Relations between the global structure of the gauge group in elliptic F-theory compactifications, fractional null string junctions, and the Mordell-Weil lattice of rational sections are discussed. We extend results in the literature, which pertain primarily to rational elliptic surfaces and obtain π(G̃) where G̃ is the semi-simple part of the gauge group. We show how to obtain the full global str...

In this note we show that if k is an algebraic number field with algebraic closure I and M is a finitely generated, free Zrmodule with continuous Ga\(k/k)action, then the continuous Galois cohomology group Hl(k, M) is a finitely generated Z,-module under certain conditions on M (see Theorem 1 below). Also, we present a simpler construction of a mapping due to S. Bloch which relates torsion alge...

Periodic points are points on Veech surfaces, whose orbit under the group of affine diffeomorphisms is finite. We characterise those points as being torsion points if the Veech surfaces is suitably mapped to its Jacobian or an appropriate factor thereof. For a primitive Veech surface in genus two we show that the only periodic points are the Weierstraß points and the singularities. Our main too...

Faltings' theorem states that curves of genus g > 2 have finitely many rational points. Using the ideas of Faltings, Mumford, Parshin and Raynaud, one obtains an upper bound on the upper bound on the number of rational points [Szp85], XI, §2, but this bound is too large to be used in any reasonable sense. In 1985, Coleman showed [Col85] that Chabauty's method, which works when the Mordell-Weil ...

Fix the data (p,K,E) where p is a prime number, K a number field, and E an elliptic curve over Q. Let K∞/K denote the maximal Zp-power extension of K. Recent work provides, in some instances, detailed information about p-adic completions of Mordell-Weil groups and their associated p-adic height pairings, and the p-primary Shafarevich-Tate groups and their associated Cassels pairings, over inter...

We describe all the elliptic fibrations with section on the Kummer surface X of the Jacobian of a very general curve C of genus 2 over an algebraically closed field of characteristic 0, modulo the automorphism group of X and the symmetric group on the Weierstrass points of C. In particular, we compute elliptic parameters and Weierstrass equations for the 25 different fibrations and analyze the ...

We prove that when all hyperelliptic curves of genus n ≥ 1 having a rational Weierstrass point are ordered by height, the average size of the 2-Selmer group of their Jacobians is equal to 3. It follows that (the limsup of) the average rank of the Mordell-Weil group of their Jacobians is at most 3/2. The method of Chabauty can then be used to obtain an effective bound on the number of rational p...

We consider the local-global principle for divisibility in Mordell-Weil group of a CM elliptic curve defined over number field. For each prime p we give lower bounds on degree d field which there exists gives counterexample to by power p. As corollary deduce that are at most finitely many curves (with or without CM) counterexamples with p>2d+1. also powers 7 holds all quadratic fields.

For an elliptic curve E defined over a fieldK, the Tate-Shafarevich group X(E/K) encodes important arithmetic and geometric information. An important conjecture of Tate and Shafarevich states X(E/K) is always finite. Supporting this conjecture is a cohomological analogy between Mordell-Weil groups of elliptic curves and unit groups of number fields. In this note, we follow the analogy to constr...

We determine all complex K3 surfaces with Picard rank 20 over Q. Here the NéronSeveri group has rank 20 and is generated by divisors which are defined over Q. Our proof uses modularity, the Artin-Tate conjecture and class group theory. With different techniques, the result has been established by Elkies to show that Mordell-Weil rank 18 over Q is impossible for an elliptic K3 surface. We also a...