Showing papers on "Ring of integers published in 2010"

Ranks of twists of elliptic curves and Hilbert's tenth problem

Abstract: In this paper we investigate the 2-Selmer rank in families of quadratic twists of elliptic curves over arbitrary number fields. We give sufficient conditions on an elliptic curve so that it has twists of arbitrary 2-Selmer rank, and we give lower bounds for the number of twists (with bounded conductor) that have a given 2-Selmer rank. As a consequence, under appropriate hypotheses we can find many twists with trivial Mordell-Weil group, and (assuming the Shafarevich-Tate conjecture) many others with infinite cyclic Mordell-Weil group. Using work of Poonen and Shlapentokh, it follows from our results that if the Shafarevich-Tate conjecture holds, then Hilbert’s Tenth Problem has a negative answer over the ring of integers of every number field.

Semistable models for modular curves of arbitrary level

Abstract: We produce an integral model for the modular curve $X(Np^m)$ over the ring of integers of a sufficiently ramified extension of $\mathbf{Z}_p$ whose special fiber is a {\em semistable curve} in the sense that its only singularities are normal crossings. This is done by constructing a semistable covering (in the sense of Coleman) of the supersingular part of $X(Np^m)$, which is a union of copies of a Lubin-Tate curve. In doing so we tie together nonabelian Lubin-Tate theory to the representation-theoretic point of view afforded by Bushnell-Kutzko types. For our analysis it was essential to work with the Lubin-Tate curve not at level $p^m$ but rather at infinite level. We show that the infinite-level Lubin-Tate space (in arbitrary dimension, over an arbitrary nonarchimedean local field) has the structure of a perfectoid space, which is in many ways simpler than the Lubin-Tate spaces of finite level.

The Cohomology of Lattices in SL(2, ℂ)

Abstract: This paper contains both theoretical results and experimental data on the behavior of the dimensions of the cohomology spaces H 1(Γ,E n ), where Γ is a lattice in SL(2,ℂ) and , n ∈ ℕ ∪ {0}, is one of the standard self-dual modules In the case Γ = SL(2,O) for the ring of integers O in an imaginary quadratic number field, we make the theory of lifting explicit and obtain lower bounds linear in n We present a large amount of experimental data for this case, as well as for some geometrically constructed and mostly nonarithmetic groups The computations for SL(2,O) lead us to discover two instances with nonlifted classes in the cohomology We also derive an upper bound of size O(n 2/ log n) for any fixed lattice Γ in the general case We discuss a number of new questions and conjectures suggested by our results and our experimental data

La droite de Berkovich sur Z

Abstract: We study here the Berkovich line over the ring of integers of a number field. It is a natural object which contains complex and non-Archimedean analytic spaces associated to each place. We prove that this line satisfies good topological and algebraic properties and exhibit a few examples of Stein spaces that lie in it. We derive applications to the study of convergent arithmetic power series: choice of zeroes and poles, noetherianity of global rings and inverse Galois problem. Typical examples of such power series are given by analytic functions on the open complex unit disk whose Taylor development in 0 has integer coefficients.

Okutsu invariants and Newton polygons

Abstract: Let K be a local field of characteristic zero, O its ring of integers and F(x) a monic irreducible polynomial with coefficients in O. K. Okutsu attached to F(x) certain primitive divisor polynomials F_1(x),..., F_r(x), that are specially close to F(x) with respect to their degree. In this paper we characterize the Okutsu families [F_1,..., F_r] in terms of certain Newton polygons of higher order, and we derive some applications: closed formulas for certain Okutsu invariants, the discovery of new Okutsu invariants, or the construction of Montes approximations to F(x); these are monic irreducible polynomials sufficiently close to F(x) to share all its Okutsu invariants. This perspective widens the scope of applications of Montes' algorithm, which can be reinterpreted as a tool to compute the Okutsu polynomials and a Montes approximation, for each irreducible factor of a monic separable polynomial f(x) in O[x].

An even unimodular 72-dimensional lattice of minimum 8

Abstract: An even unimodular 72-dimensional lattice $\Gamma $ having minimum 8 is constructed as a tensor product of the Barnes lattice and the Leech lattice over the ring of integers in the imaginary quadratic number field with discriminant $-7$ The automorphism group of $\Gamma $ contains the absolutely irreducible rational matrix group $(\PSL_2(7) \times \SL _2(25)) : 2$

Simulation vs. Equivalence.

Abstract: For several semirings S, two weighted finite automata with multiplicities in S are equivalent if and only if they can be connected by a chain of simulations. Such a semiring S is called "proper". It is known that the Boolean semiring, the semiring of natural numbers, the ring of integers, all finite commutative positively ordered semirings and all fields are proper. The semiring S is Noetherian if every subsemimodule of a finitely generated S-semimodule is finitely generated. First, it is shown that all Noetherian semirings and thus all commutative rings and all finite semirings are proper. Second, the tropical semiring is shown not to be proper. So far there has not been any example of a semiring that is not proper.

Exterior Powers of Barsotti-Tate Groups

Abstract: Let $ \CO $ be the ring of integers of a non-Archimedean local field and $ \pi $ a fixed uniformizer of $ \CO $. We establish three main results. The first one states that the exterior powers of a $ \pi $-divisible $ \CO $-module scheme of dimension at most 1 over a field exist and commute with algebraic field extensions. The second one states that the exterior powers of a $p$-divisible group of dimension at most 1 over arbitrary base exist and commute with arbitrary base change. The third one states that when $ \CO $ has characteristic zero, then the exterior powers of $ \pi $-divisible groups with scalar $ \CO $-action and dimension at most 1 over a locally Noetherian base scheme exist and commute with arbitrary base change. We also calculate the height and dimension of the exterior powers in terms of the height of the given $p$-divisible group or $ \pi $-divisible $ \CO $-module scheme.

The big de Rham-Witt complex

Abstract: This paper gives a new and direct construction of the multi-prime big de Rham-Witt complex which is defined for every commutative and unital ring; the original construction by the author and Madsen relied on the adjoint functor theorem and accordingly was very indirect. (The construction given here also corrects the 2-torsion which was not quite correct in the original version.) The new construction is based on the theory of modules and derivations over a lambda-ring which is developed first. The main result in this first part of the paper is that the universal derivation of a lambda-ring is given by the universal derivation of the underlying ring together with an additional structure depending on the lambda-ring structure in question. In the case of the ring of big Witt vectors, this additional structure gives rise to divided Frobenius operators on the module of K\"ahler differentials. It is the existence of these divided Frobenius operators that makes the new construction of the big de Rham-Witt complex possible. It is further shown that the big de Rham-Witt complex behaves well with respect to \'etale maps, and finally, the big de Rham-Witt complex of the ring of integers is explicitly evaluated. The latter complex may be interpreted as the complex of differentials along the leaves of a foliation of Spec Z.

Density results for automorphic forms on Hilbert modular groups II

Abstract: We obtain an asymptotic formula for a weighted sum over cuspidal eigenvalues in a specific region, for SL2 over a totally real number field F, with a discrete subgroup of Hecke type Γ0(I) for a non-zero ideal I in the ring of integers of F. The weights are products of Fourier coefficients. This implies in particular the existence of infinitely many cuspidal automorphic representations with multi-eigenvalues in various regions growing to infinity. For instance, in the quadratic case, the regions include floating boxes, floating balls, sectors, slanted strips (see §1.2.4–1.2.13) and products of prescribed small intervals for all but one of the infinite places of F. The main tool in the derivation is a sum formula of Kuznetsov type (Sum formula for SL2 over a totally real number field, Theorem 2.1).

On badly approximable complex numbers

Abstract: We show that the set of complex numbers which are badly approximable by ratios of elements of the ring of integers in , where D ∈ {1, 2, 3, 7, 11, 19, 43, 67, 163} has maximal Hausdorff dimension. In addition, the intersection of these sets is shown to have maximal dimension. The results remain true when the sets in question are intersected with a suitably regular fractal set.

Modular symbols over number fields

Abstract: Let K be a number field, R its ring of integers. For some classes of fields, spaces of cusp forms of weight 2 for GL(2;K) have been computed using methods based on modular symbols. J.E. Cremona [9] began the programme of extending the classical methods over Q to the case of imaginary quadratic fields. This work was continued by some of his Ph.D. students [35, 6, 22], and results have been obtained for some imaginary quadratic fields with small class number. More recently, P. Gunnells and D. Yasaki [18] have developed related algorithms for real quadratic fields. The aim of this thesis is to contribute to the extension of the modular symbols method, when possible developing algorithms and implementations for effective computations. Some parts of the theory are purely algebraic and can be extended to all number fields. We generalise the theory for cusps and Manin symbols; we also describe a generalisation of Atkin-Lehner involutions and study other normaliser elements. On the other hand, all previous explicit computations for the imaginary quadratic field case were done only for specific fields. In the last part of this thesis we begin work towards a general implementation of the techniques used in this case. In particular, we are able to compute a fundamental domain of the hyperbolic 3-space for any imaginary quadratic field. Implementations of the algorithms described in this thesis have been written by the author in the open-source mathematics software Sage [31].

On rings of integers generated by their units

Abstract: We give an affirmative answer to the following question by Jarden and Narkiewicz: Is it true that every number field has a finite extension L such that the ring of integers of L is generated by its units (as a ring)? As a part of the proof, we generalize a theorem by Hinz on power-free values of polynomials in number fields.

An efficient seventh power residue symbol algorithm

Abstract: Power residue symbols and their reciprocity laws have applications not only in number theory, but also in other fields like cryptography. A crucial ingredient in certain public key cryptosystems is a fast algorithm for computing power residue symbols. Such algorithms have only been devised for the Jacobi symbol as well as for cubic and quintic power residue symbols, but for no higher powers. In this paper, we provide an efficient procedure for computing 7th power residue symbols. The method employs arithmetic in the field ℚ(ζ), with ζ a primitive 7th root of unity, and its ring of integers ℤ[ζ]. We give an explicit characterization for an element in ℤ[ζ] to be primary, and provide an algorithm for finding primary associates of integers in ℤ[ζ]. Moreover, we formulate explicit forms of the complementary laws to Kummer's 7th degree reciprocity law, and use Lenstra's norm-Euclidean algorithm in the cyclotomic field.

On the Iteration of a Function Related to Euler's φ-Function

Abstract: A unit x in a commutative ring R with identity is called exceptional if 1−x is also a unit in R. For any integer n ≥ 2, define φe(n) to be the number of exceptional units in the ring of integers modulo n. Following work of Shapiro, Mills, Catlin and Noe on iterations of Euler’s φ-function, we develop analogous results on iterations of the function φe, when restricted to a particular subset of the positive integers.

Nonbijective idempotents preservers over semirings

Abstract: We classify linear maps which preserve idempotents on n◊n matrices over some classes of semirings. Our results include many known semirings like the semiring of all nonnegative integers, the semiring of all nonnegative reals, any unital commutative ring, which is zero divisor free and of characteristic not two (not necessarily a principal ideal domain), and the ring of integers modulo m, where m is a product of distinct odd primes.

The basic geometry of Witt vectors, II: Spaces

Abstract: This is an account of the algebraic geometry of Witt vectors and arithmetic jet spaces. The usual, "p-typical" Witt vectors of p-adic schemes of finite type are already reasonably well understood. The main point here is to generalize this theory in two ways. We allow not just p-typical Witt vectors but those taken with respect to any set of primes in any ring of integers in any global field, for example. This includes the "big" Witt vectors. We also allow not just p-adic schemes of finite type but arbitrary algebraic spaces over the ring of integers in the global field. We give similar generalizations of Buium's formal arithmetic jet functor, which is dual to the Witt functor. We also give concrete geometric descriptions of Witt spaces and arithmetic jet spaces and investigate whether a number of standard geometric properties are preserved by these functors.

Hodge classes on certain hyperelliptic prymians

Abstract: Let $n=2g+2$ be a positive even integer, $f(x)$ a degree $n$ complex polynomial without multiple roots and $C_f: y^2=f(x)$ the corresponding genus $g$ hyperelliptic curve over the field $\C$ of complex numbers. Let a $(g-1)$-dimensional complex abelian variety $P$ be a Prym variety of $C_f$ that corresponds to a unramified double cover of $C_f$. Suppose that there exists a subfield $K$ of $\C$ such that $f(x)$ lies in $K[x]$, is irreducible over $K$ and its Galois group is the full symmetric group. Assuming that $g>2$, we prove that $End(P)$ is either the ring of integers $Z$ or the direct sum of two copies of $Z$; in addition, in both cases the Hodge group of $P$ is "as large as possible". In particular, the Hodge conjecture holds true for all self-products of $P$.

On nearby cycles and -modules of log schemes in characteristic p>0

Abstract: Let K be a complete discrete valuation field of mixed characteristic (0,p) with a perfect residue field k. For a semi-stable scheme over the ring of integers OK of K or, more generally, for a log smooth scheme of semi-stable type over k, we define nearby cycles as a single 𝒟-module endowed with a monodromy ∂logt, whose cohomology should give the log crystalline cohomology. We also explicitly describe the monodromy filtration of the 𝒟-module with respect to the endomorphism ∂logt, and construct a weight spectral sequence for the cohomology of the nearby cycles.

A simple group generated by involutions interchanging residue classes of the integers

Abstract: We present a countable simple group which arises in a natural way from the arithmetical structure of the ring of integers

Arithmetic Bounds-Lenstra"s Constant and Torsion of K-Groups

Abstract: This thesis is concerned with computations of bounds for two different arithmetic invariants. In both cases it is done with the intention of proving some algebraic or arithmetic properties for number fields. The first part is devoted to computations of lower bounds for the Lenstra's constant. For a number field K the Lenstra's constant is denoted Λ(K) and defined as the length of the largest exceptional sequence in K. An exceptional sequence is a set of units in K such that for any two among them their difference is a unit as well. H.W. Lenstra showed that if Λ(K) is large enough – bigger than a constant depending on the degree and the discriminant of K – then the ring of integers of K is Euclidean with respect to the norm. Using computer software PARI/GP and some algorithms from graph theory we construct exceptional sequences in number fields having a small discriminant. These exceptional sequences yield lower bounds for Lenstra's constant which are large enough to prove the existence of 42 new Euclidean number fields of degree 8 to 12. The aim of the second part of this thesis is proving upper bounds for the torsion part of the K-groups of a number field ring of integers. A method due to C. Soule yields bounds for the torsion of these K-groups depending on an invariant of hermitian lattices over number fields. Firstly we describe some properties of rank one hermitian lattices, especially of ideal lattices. Secondly we apply these properties to arbitrary rank hermitian lattices and this implies a significant improvement of the upper bounds for their invariants and accordingly for the torsion of K-groups. The progress mainly achieves much lower contributions of the number field attributes, particularly the degree and the absolute discriminant.

Ramication of truncated discrete valuation rings: a survey

Abstract: A truncated discrete valuation ring is a commutative ring which is isomorphic to a quotient of nite length of a complete discrete valuation ring. We give an equivalence between the category of at most a-ramied nite separable extensions of a complete discrete valuation eld K and the category of at most a-ramied nite extensions of the \lengtha truncation" of the integer ring of K. This extends a theorem of Deligne in which he proved this fact assuming the residue eld is perfect. Our theory depends heavily on Abbes-Saito’s ramication theory.

Recursive sequences and polynomial congruences

Abstract: We consider the periodicity of recursive sequences defined by linear homogeneous recurrence relations of arbitrary order, when they are reduced modulo a positive integer m. We show that the period of such a sequence with characteristic polynomial f can be expressed in terms of the order of!D xCh fi as a unit in the quotient ring ZmT!UDZmTxU=h fi. When mD p is prime, this order can be described in terms of the factorization of f in the polynomial ring Z pTxU. We use this connection to develop efficient algorithms for determining the factorization types of monic polynomials of degree k 5 in Z pTxU. This article grew out of an undergraduate research project, performed by the second author under the direction of the first, to determine if results about the periodicity of second-order linear homogeneous recurrence relations modulo positive integers could be extended to higher orders. We arrived, somewhat unexpectedly, at algorithms to determine the degrees of the irreducible factors of quintic and smaller degree polynomials modulo prime numbers. The algebraic properties of certain finite rings, particularly automorphisms of those rings, provided the connection between these two topics. To illustrate some of the ideas in this article, we begin with the famous example of the Fibonacci sequence, defined by FnD Fn 1C Fn 2 with F0D 0 and F1D 1. If, for some positive integer m, we replace each Fn by its remainder on division by m, we obtain a new sequence of integers. For example, the Fibonacci sequence modulo mD 10 begins 0; 1; 1; 2; 3; 5; 8; 3; 1; 4; 5; 9; 4; 3; 7; 0; 7; 7; 4; 1; 5; 6; 1; 7; 8; 5;:::; with the n-th term simply the last digit of Fn. We can also view such a sequence as having terms in ZmDZ=hmi, the ring of integers modulo m. This has the advantage

Steinitz classes of some abelian and nonabelian extensions of even degree

Abstract: The Steinitz class of a number field extension K/k is an ideal class in the ring of integers O_k of k, which, together with the degree [K:k] of the extension determines the O_k-module structure of O_K. We call R_t(k,G) the classes which are Steinitz classes of a tamely ramified G-extension of k. We will say that those classes are realizable for the group G; it is conjectured that the set of realizable classes is always a group. In this paper we will develop some of the ideas contained in arXiv:0910.5080 to obtain some results in the case of groups of even order. In particular we show that to study the realizable Steinitz classes for abelian groups, it is enough to consider the case of cyclic groups of 2-power degree.