Showing papers on "Divisor published in 2012"

The greatest Ricci lower bound, conical Einstein metrics and the Chern number inequality

Abstract: We partially confirm a conjecture of Donaldson relating the greatest Ricci lower bound $R(X)$ to the existence of conical Kahler-Einstein metrics on a Fano manifold $X$. In particular, if $D\in |-K_X|$ is a smooth simple divisor and the Mabuchi $K$-energy is bounded below, then there exists a unique conical Kahler-Einstein metric satisfying $Ric(g) = \beta g + (1-\beta) [D]$ for any $\beta \in (0,1)$. We also construct unique smooth conical toric Kahler-Einstein metrics with $\beta=R(X)$ and a unique effective Q-divisor $D\in [-K_X]$ for all toric Fano manifolds. Finally we prove a Miyaoka-Yau type inequality for Fano manifolds with $R(X)=1$.

On the Hidden Shifted Power Problem

Abstract: We consider the problem of recovering a hidden element $s$ of a finite field $\mathbb{F}_q$ of $q$ elements from queries to an oracle that for a given $x \in \mathbb{F}_q$ returns $(x+s)^e$ for a given divisor $e \mid q-1$. We use some techniques from additive combinatorics and analytic number theory that lead to more efficient algorithms than the naive interpolation algorithm; for example, they use substantially fewer queries to the oracle.

Asymptotically cylindrical Calabi-Yau manifolds

Abstract: Let $M$ be a complete Ricci-flat Kahler manifold with one end and assume that this end converges at an exponential rate to $[0,\infty) \times X$ for some compact connected Ricci-flat manifold $X$. We begin by proving general structure theorems for $M$; in particular we show that there is no loss of generality in assuming that $M$ is simply-connected and irreducible with Hol$(M)$ $=$ SU$(n)$, where $n$ is the complex dimension of $M$. If $n > 2$ we then show that there exists a projective orbifold $\bar{M}$ and a divisor $\bar{D}$ in $|{-K_{\bar{M}}}|$ with torsion normal bundle such that $M$ is biholomorphic to $\bar{M}\setminus\bar{D}$, thereby settling a long-standing question of Yau in the asymptotically cylindrical setting. We give examples where $\bar{M}$ is not smooth: the existence of such examples appears not to have been noticed previously. Conversely, for any such pair $(\bar{M}, \bar{D})$ we give a short and self-contained proof of the existence and uniqueness of exponentially asymptotically cylindrical Calabi-Yau metrics on $\bar{M}\setminus\bar{D}$.

Odd perfect numbers are greater than $10^{1500}$

Abstract: Brent, Cohen, and te Riele proved in 1991 that an odd perfect number N is greater than 10^300. We modify their method to obtain N > 10^1500. We also obtain that N has at least 101 not necessarily distinct prime factors and that its largest component (i.e. divisor p^a with p prime) is greater than 10^62.

Perfect Gaussian Integer Sequences of Odd Prime Length

Abstract: A Gaussian integer is a complex number whose real and imaginary parts are both integers. A Gaussian integer sequence is called perfect (odd perfect) if the out-of-phase values of the periodic (odd periodic) autocorrelation function are equal to zero. In this letter, for any odd prime p, using the cyclotomic classes of order 2 and 4 with respect to GF(p), we propose perfect and odd perfect Gaussian integer sequences of length p. Several examples are also given.

Conical Kahler-Einstein metric revisited

Abstract: In this paper we introduce the "interpolation-degneration" strategy to study Kahler-Einstein metrics on a smooth Fano manifold with cone singularities along a smooth divisor that is proportional to the anti-canonical divisor. By "interpolation" we show the angles in $(0, 2\pi]$ that admit a conical Kahler-Einstein metric form an interval; and by "degeneration" we figure out the boundary of the interval. As a first application, we show that there exists a Kahler-Einstein metric on $P^2$ with cone singularity along a smooth conic (degree 2) curve if and only if the angle is in $(\pi/2, 2\pi]$. When the angle is $2\pi/3$ this proves the existence of a Sasaki-Einstein metric on the link of a three dimensional $A_2$ singularity, and thus answers a problem posed by Gauntlett-Martelli-Sparks-Yau. As a second application we prove a version of Donaldson's conjecture about conical Kahler-Einstein metrics in the toric case using Song-Wang's recent existence result of toric invariant conical Kahler-Einstein metrics.

Inoue type manifolds and Inoue surfaces: a connected component of the moduli space of surfaces with K^2 = 7, p_g=0

Abstract: We show that a family of minimal surfaces of general type with p_g = 0, K^2=7, constructed by Inoue in 1994, is indeed a connected component of the moduli space: indeed that any surface which is homotopically equivalent to an Inoue surface belongs to the Inoue family. The ideas used in order to show this result motivate us to give a new definition of varieties, which we propose to call Inoue-type manifolds: these are obtained as quotients \hat{X} / G, where \hat{X} is an ample divisor in a K(\Gamma, 1) projective manifold Z, and G is a finite group acting freely on \hat{X} . For these type of manifolds we prove a similar theorem to the above, even if weaker, that manifolds homotopically equivalent to Inoue-type manifolds are again Inoue-type manifolds.

G-flux and spectral divisors

Abstract: We propose a construction of G-flux in singular elliptic Calabi-Yau fourfold compactifications of F-theory, which in the local limit allow a spectral cover description. The main tool of construction is the so-called spectral divisor in the resolved Calabi-Yau geometry, which in the local limit reduces to the Higgs bundle spectral cover. We exemplify the workings of this in the case of an E 6 singularity by constructing the resolved geometry, the spectral divisor and in the local limit, the spectral cover. The G-flux constructed with the spectral divisor is shown to be equivalent to the direct construction from suitably quantized linear combinations of holomorphic surfaces in the resolved geometry, and in the local limit reduces to the spectral cover flux.

Primitive divisors of Lucas and Lehmer sequences, II

Abstract: Let $\al$ and $\be$ be conjugate complex algebraic integers which generate Lucas or Lehmer sequences. We present an algorithm to search for elements of such sequences which have no primitive divisors. We use this algorithm to prove that for all $\al$ and $\be$ with $\hgt(\be/\al) \leq 4$, the $n$-th element of these sequences has a primitive divisor for $n > 30$. In the course of proving this result, we give an improvement of a result of Stewart concerning more general sequences.

A New Unicity Theorem and Erdos' Problem for Polarized Semi-Abelian Varieties

Abstract: In 1988 Erdos asked if the prime divisors of x n − 1 for all n = 1, 2, … determine the given integer x; the problem was affirmatively answered by Corrales-Rodriganez and Schoof (J Number Theory 64:276–290, 1997) [but a solution could also be deduced from an earlier result of Schinzel (Bull Acad Polon Sci 8:307–309, 2007)] together with its elliptic version. Analogously, Yamanoi (Forum Math 16:749–788, 2004) proved that the support of the pulled-back divisor f * D of an ample divisor on an abelian variety A by an algebraically non-degenerate entire holomorphic curve f : C → A essentially determines the pair (A, D). By making use of the main theorem of Noguchi (Forum Math 20:469–503, 2008) we here deal with this problem for semi-abelian varieties; namely, given two polarized semi-abelian varieties (A 1, D 1), (A 2, D 2) and algebraically non-degenerate entire holomorphic curves f i : C → A i , i = 1, 2, we classify the cases when the inclusion $${{\rm{Supp}}\, f_1^*D_1\subset {\rm{Supp}}\, f_2^* D_2}$$ holds. We shall remark in §5 that these methods yield an affirmative answer to a question of Lang formulated in 1966. Our answer is more general and more geometric than the original question. Finally, we interpret the main result of Corvaja and Zannier (Invent Math 149:431–451, 2002) to provide an arithmetic counterpart in the toric case.

Uniform estimates for primitive divisors in elliptic divisibility sequences

Abstract: Let P be a nontorsion rational point on an elliptic curve E, given by a minimal Weierstrass equation, and write the first coordinate of nP as A n ∕ D n 2, a fraction in lowest terms. The sequence of values D n is the elliptic divisibility sequence (EDS) associated to P. A prime p is a primitive divisor of D n if p divides D n , and p does not divide any earlier term in the sequence. The Zsigmondy set for P is the set of n such that D n has no primitive divisors. It is known that Z is finite. In the first part of the paper we prove various uniform bounds for the size of the Zsigmondy set, including (1) if the j-invariant of E is integral, then the size of the Zsigmondy set is bounded independently of E and P, and (2) if the abc Conjecture is true, then the size of the Zsigmondy set is bounded independently of E and P for all curves and points. In the second part of the paper, we derive upper bounds for the maximum element in the Zsigmondy set for points on twists of a fixed elliptic curve.

Polyhedral divisors and torus actions of complexity one over arbitrary fields

Abstract: We show that the presentation of affine $\mathbb{T}$-varieties of complexity one in terms of polyhedral divisors holds over an arbitrary field. We also describe a class of multigraded algebras over Dedekind domains. We study how the algebra associated to a polyhedral divisor changes when we extend the scalars. As another application, we provide a combinatorial description of affine $\mathbf{G}$-varieties of complexity one over a field, where $\mathbf{G}$ is a (not-nescessary split) torus, by using elementary facts on Galois descent. This class of affine $\mathbf{G}$-varieties is described via a new combinatorial object, which we call (Galois) invariant polyhedral divisor.

Correctly rounded constant integer division via multiply-add

Abstract: Implementing integer division in hardware is expensive when compared to multiplication. In the case where the divisor is a constant, expensive integer division algorithms can be replaced by cheaper integer multiplications and additions. This paper presents the conditions for multiply-add schemes to perform correctly rounded unsigned invariant integer division under one of three rounding modes. We propose a heuristic to explore the space of implementations meeting the conditions we derive. Experiments show that an average speed up of 20% and area reduction of 50% can be achieved compared to existing correctly rounded approaches. Extension to two's complement numbers is also presented.

Simple normal crossing Fano varieties and log Fano manifolds

Abstract: A projective log variety (X, D) is called "a log Fano manifold" if X is smooth and if D is a reduced simple normal crossing divisor on X with -(K_X+D) ample. The n-dimensional log Fano manifolds (X, D) with nonzero D are classified in this article when the log Fano index r of (X, D) satisfies either r\geq n/2 with \rho(X)\geq 2 or r\geq n-2. This result is a partial generalization of the classification of logarithmic Fano threefolds by Maeda.

A classification of terminal quartic 3-folds and applications to rationality questions

Abstract: This paper studies the birational geometry of terminal Gorenstein Fano 3-folds. If Y is not $${\mathbb{Q}}$$ -factorial, in most cases, it is possible to describe explicitly the divisor class group Cl Y by running a Minimal Model Program on X, a small $${\mathbb{Q}}$$ -factorialization of Y. In this case, the generators of Cl Y/ Pic Y are “topological traces” of K-negative extremal contractions on X. One can show, as an application of these methods, that a number of families of non-factorial terminal Gorenstein Fano 3-folds are rational. In particular, I give some examples of rational quartic hypersurfaces $${Y_4 \subset \mathbb{P}^4}$$ with rk Cl Y = 2 and show that when rk Cl Y ≥ 6, Y is always rational.

G-flux and Spectral Divisors

Abstract: We propose a construction of G-flux in singular elliptic Calabi-Yau fourfold compactifications of F-theory, which in the local limit allow a spectral cover description. The main tool of construction is the so-called spectral divisor in the resolved Calabi-Yau geometry, which in the local limit reduces to the Higgs bundle spectral cover. We exemplify the workings of this in the case of an E_6 singularity by constructing the resolved geometry, the spectral divisor and in the local limit, the spectral cover. The G-flux constructed with the spectral divisor is shown to be equivalent to the direct construction from suitably quantized linear combinations of holomorphic surfaces in the resolved geometry, and in the local limit reduces to the spectral cover flux.

Sieve Methods for Odd Perfect Numbers

Abstract: Using a new factor chain argument, we show that 5 does not divide an odd perfect number indivisible by a sixth power. Applying sieve techniques, we also find an upper bound on the smallest prime divisor. Putting this together we prove that an odd perfect number must be divisible by the sixth power of a prime or its smallest prime factor lies in the range 10^8 < p < 10^1000. These results are generalized to much broader situations.

Chern classes of free hypersurface arrangements

Abstract: The Chern class of the sheaf of logarithmic derivations along a simple normal crossing divisor equals the Chern-Schwartz-MacPherson class of the complement of the divisor. We extend this equality to more general divisors, which are locally analytically isomorphic to free hyperplane arrangements.

The double ramification cycle and the theta divisor

Abstract: We compute the classes of universal theta divisors of degrees zero and g-1 over the Deligne-Mumford compactification of the moduli space of curves, with various integer weights on the points, in particular reproving a recent result of M\"uller. We also obtain a formula for the class in the Chow ring of the moduli space of curves of compact type of the double ramification locus, given by the condition that a fixed linear combination of the marked points is a principal divisor, reproving a recent result of Hain. Our approach for computing the theta divisor is more direct, via test curves and the geometry of the theta divisor, and works easily over the entire Deligne-Mumford compactification. We use our extended result in another paper to study the partial compactification of the double ramification cycle.

Some arithmetic identities involving divisor functions

Abstract: For a positive integer $n$, let $\sigma(n):= \sum_{d \in \mathb{N}, d|n} d$. The explicit evaluation of such arithmetic sums as $\sum_{(a,b,c) \in \ABIFnn^3, a+2b+4c=n} \sigma(a)\sigma(b) \sigma(c)$ and $\sum_{(a,b) \in \ABIFnn^2, a+2b=n} a \sigma(a)\sigma(b)$ is carried out for all positive integers $n$.

On the canonical decomposition of generalized modular functions

Abstract: The authors have conjectured (\cite{KoM}) that if a normalized generalized modular function (GMF) $f$, defined on a congruence subgroup $\Gamma$, has integral Fourier coefficients, then $f$ is classical in the sense that some power $f^m$ is a modular function on $\Gamma$. A strengthened form of this conjecture was proved (loc cit) in case the divisor of $f$ is \emph{empty}. In the present paper we study the canonical decomposition of a normalized parabolic GMF $f = f_1f_0$ into a product of normalized parabolic GMFs $f_1, f_0$ such that $f_1$ has \emph{unitary character} and $f_0$ has \emph{empty divisor}. We show that the strengthened form of the conjecture holds if the first "few" Fourier coefficients of $f_1$ are algebraic. We deduce proofs of several new cases of the conjecture, in particular if either $f_0=1$ or if the divisor of $f$ is concentrated at the cusps of $\Gamma$.

Primitive divisors of certain elliptic divisibility sequences

Abstract: Let $P$ be a non-torsion point on the elliptic curve $E_{a}: y^{2}=x^{3}+ax$. We show that if $a$ is fourth-power-free and either $n>2$ is even or $n>1$ is odd with $x(P)<0$ or $x(P)$ a perfect square, then the $n$-th element of the elliptic divisibility sequence generated by $P$ always has a primitive divisor.

On certain arithmetic functions involving the greatest common divisor

Abstract: The paper deals with asymptotics for a class of arithmetic functions which describe the value distribution of the greatest-common-divisor function Typically, they are generated by a Dirichlet series whose analytic behavior is determined by the factor ζ2(s)ζ(2s − 1) Furthermore, multivariate generalizations are considered

On the Dirichlet divisor problem in short intervals

Abstract: We present several new results involving $\Delta(x+U)-\Delta(x)$, where $U = o(x)$ and $$ \Delta(x):=\sum_{n\le x}d(n)-x\log x-(2\gamma-1)x $$ is the error term in the classical Dirichlet divisor problem.