Zariski-closure of ...

Let a < b be coprime positive integers. Armstrong, Rhoades, and Williams (2013) defined a set NC(a, b) of ‘rational noncrossing partitions’, which form a subset of the ordinary noncrossing partitions of {1, 2, . . . , b− 1}. Confirming a conjecture of Armstrong et. al., we prove that NC(a, b) is closed under rotation and prove an instance of the cyclic sieving phenomenon for this rotational act...