On a Refined Stark Conjecture for Function Fields

We prove that a refinement of Stark's Conjecture formulated by Rubin in Ann. Inst Fourier 4 (1996) is true up to primes dividing the order of the Galois group, for finite, Abelian extensions of function fields over finite fields. We also show that in the case of constant field extensions, a statement stronger than Rubin's holds true.     

On Normal Bases for Finite Commutative Rings

This paper is devoted to the introduction of extension rings S : = R[x]/gR[x] with a suitable polynomial g ? R[x] of arbitrary commutative rings R with identity and to the development of a normal basis concept of S over R, which is similar to that of Galois extensions of finite fields. We prove new results for Galois extensions of local rings and apply them together with the Chinese remainder theorem to solve the above task in a constructive way.     

CYCLOTOMIC ELEMENTS IN K_2F,REVISITED

Basing on results of Xu and Qin [10], and Guo [12] on cyclotomic elements in K2F for local fields F, we prove that every element in K2Q is a finite or infinite product of cyclotomic elements. Next, we extend this result to finite extensions of Q satisfying some additional conditions.     

Finite descent obstruction for curves over function fields

We prove that a form of finite Galois descent obstruction is the only obstruction to the existence of integral points on integral models of twists of modular curves over function fields.     

Analysis and Prbability over Infinite Extensions of a Local Field

We consider an infinite extension K of a local field of zero characteristic which is a union of an increasing sequence of finite extensions. K is equipped with an inductive limit topology; its conjugate K; is a completion of K with respect to a topology given by certain explicitly written semi-norms. We construct and study a Gaussian measure, a Fourier transform, a fractional differentiation operator and a cadlag Markov process on K. If we deal with Galois extensions then all these objects are Galois-invariant.     

Projective Extensions of Fields

Every field K admits proper projective extensions, that is,Galois extensions where the Galois group is a non-trivial projectivegroup, unless K is separably closed or K is a pythagorean formallyreal field without cyclic extensions of odd degree. As a consequence,it turns out that almost all absolute Galois groups decomposeas proper semidirect products. We show that each local field has a unique maximal projectiveextension, and that the same holds for each global field ofpositive characteristic. In characteristic 0, we prove thatLeopoldt's conjecture for all totally real number fields isequivalent to the statement that, for all totally real numberfields, all projective extensions are cyclotomic. So the realizabilityof any non-procyclic projective group as Galois group over Qproduces counterexamples to the Leopoldt conjecture.     

Congruences between derivatives of geometric <Emphasis Type="Italic">L</Emphasis>-functions

We prove a natural refinement of a theorem of Lichtenbaum describing the leading terms of Zeta functions of curves over finite fields in terms of Weil-étale cohomology. We then use this result to prove the validity of Chinburg’s Ω(3)-Conjecture for all abelian extensions of global function fields, to prove natural refinements and generalisations of the refined Stark conjectures formulated by, amongst others, Gross, Tate, Rubin and Popescu, to prove a variety of explicit restrictions on the Galois module structure of unit groups and divisor class groups and to describe explicitly the Fitting ideals of certain Weil-étale cohomology groups. In an Appendix coauthored with K.F. Lai and K.-S. Tan we also show that the main conjectures of geometric Iwasawa theory can be proved without using either crystalline cohomology or Drinfeld modules.     

Cohomology and the normal basis theorem

We use an inverse limit version of Hilbert's theorem 90 to obtain a cohomological proof, based on Galois descent, of Lenstra's version of the normal basis theorem for infinite Galois field extensions.     

Galois theory for bialgebroids, depth two and normal Hopf subalgebras

We reduce certain proofs in [16, 11, 12] to depth two quasibases from one side only, a minimalistic approach which leads to a characterization of Galois extensions for finite projective bialgebroids without the Frobenius extension property. We prove that a proper algebra extension is a leftT-Galois extension for some right finite projective left bialgebroid over some algebraR if and only if it is a left depth two and left balanced extension. Exchanging left and right in this statement, we have a characterization of right Galois extensions for left finite projective right bialgebroids. Looking to examples of depth two, we establish that a Hopf subalgebra is normal if and only if it is a Hopf-Galois extension. We characterize finite weak Hopf-Galois extensions using an alternate Galois canonical mapping with several corollaries: that these are depth two and that surjectivity of the Galois mapping implies its bijectivity.

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Sunto Riduciamo alcune prove di [16,11,12] a quasibasi di profondità due da un lato solo, un approccio minimalistico che conduce ad una caratterizzazione di estensioni di Galois per bialgebroidi proietivi finiti senza la proprietà di estensione di Frobenius. Dimostriamo che un'algebra che sia un'estensione propria è un'estensioneT-Galois sinistra per qualche bialgebroide finito proiettivo a sinistra su qualche algebraR se, e solo se, è un'estensione di profondità due a sinistra e bilanciata a sinistra. Scambiando destra e sinistra nell'enunciato, otteniamo una caratterizzazione di estensioni di Galois destre per bialgebroidi finiti proiettivi a destra. Guardando ad esempi di profondità due, otteniamo che una sottoalgebra di Hopf è normale se, e solo se, è un'estensione Hopf-Galois. Caratterizziamo le estensioni Hopf-Galois deboli finite usando un'applicazione canonica di Galois alternativa ottenendo parecchi corollari: queste sono di profondità due e la suriettività dell'applicazione di Galois implica la sua biiettività.

     

The amenability and non-amenability of skew fields

We investigate the amenability of skew field extensions of the complex numbers. We prove that all skew fields of finite Gelfand-Kirillov transcendence degree are amenable. However there are both amenable and non-amenable finitely generated skew fields of infinite Gelfand-Kirillov transcendence degree.

     

Prehomogeneous vector spaces and field extensions

Summary Letk be an infinite field of characteristic not equal to 2, 3, 5. In this paper, we construct a natural map from the set of orbits of certain prehomogeneous vector spaces to the set of isomorphism classes of Galois extensions ofk which are splitting fields of equations of certain degrees, and prove that the inverse image of this map corresponds bijectively with conjugacy classes of Galois homomorphisms.Oblatum 24-I-1992 & 23-IV-1992Both authors are supported by NSF Grant DMS-8803085, DMS-9101091; The first author was partially supported by NSA grant MDA904-91-H-0041     

Arithmetik in Frobeniuserweiterungen

In partly generalization of work of H.Hasse and J.Porusch finite Galois extensions of local and number fields with Frobeniusgroups as Galoisgroups are studied in respect to discriminant-and L-function-relations, conductors, residue class degree and ramification index.     

Absolute integral closure in positive characteristic

Let R be a local Noetherian domain of positive characteristic. A theorem of Hochster and Huneke [M. Hochster, C. Huneke, Infinite integral extensions and big Cohen–Macaulay algebras, Ann. of Math. 135 (1992) 53–89] states that if R is excellent, then the absolute integral closure of R is a big Cohen–Macaulay algebra. We prove that if R is the homomorphic image of a Gorenstein local ring, then all the local cohomology (below the dimension) of such a ring maps to zero in a finite extension of the ring. As a result there follow an extension of the original result of Hochster and Huneke to the case in which R is a homomorphic image of a Gorenstein local ring, and a considerably simpler proof of this result in the cases where the assumptions overlap, e.g., for complete Noetherian local domains.     

Integrating onp-adic Lie groups

Inspired by the classical Mahler measure of a polynomial, we study the integral of the order of an arithmetic polynomial on a compactp-adic Lie group. A result of Denef and van den Dries guarantees this is always a rational number. Integrals of this kind arise naturally; for example, the local canonical height of a rational point on an elliptic curve is given by a Mahler measure. Also, the mean valuation of the normal integral generators in a finite Galois extension arises as a Mahler measure. There is interest in being able to calculate the value of this measure. We show that for some classical groups, it is possible to reduce the integral to a simpler form, one where explicit computations are feasible. The motivation comes from the calculus trick of integration by substitution, also from Weyl’s criterion. Applications are given to Galois Module Theory. Also, a close encounter with Leopoldt’s conjecture is recorded. We deduce our results on the Mahler measure from the more general setting of local zeta functions defined forp-adic Lie groups. Our techniques apply to certain zeta functions, so we state and prove our results at that level of generality in our main theorem. Thanks go to Steve Wilson, the SERC and the London Mathematical Society for the Durham Galois Modules Workshop, which inspired the results in §5. Thanks go to Alex Lubotzky and the Royal Society for making possible the visit of the second author to the Hebrew University in Jerusalem which lead to the zeta-function point of view in §1 and §2.     

Iwasawa theory of totally real fields for certain non-commutative p-extensions

In this paper, we will prove the non-commutative Iwasawa main conjecture—formulated by John Coates, Takako Fukaya, Kazuya Kato, Ramdorai Sujatha and Otmar Venjakob (2005)—for certain specific non-commutative p-adic Lie extensions of totally real fields by using theory on integral logarithms introduced by Robert Oliver and Laurence R. Taylor, theory on Hilbert modular forms introduced by Pierre Deligne and Kenneth A. Ribet, and so on. Our results give certain generalization of the recent work of Kazuya Kato on the proof of the main conjecture for Galois extensions of Heisenberg type.     

Algebraic complexities and algebraic curves over finite fields

Algebraic schemes of computation of bilinear forms over various rings of scalars are examined. The problem of minimal complexity of these schemes is considered for computation of polynomial multiplication and multiplication in commutative algebras, and finite extensions of fields. While for infinite fields minimal complexities are known (Winograd, Fiduccia, Strassen), for finite fields precise minimal complexities are not yet determined. We prove lower and upper bounds on minimal complexities. Both are linear in the number of inputs. These results are obtained using the relationship with linear coding theory and the theory of algebraic curves over finite fields.     

Self-dual normal bases for infinite galois field extensions

Let L ? K be an infinite Galois field extension with the property that every finite Galois extension M ? K, where L ? M, has a self-dual normal basis. We prove a self-dual normal basis theorem for L ? K when char (K) ≠2.     

G-Closed fields and imbeddings of quadratic number fields

A field, K, that has no extensions with Galois group isomorphic to G is called G-closed. It is proved that a finite extension of K admits an infinite number of nonisomorphic extensions with Galois group G. A trinomial of degree n is exhibited with Galois group, the symmetric group of degree n, and with prescribed discriminant. This result is used to show that any quadratic extension of an A_n-closed field admits an extension with Galois group A_n.     

On self-dual normal bases

We deal with the existence of self-dual normal basis for Galois extensions of a commutative ring. We consider commutative rings which are local, connected semi-local (under some suitable restrictions) or zero-dimensional. We show that for such kind of rings every Galois extension of odd degree has a self-dual normal basis.     

Chebyshev?s bias in Galois extensions of global function fields

We study Chebyshev?s bias in a finite, possibly nonabelian, Galois extension of global function fields. We show that, when the extension is geometric and satisfies a certain property, called, Linear Independence (LI), the less square elements a conjugacy class of the Galois group has, the more primes there are whose Frobenius conjugacy classes are equal to the conjugacy class. Our results are in line with the previous work of Rubinstein and Sarnak in the number field case and that of the first-named author in the case of polynomial rings over finite fields. We also prove, under LI, the necessary and sufficient conditions for a certain limiting distribution to be symmetric, following the method of Rubinstein and Sarnak. Examples are provided where LI is proved to hold true and is violated. Also, we study the case when the Galois extension is a scalar field extension and describe the complete result of the prime number race in that case.