Volume 2011, Issue 658

Let M be a closed orientable surface of negative curvature. A connection is said to be transparent if its parallel transport along closed geodesics is the identity. We describe all transparent SU(2)-connections and we show that they can be built up from suitable Bäcklund transformations.

The purpose of the paper is to complete several global and local results concerning parity of ranks of elliptic curves. Primarily, we show that the Shafarevich–Tate conjecture implies the parity conjecture for all elliptic curves over number fields, we give a formula for local and global root numbers of elliptic curves and complete the proof of a conjecture of Kramer and Tunnell in characteristic 0. The method is to settle the outstanding local formulae by deforming from local fields to totally real number fields and then using global parity results.

By means of a quantitative version of the Schmidt Subspace Theorem, we obtain irrationality and trancendence measures for real numbers whose expansion in an integer base has a sublinear complexity. We further give several applications of our general results to Sturmian, automatic, and morphic numbers, and to lacunary series. In particular, we extend a theorem on Sturmian numbers established by Bundschuh in 1980. We also provide a first step towards a conjecture of Becker by proving that irrational automatic real numbers are either S - or T -numbers. This improves upon a recent result of Adamczewski and Cassaigne, who established that irrational automatic real numbers cannot be Liouville numbers.

We prove that the existence of log minimal models in dimension d essentially implies the LMMP with scaling in dimension d . As a consequence we prove that a weak nonvanishing conjecture in dimension d implies the minimal model conjecture in dimension d .

We study the effect on the zeros of generating functions of sequences under certain non-linear transformations. Characterizations of Pólya–Schur type are given of the transformations that preserve the property of having only real and non-positive zeros. In particular, if a polynomial a 0 + a 1 z + ⋯ + a n z n has only real and non-positive zeros, then so does the polynomial . This confirms a conjecture of Fisk, McNamara–Sagan and Stanley, respectively. A consequence is that if a polynomial has only real and non-positive zeros, then its Taylor coefficients form an infinitely log-concave sequence. We extend the results to transcendental entire functions in the Laguerre–Pólya class, and discuss the consequences to problems on iterated Turán inequalities, studied by Craven and Csordas. Finally, we propose a new approach to a conjecture of Boros and Moll.

There are a lot of arithmetic consequences if a Galois group of a number field is of cohomological dimension ≦ 2 (cf. [Schmidt, J. reine angew. Math. 596: 115–130, 2006], [Schmidt, Doc. Math. 12: 441–471, 2007], [Schmidt, J. reine angew. Math. 640: 203–235, 2010]). But with class field theory we only have an approximate description of the relators of such groups, which makes it difficult to determine the cohomological dimension. There are several criteria (cf. [Labute, Math. 596: 155–182, 2006], [Labute and Mináč, Mild pro-2-groups and 2-extensions of ℚ with restricted ramification, 2009]) on the so called linking numbers to get cd ≦ 2. The techniques in these papers use Lie algebra theory which become much more complicated for pro-2-groups. Here we will give a more simple and direct proof of the same algebraic criteria for a pro- p -group to be of cd ≦ 2 including the case p = 2.

The word ‘double’ was used by Ehresmann to mean ‘an object X in the category of all X ’. Double categories, double groupoids and double vector bundles are instances, but the notion of Lie algebroid cannot readily be doubled in the Ehresmann sense, since a Lie algebroid bracket cannot be defined diagrammatically. In this paper we use the duality of double vector bundles to define a notion of double Lie algebroid, and we show that this abstracts the infinitesimal structure (at second order) of a double Lie groupoid. We further show that the cotangent of either Lie algebroid in a Lie bialgebroid has a double Lie algebroid structure, that reduces, in the case of a Lie bialgebra, to the classical Drinfel'd double. Thus one may say that the Drinfel'd double of a Lie bialgebroid is an Ehresmann double, and it follows that double Lie groupoids provide global models for the cotangent double of a Lie bialgebroid. A double vector bundle is called vacant if it is constructed from vector bundles A, B on a common base M as the simultaneous pullback A × M B . Given a matched pair ( A, B ) of Lie algebroids over base M , we show that A × M B has a double Lie algebroid structure, and that any double Lie algebroid structure on a vacant double vector bundle A × M B arises in this way. In particular, double Lie algebras in the sense of Lu and Weinstein, Kosmann-Schwarzbach and Magri, and Majid, have the structure of a vacant double Lie algebroid. Lastly we extend the construction of Lu (Duke Math. J. 86: 261–304, 1997) which associates a matched pair of Lie algebroids to any Poisson group action, to actions of Lie bialgebroids; this yields a double Lie algebroid which in general does not correspond to a matched pair. The methods of the paper are entirely ‘classical’ rather than utilizing super techniques.