Аржанцев Иван Владимирович

Infinite transitivity, finite generation, and Demazure roots

2018 · PREPRINT · en

An affine algebraic variety X of dimension ≥ 2 is called flexible if the subgroup SAut(X) ⊂ Aut(X) generated by the one-parameter unipotent subgroups acts m-transitively on reg (X) for any m ≥ 1. In a preceding paper ([4]) we proved that any nondegenerate toric affine variety X is flexible. Here we show that if such a toric variety X is smooth in codimension 2 then one can find a subgroup of SAut(X) generated by a finite number of one-parameter unipotent subgroups which has the same transitivity property. In fact, four such subgroups are enough for X = A^n if n ≥ 3, and just three if n = 2.

The automorphism group of a rigid affine variety

2017 · ARTICLE · en

An irreducible algebraic variety X is rigid if it admits no nontrivial action of the additive group of the ground field. We prove that the automorphism group of a rigid affine variety contains a unique maximal torus . If the grading on the algebra of regular functions defined by the action of is pointed, the group is a finite extension of . As an application, we describe the automorphism group of a rigid trinomial affine hypersurface and find all isomorphisms between such hypersurfaces.

Additive actions on toric varieties

2017 · ARTICLE · en

By an additive action on an algebraic variety of dimension we mean a regular action with an open orbit of the commutative unipotent group . We prove that if a complete toric variety admits an additive action, then it admits an additive action normalized by the acting torus. Normalized additive actions on a toric variety are in bijection with complete collections of Demazure roots of the fan . Moreover, any two normalized additive actions on are isomorphic.

On rigidity of factorial trinomial hypersurfaces

2016 · ARTICLE · en

An affine algebraic variety X is rigid if the algebra of regular functions K[X] admits no nonzero locally nilpotent derivation. We prove that a factorial trinomial hypersurface is rigid if and only if every exponent in the trinomial is at least 2.

Cox rings

2015 · BOOK · en

Cox rings are significant global invariants of algebraic varieties, naturally generalizing homogeneous coordinate rings of projective spaces. This book provides a largely self-contained introduction to Cox rings, with a particular focus on concrete aspects of the theory. Besides the rigorous presentation of the basic concepts, other central topics include the case of finitely generated Cox rings and its relation to toric geometry; various classes of varieties with group actions; the surface case; and applications in arithmetic problems, in particular Manin's conjecture. The introductory chapters require only basic knowledge in algebraic geometry. The more advanced chapters also touch on algebraic groups, surface theory, and arithmetic geometry. Each chapter ends with exercises and problems. These comprise mini-tutorials and examples complementing the text, guided exercises for topics not discussed in the text, and, finally, several open problems of varying difficulty.

Equivariant Embeddings of Commutative Linear Algebraic Groups of Corank One

2015 · ARTICLE · en

Infinite transitivity on universal torsors

2014 · ARTICLE · en

Let X be an algebraic variety covered by open charts isomorphic to the affine space and q: X' \to X be the universal torsor over X. We prove that the automorphism group of the quasiaffine variety X' acts on X' infinitely transitively. Also we find wide classes of varieties X admitting such a covering.

The automorphism group of a variety with torus action of complexity one

2014 · ARTICLE · en

Given an action of an affine algebraic group with only trivial characters on a factorial variety, we ask for categorical quotients. We characterize existence in the category of algebraic varieties. Moreover, allowing constructible sets as quotients, we obtain a more general existence result which, for example, settles the case of a finitely generated algebra of invariants. As an application, we provide a combinatorial GIT-type construction of categorial quotients for actions on, e.g. complete varieties with finitely generated Cox ring via lifting to the characteristic space.

Additive actions on projective hypersurfaces

2014 · CHAPTER · en

By an additive action on a hypersurface H in a projective space we mean an effective action of a commutative unipotent group on the projective space which leaves H invariant and acts on H with an open orbit. Brendan Hassett and Yuri Tschinkel have shown that actions of commutative unipotent groups on projective spaces can be described in terms of local algebras with some additional data. We prove that additive actions on projective hypersurfaces correspond to invariant multilinear symmetric forms on local algebras. It allows us to obtain explicit classification results for non-degenerate quadrics and quadrics of corank one.

On orbits of the automorphism group on an affine toric variety

2013 · ARTICLE · en

Let X be an affine toric variety. The total coordinates on X provide a canonical presentation !X -> X of X as a quotient of a vector space !X by a linear action of a quasitorus. We prove that the orbits of the connected component of the automorphism group Aut(X) on X coincide with the Luna strata defined by the canonical quotient presentation.