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201. CJM 1998 (vol 50 pp. 1105)

Roberts, Brooks
Tempered representations and the theta correspondence
Let $V$ be an even dimensional nondegenerate symmetric bilinear space over a nonarchimedean local field $F$ of characteristic zero, and let $n$ be a nonnegative integer. Suppose that $\sigma \in \Irr \bigl(\OO (V)\bigr)$ and $\pi \in \Irr \bigl(\Sp (n,F)\bigr)$ correspond under the theta correspondence. Assuming that $\sigma$ is tempered, we investigate the problem of determining the Langlands quotient data for $\pi$.

Categories:11F27, 22E50

202. CJM 1998 (vol 50 pp. 1007)

Elder, G. Griffith
Galois module structure of ambiguous ideals in biquadratic extensions
Let $N/K$ be a biquadratic extension of algebraic number fields, and $G=\Gal (N/K)$. Under a weak restriction on the ramification filtration associated with each prime of $K$ above $2$, we explicitly describe the $\bZ[G]$-module structure of each ambiguous ideal of $N$. We find under this restriction that in the representation of each ambiguous ideal as a $\bZ[G]$-module, the exponent (or multiplicity) of each indecomposable module is determined by the invariants of ramification, alone. For a given group, $G$, define ${\cal S}_G$ to be the set of indecomposable $\bZ[G]$-modules, ${\cal M}$, such that there is an extension, $N/K$, for which $G\cong\Gal (N/K)$, and ${\cal M}$ is a $\bZ[G]$-module summand of an ambiguous ideal of $N$. Can ${\cal S}_G$ ever be infinite? In this paper we answer this question of Chinburg in the affirmative.

Keywords:Galois module structure, wild ramification
Categories:11R33, 11S15, 20C32

203. CJM 1998 (vol 50 pp. 794)

Louboutin, Stéphane
Upper bounds on $|L(1,\chi)|$ and applications
We give upper bounds on the modulus of the values at $s=1$ of Artin $L$-functions of abelian extensions unramified at all the infinite places. We also explain how we can compute better upper bounds and explain how useful such computed bounds are when dealing with class number problems for $\CM$-fields. For example, we will reduce the determination of all the non-abelian normal $\CM$-fields of degree $24$ with Galois group $\SL_2(F_3)$ (the special linear group over the finite field with three elements) which have class number one to the computation of the class numbers of $23$ such $\CM$-fields.

Keywords:Dedekind zeta function, Dirichlet series, $\CM$-field, relative class number
Categories:11M20, 11R42, 11Y35, 11R29

204. CJM 1998 (vol 50 pp. 465)

Balog, Antal
Six primes and an almost prime in four linear equations
There are infinitely many triplets of primes $p,q,r$ such that the arithmetic means of any two of them, ${p+q\over2}$, ${p+r\over2}$, ${q+r\over2}$ are also primes. We give an asymptotic formula for the number of such triplets up to a limit. The more involved problem of asking that in addition to the above the arithmetic mean of all three of them, ${p+q+r\over3}$ is also prime seems to be out of reach. We show by combining the Hardy-Littlewood method with the sieve method that there are quite a few triplets for which six of the seven entries are primes and the last is almost prime.}

Categories:11P32, 11N36

205. CJM 1998 (vol 50 pp. 563)

Goldston, D. A.; Yildirim, C. Y.
Primes in short segments of arithmetic progressions
Consider the variance for the number of primes that are both in the interval $[y,y+h]$ for $y \in [x,2x]$ and in an arithmetic progression of modulus $q$. We study the total variance obtained by adding these variances over all the reduced residue classes modulo $q$. Assuming a strong form of the twin prime conjecture and the Riemann Hypothesis one can obtain an asymptotic formula for the total variance in the range when $1 \leq h/q \leq x^{1/2-\epsilon}$, for any $\epsilon >0$. We show that one can still obtain some weaker asymptotic results assuming the Generalized Riemann Hypothesis (GRH) in place of the twin prime conjecture. In their simplest form, our results are that on GRH the same asymptotic formula obtained with the twin prime conjecture is true for ``almost all'' $q$ in the range $1 \leq h/q \leq h^{1/4-\epsilon}$, that on averaging over $q$ one obtains an asymptotic formula in the extended range $1 \leq h/q \leq h^{1/2-\epsilon}$, and that there are lower bounds with the correct order of magnitude for all $q$ in the range $1 \leq h/q \leq x^{1/3-\epsilon}$.

Category:11M26

206. CJM 1998 (vol 50 pp. 412)

McIntosh, Richard J.
Asymptotic transformations of $q$-series
For the $q$-series $\sum_{n=0}^\infty a^nq^{bn^2+cn}/(q)_n$ we construct a companion $q$-series such that the asymptotic expansions of their logarithms as $q\to 1^{\scriptscriptstyle -}$ differ only in the dominant few terms. The asymptotic expansion of their quotient then has a simple closed form; this gives rise to a new $q$-hypergeometric identity. We give an asymptotic expansion of a general class of $q$-series containing some of Ramanujan's mock theta functions and Selberg's identities.

Categories:11B65, 33D10, 34E05, 41A60

207. CJM 1998 (vol 50 pp. 74)

Flicker, Yuval Z.
Elementary proof of the fundamental lemma for a unitary group
The fundamental lemma in the theory of automorphic forms is proven for the (quasi-split) unitary group $U(3)$ in three variables associated with a quadratic extension of $p$-adic fields, and its endoscopic group $U(2)$, by means of a new, elementary technique. This lemma is a prerequisite for an application of the trace formula to classify the automorphic and admissible representations of $U(3)$ in terms of those of $U(2)$ and base change to $\GL(3)$. It compares the (unstable) orbital integral of the characteristic function of the standard maximal compact subgroup $K$ of $U(3)$ at a regular element (whose centralizer $T$ is a torus), with an analogous (stable) orbital integral on the endoscopic group $U(2)$. The technique is based on computing the sum over the double coset space $T\bs G/K$ which describes the integral, by means of an intermediate double coset space $H\bs G/K$ for a subgroup $H$ of $G=U(3)$ containing $T$. Such an argument originates from Weissauer's work on the symplectic group. The lemma is proven for both ramified and unramified regular elements, for which endoscopy occurs (the stable conjugacy class is not a single orbit).

Categories:22E35, 11F70, 11F85, 11S37

208. CJM 1997 (vol 49 pp. 1265)

Snaith, V. P.
Hecke algebras and class-group invariant
Let $G$ be a finite group. To a set of subgroups of order two we associate a $\mod 2$ Hecke algebra and construct a homomorphism, $\psi$, from its units to the class-group of ${\bf Z}[G]$. We show that this homomorphism takes values in the subgroup, $D({\bf Z}[G])$. Alternative constructions of Chinburg invariants arising from the Galois module structure of higher-dimensional algebraic $K$-groups of rings of algebraic integers often differ by elements in the image of $\psi$. As an application we show that two such constructions coincide.

Categories:16S34, 19A99, 11R65

209. CJM 1997 (vol 49 pp. 1139)

Kraus, Alain
Majorations effectives pour l'équation de Fermat généralisée
Soient $A$, $B$ et $C$ trois entiers non nuls premiers entre eux deux \`a deux, et $p$ un nombre premier. Comme cons\'equence des travaux de A. Wiles et F. Diamond sur la conjecture de Taniyama-Weil, on explicite une constante $f(A,B,C)$ telle que, sous certaines conditions portant sur $A$, $B$ et $C$, l'\'equation $Ax^p+By^p+Cz^p=0$ n'a aucune solution non triviale dans $\Z$, si $p$ est $>f(A,B,C)$. On d\'emontre par ailleurs, sans condition suppl\'ementaire portant sur $A$, $B$ et $C$, que cette \'equation n'a aucune solution non triviale dans $\Z$, si $p$ divise $xyz$, et si $p$ est $>f(A,B,C)$.

Category:11G

210. CJM 1997 (vol 49 pp. 887)

Borwein, Peter; Pinner, Christopher
Polynomials with $\{ 0, +1, -1\}$ coefficients and a root close to a given point
For a fixed algebraic number $\alpha$ we discuss how closely $\alpha$ can be approximated by a root of a $\{0,+1,-1\}$ polynomial of given degree. We show that the worst rate of approximation tends to occur for roots of unity, particularly those of small degree. For roots of unity these bounds depend on the order of vanishing, $k$, of the polynomial at $\alpha$. In particular we obtain the following. Let ${\cal B}_{N}$ denote the set of roots of all $\{0,+1,-1\}$ polynomials of degree at most $N$ and ${\cal B}_{N}(\alpha,k)$ the roots of those polynomials that have a root of order at most $k$ at $\alpha$. For a Pisot number $\alpha$ in $(1,2]$ we show that \[ \min_{\beta \in {\cal B}_{N}\setminus \{ \alpha \}} |\alpha -\beta| \asymp \frac{1}{\alpha^{N}}, \] and for a root of unity $\alpha$ that \[ \min_{\beta \in {\cal B}_{N}(\alpha,k)\setminus \{\alpha\}} |\alpha -\beta|\asymp \frac{1}{N^{(k+1) \left\lceil \frac{1}{2}\phi (d)\right\rceil +1}}. \] We study in detail the case of $\alpha=1$, where, by far, the best approximations are real. We give fairly precise bounds on the closest real root to 1. When $k=0$ or 1 we can describe the extremal polynomials explicitly.

Keywords:Mahler measure, zero one polynomials, Pisot numbers, root separation
Categories:11J68, 30C10

211. CJM 1997 (vol 49 pp. 641)

Burris, Stanley; Compton, Kevin; Odlyzko, Andrew; Richmond, Bruce
Fine spectra and limit laws II First-order 0--1 laws.
Using Feferman-Vaught techniques a condition on the fine spectrum of an admissible class of structures is found which leads to a first-order 0--1 law. The condition presented is best possible in the sense that if it is violated then one can find an admissible class with the same fine spectrum which does not have a first-order 0--1 law. If the condition is satisfied (and hence we have a first-order %% 0--1 law)

Categories:03N45, 11N45, 11N80, 05A15, 05A16, 11M41, 11P81

212. CJM 1997 (vol 49 pp. 722)

Elder, G. Griffith; Madan, Manohar L.
Galois module structure of the integers in wildly ramified $C_p\times C_p$ extensions
Let $L/K$ be a finite Galois extension of local fields which are finite extensions of $\bQ_p$, the field of $p$-adic numbers. Let $\Gal (L/K)=G$, and $\euO_L$ and $\bZ_p$ be the rings of integers in $L$ and $\bQ_p$, respectively. And let $\euP_L$ denote the maximal ideal of $\euO_L$. We determine, explicitly in terms of specific indecomposable $\bZ_p[G]$-modules, the $\bZ_p[G]$-module structure of $\euO_L$ and $\euP_L$, for $L$, a composite of two arithmetically disjoint, ramified cyclic extensions of $K$, one of which is only weakly ramified in the sense of Erez \cite{erez}.

Keywords:Galois module structure---integral representation.
Categories:11S15, 20C32

213. CJM 1997 (vol 49 pp. 749)

Howe, Lawrence
Twisted Hasse-Weil $L$-functions and the rank of Mordell-Weil groups
Following a method outlined by Greenberg, root number computations give a conjectural lower bound for the ranks of certain Mordell-Weil groups of elliptic curves. More specifically, for $\PQ_{n}$ a \pgl{{\bf Z}/p^{n}{\bf Z}}-extension of ${\bf Q}$ and $E$ an elliptic curve over {\bf Q}, define the motive $E \otimes \rho$, where $\rho$ is any irreducible representation of $\Gal (\PQ_{n}/{\bf Q})$. Under some restrictions, the root number in the conjectural functional equation for the $L$-function of $E \otimes \rho$ is easily computible, and a `$-1$' implies, by the Birch and Swinnerton-Dyer conjecture, that $\rho$ is found in $E(\PQ_{n}) \otimes {\bf C}$. Summing the dimensions of such $\rho$ gives a conjectural lower bound of $$ p^{2n} - p^{2n - 1} - p - 1 $$ for the rank of $E(\PQ_{n})$.

Categories:11G05, 14G10

214. CJM 1997 (vol 49 pp. 468)

Burris, Stanley; Sárközy, András
Fine spectra and limit laws I. First-order laws
Using Feferman-Vaught techniques we show a certain property of the fine spectrum of an admissible class of structures leads to a first-order law. The condition presented is best possible in the sense that if it is violated then one can find an admissible class with the same fine spectrum which does not have a first-order law. We present three conditions for verifying that the above property actually holds. The first condition is that the count function of an admissible class has regular variation with a certain uniformity of convergence. This applies to a wide range of admissible classes, including those satisfying Knopfmacher's Axiom A, and those satisfying Bateman and Diamond's condition. The second condition is similar to the first condition, but designed to handle the discrete case, {\it i.e.}, when the sizes of the structures in an admissible class $K$ are all powers of a single integer. It applies when either the class of indecomposables or the whole class satisfies Knopfmacher's Axiom A$^\#$. The third condition is also for the discrete case, when there is a uniform bound on the number of $K$-indecomposables of any given size.

Keywords:First order limit laws, generalized number theory
Categories:O3C13, 11N45, 11N80, 05A15, 05A16, 11M41, 11P81

215. CJM 1997 (vol 49 pp. 499)

Fitzgerald, Robert W.
Gorenstein Witt rings II
The abstract Witt rings which are Gorenstein have been classified when the dimension is one and the classification problem for those of dimension zero has been reduced to the case of socle degree three. Here we classifiy the Gorenstein Witt rings of fields with dimension zero and socle degree three. They are of elementary type.

Categories:11E81, 13H10

216. CJM 1997 (vol 49 pp. 283)

McCall, Thomas M.; Parry, Charles J.; Ranalli, Ramona R.
The $2$-rank of the class group of imaginary bicyclic biquadratic fields
A formula is obtained for the rank of the $2$-Sylow subgroup of the ideal class group of imaginary bicyclic biquadratic fields. This formula involves the number of primes that ramify in the field, the ranks of the $2$-Sylow subgroups of the ideal class groups of the quadratic subfields and the rank of a $Z_2$-matrix determined by Legendre symbols involving pairs of ramified primes. As applications, all subfields with both $2$-class and class group $Z_2 \times Z_2$ are determined. The final results assume the completeness of D.~A.~Buell's list of imaginary fields with small class numbers.

Categories:11R16, 11R29, 11R20

217. CJM 1997 (vol 49 pp. 405)

Vulakh, L. Ya.
On Hurwitz constants for Fuchsian groups
Explicit bounds for the Hurwitz constants for general cofinite Fuchsian groups have been found. It is shown that the bounds obtained are exact for the Hecke groups and triangular groups with signature $(0:2,p,q)$.

Categories:11J04, 20H10
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