On class numbers of quadratic extensions¶over function fields

We consider class numbers of quadratic extensions over a fixed function field. We will show that there exist infinitely many quadratic extensions which have class numbers not being divisible by 3 and satisfy prescribed ramification conditions. Received: 24 October 1997 / Revised version: 26 February 1998     

A procedure to calculate torsion¶of elliptic curves over ℚ

We present an algorithm which uses the analytic parameterization of elliptic curves to rapidly calculate torsion subgroups, and calculate its running time. This algorithm is much faster than the “traditional” Lutz–Nagell algorithm used by most computer algebra systems to calculate torsion subgroups. Received: 7 August 1997 / Revised version: 28 November 1997     

On primitive roots of tori: The case of function fields

We generalize Bilharz's Theorem for to all one-dimensional tori over global function fields of finite constant field. As an application, we also derive an analogue, in the setting of function fields, of a theorem (Chen-Kitaoka-Yu, Roskam) on the distribution of fundamental units modulo primes. Received: 16 October 2000 / Published online: 2 December 2002 Research partially supported by National Science Council, Rep. of China.     

Corrected collocation methods for periodic pseudodifferential equations

Summary. We propose an approximation method for periodic pseudodifferential equations, which yields higher convergence rates in Sobolev spaces with negative order than the collocation method. The main idea consists in correcting the usual collocation solution in a certain way by the solution of a small Galerkin system for the same equation. Both trigonometric and spline approximation methods are considered. In most of the cases our convergence result even improves that of the qualocation method. Received January 3, 1994 / Revised version received August 17, 1994     

Fully discrete spectral boundary integral methods for Helmholtz problems on smooth closed surfaces in ${\mathbb R}^3$

Summary. In this paper we describe and analyse a class of spectral methods, based on spherical polynomial approximation, for second-kind weakly singular boundary integral equations arising from the Helmholtz equation on smooth closed 3D surfaces diffeomorphic to the sphere. Our methods are fully discrete Galerkin methods, based on the application of special quadrature rules for computing the outer and inner integrals arising in the Galerkin matrix entries. For the outer integrals we use, for example, product-Gauss rules. For the inner integrals, a variant of the classical product integration procedure is employed to remove the singularity arising in the kernel. The key to the analysis is a recent result of Sloan and Womersley on the norm of discrete orthogonal projection operators on the sphere. We prove that our methods are stable for continuous data and superalgebraically convergent for smooth data. Our theory includes as a special case a method closely related to one of those proposed by Wienert (1990) for the fast computation of direct and inverse acoustic scattering in 3D. Received May 29, 2000 / Revised version received March 26, 2001/ Published online December 18, 2001     

Initial layers of Zp-extensions and Greenberg's conjecture

Let p be a prime number, K a finite abelian extension of Q containing p-th roots of unity and K _n the n-th layer of the cyclotomic Z _p -extension of K. Under some conditions we construct an element of K _n from an ideal class of the maximal real subfield of K _n . We determine whether its p-th root is contained by some Z _p -extension of K _n or not for each n, using the zero of p-adic L-function and the order of the ideal class group of the maximal real subfield of K _m for sufficiently large m. Received: 13 February 1998 / Revised version: 30 September 1998     

Relèvement global d'extensions locales: quelques problèmes de plongement

Résumé. Soit L/K une extension galoisienne finie de corps p-adiques, et soit F un corps de nombres totalement réel, dont le complété en une place v est isomorphe à {it K}. Si p est impair, nous montrons qu'il existe une extension galoisienne finie E de F, totalement réelle, de même degré que L sur K, et dont le complété en v est isomorphe àL ; quand p vaut 2, nous prouvons une version affaiblie. Ces résultats interviennent dans une preuve des conjectures de Langlands pour sur les corps p-adiques. Received March 27, 2000 / Revised May 8, 2000 / Published online December 8, 2000     

The associated primes of local cohomology modules over rings of small dimension

Let R be a commutative Noetherian local ring of dimension d, I an ideal of R, and M a finitely generated R-module. We prove that the set of associated primes of the local cohomology module H ⁱ _I (M) is finite for all i≥ 0 in the following cases: (1) d≤ 3; (2) d= 4 and $R$ is regular on the punctured spectrum; (3) d= 5, R is an unramified regular local ring, and M is torsion-free. In addition, if $d>0$ then H ^{d − 1} _I (M) has finite support for arbitrary R, I, and M. Received: 31 October 2000 / Revised version: 8 January 2001     

Semiconvergence of nonnegative splittings for singular matrices

Summary. In this paper, we discuss semiconvergence of the matrix splitting methods for solving singular linear systems. The concepts that a splitting of a matrix is regular or nonnegative are generalized and we introduce the terminologies that a splitting is quasi-regular or quasi-nonnegative. The equivalent conditions for the semiconvergence are proved. Comparison theorem on convergence factors for two different quasi-nonnegative splittings is presented. As an application, the semiconvergence of the power method for solving the Markov chain is derived. The monotone convergence of the quasi-nonnegative splittings is proved. That is, for some initial guess, the iterative sequence generated by the iterative method introduced by a quasi-nonnegative splitting converges towards a solution of the system from below or from above. Received August 19, 1997 / Revised version received August 20, 1998 / Published online January 27, 2000     

Multiplicities of mod p Galois representations

We study the multiplicity with which 2-dimensional mod p Galois representations occur in Jacobians of modular curves. Received: 26 February 1997 / Revised version: 2 July 1997     

Ramification de certaines extensions de Lie p-adiques

Let G be a p-adic Lie group and let K be a finite extension of the p-adic number field ℚ _p . There are finitely many filtrations of G which could be ramification filtrations of totally ramified Galois extensions of K with Galois group G. Received: 19 October 1998     

Galois modular representation of associated Jacobians in the tamely ramified cyclic case

Let L/K be an ℓ-cyclic extension with Galois group G of algebraic function fields over an algebraically closed field k of characteristic p ≠  ℓ. In this paper, the -module structure of the ℓ-torsion of the Jacobian associated to L is explicitly determined.     

Exponents of the ideal class groups of CM number fields

Since class numbers of CM number fields of a given degree go to infinity with the absolute values of their discriminants, it is reasonable to ask whether the same conclusion still holds true for the exponents of their ideal class groups. We prove that under the assumption of the Generalized Riemann Hypothesis this is indeed the case. Received: 8 May 2001; in final form: 15 April 2002/Published online: 8 November 2002     

Théorie d'Iwasawa des corps p-rationnels et p-birationnels

We study Iwasawa theory for p-rational and p-birational fields. A classical invariant characterises them and, in the case of CM-fields, this gives an explicit characterisation. We show how to compute those fields and and give numerical examples for small degrees.

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Received: 20 May 1997 / Revised version: 9 April 1998     

On Finite Pseudorandom Binary Sequences, V.On (nα) and (n 2α) Sequences

 Let k be a positive integer and α be a real number, and for if the fractional part of is <1/2 and e _n =−1 if it is ≥1/2. The pseudorandom properties of the sequence are studied. As measures of pseudorandomness, the regularity of the distribution relative to arithmetic progressions and the correlation are used. Here the special cases k=1 and k=2 are studied (while the case k>2 will be studied in the sequel). (Received 23 April 1999)     

Sums and Differences of Two Cubic Polynomials

 When f(x) is a cubic polynomial with integral coefficients, we show that almost all integers represented as the sum or difference of two values of f(x), with , are thus represented essentially uniquely. (Received 18 January 1999; in revised form 17 May 1999)