4 e série, t. 42, 2009, p. 783 à 835 CONVEX BODIES ASSOCIATED TO LINEAR SERIES ʙʏ Rʙʀ LAZARSFELD ɴ Mɪʀ MUSTAT ,Ȃ Aʙʀ. -In his work on log-concavity of multiplicities, Okounkov showed in passing that one could associate a convex body to a linear series on a projective variety, and then use convex geometry to study such linear systems. Although Okounkov was essentially working in the classical setting of ample line bundles, it turns out that the construction goes through for an arbitrary big divisor. Moreover, this viewpoint renders transparent many basic facts about asymptotic invariants of linear series, and opens the door to a number of extensions. The purpose of this paper is to initiate a systematic development of the theory, and to give some applications and examples.R. -Dans son travail sur la log-concavité des multiplicités, Okounkov montre au passage que l'on peut associer un corps convexe à un système linéaire sur une variété projective, puis utiliser la géométrie convexe pour étudier ces systèmes linéaires. Bien qu'Okounkov travaille essentiellement dans le cadre classique des fibrés en droites amples, il se trouve que sa construction s'étend au cas d'un grand diviseur arbitraire. De plus, ce point de vue permet de rendre transparentes de nombreuses propriétés de base des invariants asymptotiques des systèmes linéaires, et ouvre la porte à de nombreuses extensions. Le but de cet article est d'initier un développement systématique de la théorie et de donner quelques applications et exemples.

We study asymptotic jumping numbers for graded sequences of ideals, and show that every such invariant is computed by a suitable real valuation of the function field. We conjecture that every valuation that computes an asymptotic jumping number is necessarily quasi-monomial. This conjecture holds in dimension two. In general, we reduce it to the case of affine space and to graded sequences of valuation ideals. Along the way, we study the structure of a suitable valuation space.

The F -thresholds are characteristic p analogs of the jumping coefficients for multiplier ideals in characteristic zero. We give an alternative description of the F -thresholds of an ideal in a regular and F -finite ring R, which enables us to settle two open questions posed in [MTW]. Namely, we show that if the ring is, in addition, essentially of finite type over a field, then the F -thresholds are rational and discrete.

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We introduce the notion of Bernstein-Sato polynomial of an arbitrary variety (which is not necessarily reduced nor irreducible), using the theory of V -filtrations of M. Kashiwara and B. Malgrange. We prove that the decreasing filtration by multiplier ideals coincides essentially with the restriction of the V -filtration. This implies a relation between the roots of the Bernstein-Sato polynomial and the jumping coefficients of the multiplier ideals, and also a criterion for rational singularities in terms of the maximal root of the polynomial in the case of a reduced complete intersection. These are generalizations of the hypersurface case. We can calculate the polynomials explicitly in the case of monomial ideals.

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We associate a weight function to pairs (X, ω) consisting of a smooth and proper variety X over a complete discretely valued field and a pluricanonical form ω on X. This weight function is a real-valued function on the non-Archimedean analytification of X. It is piecewise affine on the skeleton of any regular model with strict normal crossings of X, and strictly ascending as one moves away from the skeleton. We apply these properties to the study of the Kontsevich-Soibelman skeleton of (X, ω), and we prove that this skeleton is connected when X has geometric genus one and ω is a canonical form on X. This result can be viewed as an analog of the Shokurov-Kollár connectedness theorem in birational geometry.

No abstract