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Limits of log canonical thresholds

Tommaso de Fernex, Mircea Mustață (2009)

Annales scientifiques de l'École Normale Supérieure

Let 𝒯 n denote the set of log canonical thresholds of pairs ( X , Y ) , with X a nonsingular variety of dimension n , and Y a nonempty closed subscheme of X . Using non-standard methods, we show that every limit of a decreasing sequence in 𝒯 n lies in 𝒯 n - 1 , proving in this setting a conjecture of Kollár. We also show that 𝒯 n is closed in 𝐑 ; in particular, every limit of log canonical thresholds on smooth varieties of fixed dimension is a rational number. As a consequence of this property, we see that in order to check...

Line bundles with partially vanishing cohomology

Burt Totaro (2013)

Journal of the European Mathematical Society

Define a line bundle L on a projective variety to be q -ample, for a natural number q , if tensoring with high powers of L kills coherent sheaf cohomology above dimension q . Thus 0-ampleness is the usual notion of ampleness. We show that q -ampleness of a line bundle on a projective variety in characteristic zero is equivalent to the vanishing of an explicit finite list of cohomology groups. It follows that q -ampleness is a Zariski open condition, which is not clear from the definition.

Linear bounds for levels of stable rationality

Fedor Bogomolov, Christian Böhning, Hans-Christian Graf von Bothmer (2012)

Open Mathematics

Let G be one of the groups SLn(ℂ), Sp2n (ℂ), SOm(ℂ), Om(ℂ), or G 2. For a generically free G-representation V, we say that N is a level of stable rationality for V/G if V/G × ℙN is rational. In this paper we improve known bounds for the levels of stable rationality for the quotients V/G. In particular, their growth as functions of the rank of the group is linear for G being one of the classical groups.

Linear Fractional Recurrences: Periodicities and Integrability

Eric Bedford, Kyounghee Kim (2011)

Annales de la faculté des sciences de Toulouse Mathématiques

Linear fractional recurrences are given as z n + k = A ( z ) / B ( z ) , where A ( z ) and B ( z ) are linear functions of z n , z n + 1 , , z n + k - 1 . In this article we consider two questions about these recurrences: (1) Find A ( z ) and B ( z ) such that the recurrence is periodic; and (2) Find (invariant) integrals in case the induced birational map has quadratic degree growth. We approach these questions by considering the induced birational map and determining its dynamical degree. The first theorem shows that for each k there are k -step linear fractional recurrences...

Linear free divisors and the global logarithmic comparison theorem

Michel Granger, David Mond, Alicia Nieto-Reyes, Mathias Schulze (2009)

Annales de l’institut Fourier

A complex hypersurface D in n is a linear free divisor (LFD) if its module of logarithmic vector fields has a global basis of linear vector fields. We classify all LFDs for n at most 4 .By analogy with Grothendieck’s comparison theorem, we say that the global logarithmic comparison theorem (GLCT) holds for D if the complex of global logarithmic differential forms computes the complex cohomology of n D . We develop a general criterion for the GLCT for LFDs and prove that it is fulfilled whenever the...

Linear maps preserving orbits

Gerald W. Schwarz (2012)

Annales de l’institut Fourier

Let H GL ( V ) be a connected complex reductive group where V is a finite-dimensional complex vector space. Let v V and let G = { g GL ( V ) g H v = H v } . Following Raïs we say that the orbit H v is characteristic for H if the identity component of G is H . If H is semisimple, we say that H v is semi-characteristic for H if the identity component of G is an extension of H by a torus. We classify the H -orbits which are not (semi)-characteristic in many cases.

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