Search

We show, among other things, that for each integer $$n \ge 3$$ , there is a smooth complex projective rational variety of dimension n, with discrete non-finitely generated automorphism group and with infinitely many mutually non-isomorphic real forms. Our result is inspired by the work of Lesieutre and the work of Dinh and Oguiso.

В работе исследуются многообразия представлений двух классов конечно порожденных групп.Первый класс состоит из групп с копредставлением\begin{gather*}G = \langle a_1,\ldots,a_s,b_1,\ldots,b_k,x_1,\ldots,x_g\mid\\ a_1^{m_1}=\ldots=a_s^{m_s}= x_1^2\ldots x_g^2 W(a_1,\ldots,a_s,b_1,\ldots,b_k)=1\rangle,\end{gather*}где $g\ge 3$, $m_i\ge 2$ для $i=1,\ldots,s$ и$W(a_1,\ldots,a_s,b_1,\ldots,b_k)$ --- элемент в нормальной формев свободном произведении циклических групп $H=\langle a_1\mid a_1^{m_1}\rangle\ast\ldots\ast\langle a_s\mid a_s^{m_s}\rangle\ast\langle b_1\rangle\ast\ldots\ast\langle b_k\rangle$.Второй класс состоит из групп с копредставлением$$G(p,q) = \langle a_1,\ldots,a_s,b_1,\ldots,b_k,x_1,\ldots,x_g,t\mid a_1^{m_1}=\ldots=a_s^{m_s}=1,\ tU^pt^{-1}=U^q \rangle,$$где $p$ и $q$ --- целые числа, такие, что $p>|q|\geq1$, $(p,q)=1$, $m_i\ge 2$ для $i=1,\ldots,s$, \linebreak $g\ge 3$,$U=x_1^2\ldots x_g^2W(a_1,\ldots,a_s,b_1,\ldots,b_k)$ и $W(a_1,\ldots,a_s,b_1,\ldots,b_k)$ --- элемент, определенный выше.Найдены неприводимые компоненты многообразий представлений $R_n(G)$ и $R_n(G(p,q))$, вычислены их размерности и доказано, что каждая неприводимаякомпонента является рациональным многообразием.

Let G be an algebraic group over F and p a prime integer. We introduce the notion of a p-retract rational variety and prove that if Y → X is a p-versal G-torsor, then BG is a stable p-retract of X. It follows that the classifying space BG is p-retract rational if and only if there is a p-versal G-torsor Y → X with X a rational variety, that is, all G-torsors over infinite fields are rationally parameterized. In particular, for such groups G the unramified Galois cohomology group $$ {H}_{\mathrm{nr}}^n $$ (F(BG), ℚp/ℤp(j)) coincides with Hn(F, ℚp/ℤp(j)).

Let $X$ be a smooth Fano threefold. We show that $X$ admits a non-isomorphic surjective endomorphism if and only if $X$ is either a toric variety or a product of $\mathbb{P}^1$ and a del Pezzo surface; in this case, $X$ is a rational variety. We further show that $X$ admits a polarized (or amplified) endomorphism if and only if $X$ is a toric variety., Minor revision, 34 pages, Mathematische Annalen (to appear)

Let$$\mathbb R(C)$$R(C)be the function field of a smooth, irreducible projective curve over$$\mathbb R$$R. LetXbe a smooth, projective, geometrically irreducible variety equipped with a dominant morphismfonto a smooth projective rational variety with a smooth generic fibre over$$\mathbb R(C)$$R(C). Assume that the cohomological obstruction introduced by Colliot-Thélène is the only one to the local-global principle for rational points for the smooth fibres offover$$\mathbb R(C)$$R(C)-valued points. Then we show that the same holds forX, too, by adopting the fibration method similarly to Harpaz–Wittenberg.

By the geometry of the 3-fold quadric we show that the coarse moduli space of genus g ineffective spin hyperelliptic curves with two marked points is a rational variety for every $g \geq 2$., Comment: 29 pages, 3 figures

In a prior paper the authors obtained a four-dimensional discrete integrable dynamical system by the traveling wave reduction from the lattice super-KdV equation in a case of finitely generated Grassmann algebra. The system is a coupling of a Quispel-Roberts-Thompson map and a linear map but does not satisfy the singularity confinement criterion. It was conjectured that the dynamical degree of this system grows quadratically. In this paper, constructing a rational variety where the system is lifted to an algebraically stable map and using the action of the map on the Picard lattice, we prove this conjecture. We also show that invariants can be found through the same technique.

We present a new algorithm to decide whether two rational parametric curves are related by a projective transformation and detect all such projective equivalences. Given two rational curves, we derive a system of polynomial equations whose solutions define linear rational transformations of the parameter domain, such that each transformation corresponds to a projective equivalence between the two curves. The corresponding projective mapping is then found by solving a small linear system of equations. Furthermore we investigate the special cases of detecting affine equivalences and symmetries as well as polynomial input curves. The performance of the method is demonstrated by several numerical examples.

We show that, in any dimension greater than one, there are an abelian variety, a smooth rational variety and a Calabi-Yau manifold, with primitive birational automorphisms of first dynamical degree $>1$. We also show that there are smooth complex projective Calabi-Yau manifolds and smooth rational manifolds, of any even dimension, with primitive biregular automorphisms of positive topological entropy.

A smooth variety is called uniformly rational if every point admits a Zariski open neighborhood isomorphic to a Zariski open subset of the affine space. In this note we show that every smooth and rational affine variety endowed with an algebraic torus action such that the algebraic quotient has dimension 0 or 1 is uniformly rational., Comment: 4 pages

Representation and character varieties of the Baumslag–Solitar groups BS(p, q) are analyzed. Irreducible components of these varieties are found, and their dimension is calculated. It is proved that all irreducible components of the representation variety Rn(BS(p, q)) are rational varieties of dimension n2, and each irreducible component of the character variety Xn(BS(p, q)) is a rational variety of dimension k ≤ n. The smoothness of irreducible components of the variety Rns (BS(p, q)) of irreducible representations is established, and it is proved that all irreducible components of the variety Rns (BS(p, q)) are isomorphic to A1 {0}.

In this paper we study an instance of projective Reed-Muller type codes, i.e., codes obtained by the evaluation of homogeneous polynomials of a fixed degree in the points of a projective variety. In our case the variety is an important example of a determinantal variety, namely the projective surface known as rational normal scroll, defined over a finite field, which is the basic underlining algebraic structure of this work. We determine the dimension and a lower bound for the minimum distance of the codes, and in many cases we also find the exact value of the minimum distance. To obtain the results we use some methods from Grobner bases theory.

We give an explicit description of the divisor class groups of rational trinomial varieties. As an application, we relate the iteration of Cox rings of any rational variety with torus action of complexity one to that of a Du Val surface., Comment: 17 pages

F. Campana had asked whether a certain threefold is rational. F. Catanese, K. Oguiso and T. T. Truong have recently shown that this variety is birational to a specific conic bundle threefold, which they show is unirational. Computing residues of elements in the Brauer group of the function field of the plane, I prove that that conic bundle threefold is birational to another conic bundle threefold, and the latter is clearly a rational variety.

We investigate the relationship between rational plane curves and the envelopes defined by the syzygies of their parameterizations.

Let K be a finite algebraic extension of the field of rational functions in one indeterminate over a finite field and let [Formula: see text] denote an algebraic closure of K. We count points in projective space [Formula: see text] with given height and generating a quadratic extension of K. If n > 2, we derive an asymptotic estimate for the number of such points as the height tends to infinity. Such estimates are analogous to previous results of Schmidt where the field K is replaced by the field of rational numbers ℚ.

A linear pentapod is a parallel manipulator with five collinear anchor points on the motion platform (end-effector), which are connected via \(\mathrm {S\underline{P}S}\) legs to the base. This manipulator has five controllable degrees-of-freedom and the remaining one is a free rotation around the motion platform axis (which in fact is an axial spindle). In this paper we present a rational parametrization of the singularity variety of the linear pentapod. Moreover we compute the shortest distance to this rational variety with respect to a suitable metric. Kinematically this distance can be interpreted as the radius of the maximal singularity free-sphere. Moreover we compare the result with the radius of the maximal singularity free-sphere in the position workspace and the orientation workspace, respectively.

In this Ph.D. thesis, we investigate some arithmetic properties of algebraic varieties. The thesis consists of two parts: a geometric part (over an arbitrary field) and an arithmetic part (over a number field). The geometric part is devoted to the study of the quotient by its constant part of the third unramified cohomology group of (geometrically) rational surfaces and of their universal torsors. For del Pezzo surfaces of degree at least 5, we show that this quotient is zero, except in the case of del Pezzo surfaces of degree 8 of a special type. For universal torsors as above, we show this quotient is finite and we give a sufficient condition for it to vanish. This condition involves the Galois structure of the geometrical Picard group. The arithmetic part is devoted to the study of the Brauer-Manin obstruction to strong approximation. In collaboration with C. Demarche and F. Xu, we establish the equivalence of étale Brauer-Manin obstruction and the descent obstruction. Then I establish a general theorem about strong approximation of open varieties equipped with an action of a connected linear algebraic group G and containing a G-homogeneous space as open subset.; Dans cette thèse, on s’intéresse à des propriétés arithmétiques des variétés algébriques. Elle contient deux parties : partie géométrique (sur un corps quelconque) et partie arithmétique (sur un corps de nombres). Dans la partie géométrique, j’étudie le quotient par sa partie constante du troisième groupe de cohomologie non ramifiée des surfaces (géométriquement) rationnelles et de leurs torseurs universels. Pour les surfaces de del Pezzo de degré au moins 5, je montre que ce quotient est trivial, sauf pour des surfaces de del Pezzo de degré 8 d’un type particulier. Pour les torseurs universels ci-dessus, je montre que ce quotient est fini et je donne une condition suffisante pour qu’il soit nul, en termes de la structure galoisienne du groupe de Picard géométrique de la surface. Dans la partie arithmétique, on étudie l’obstruction de Brauer–Manin à l’approximation forte. En collaboration avec C. Demarche et F. Xu, nous établissons l’équivalence de l’obstruction de Brauer-Manin étale et de l’obstruction de descente pour les variétés quasi-projectives. Ensuite, j’établis un théorème très général sur l’approximation forte pour les variétés ouvertes munies d’une action d’un groupe linéaire connexe G et dont un ouvert est un espace homogène de G.

In this Ph.D. thesis, we investigate some arithmetic properties of algebraic varieties. The thesis consists of two parts: a geometric part (over an arbitrary field) and an arithmetic part (over a number field). The geometric part is devoted to the study of the quotient by its constant part of the third unramified cohomology group of (geometrically) rational surfaces and of their universal torsors. For del Pezzo surfaces of degree at least 5, we show that this quotient is zero, except in the case of del Pezzo surfaces of degree 8 of a special type. For universal torsors as above, we show this quotient is finite and we give a sufficient condition for it to vanish. This condition involves the Galois structure of the geometrical Picard group. The arithmetic part is devoted to the study of the Brauer-Manin obstruction to strong approximation. In collaboration with C. Demarche and F. Xu, we establish the equivalence of étale Brauer-Manin obstruction and the descent obstruction. Then I establish a general theorem about strong approximation of open varieties equipped with an action of a connected linear algebraic group G and containing a G-homogeneous space as open subset.; Dans cette thèse, on s’intéresse à des propriétés arithmétiques des variétés algébriques. Elle contient deux parties : partie géométrique (sur un corps quelconque) et partie arithmétique (sur un corps de nombres). Dans la partie géométrique, j’étudie le quotient par sa partie constante du troisième groupe de cohomologie non ramifiée des surfaces (géométriquement) rationnelles et de leurs torseurs universels. Pour les surfaces de del Pezzo de degré au moins 5, je montre que ce quotient est trivial, sauf pour des surfaces de del Pezzo de degré 8 d’un type particulier. Pour les torseurs universels ci-dessus, je montre que ce quotient est fini et je donne une condition suffisante pour qu’il soit nul, en termes de la structure galoisienne du groupe de Picard géométrique de la surface. Dans la partie arithmétique, on étudie l’obstruction de Brauer–Manin à l’approximation forte. En collaboration avec C. Demarche et F. Xu, nous établissons l’équivalence de l’obstruction de Brauer-Manin étale et de l’obstruction de descente pour les variétés quasi-projectives. Ensuite, j’établis un théorème très général sur l’approximation forte pour les variétés ouvertes munies d’une action d’un groupe linéaire connexe G et dont un ouvert est un espace homogène de G.

I discuss some arithmetic aspects of higher-dimensional algebraic geometry. I focus on varieties with many rational points and on connections with classification theory and the minimal model program.

Let $M_{g, n}$ (respectively, $\overline{M_{g, n}}$) be the moduli space of smooth (respectively stable) curves of genus $g$ with $n$ marked points. Over the field of complex numbers, it is a classical problem in algebraic geometry to determine whether or not $M_{g, n}$ (or equivalently, $\overline{M_{g, n}}$) is a rational variety. Theorems of J. Harris, D. Mumford, D. Eisenbud and G. Farkas assert that $M_{g, n}$ is not unirational for any $n \geqslant 0$ if $g \geqslant 22$. Moreover, P. Belorousski and A. Logan showed that $M_{g, n}$ is unirational for only finitely many pairs $(g, n)$ with $g \geqslant 1$. Finding the precise range of pairs $(g, n)$, where $M_{g, n}$ is rational, stably rational or unirational, is a problem of ongoing interest. In this paper we address the rationality problem for twisted forms of $\overline{M_{g, n}}$ defined over an arbitrary field $F$ of characteristic $\neq 2$. We show that all $F$-forms of $\overline{M_{g, n}}$ are stably rational for $g = 1$ and $3 \leqslant n \leqslant 4$, $g = 2$ and $2 \leqslant n \leqslant 3$, $g = 3$ and $1 \leqslant n \leqslant 14$, $g = 4$ and $1 \leqslant n \leqslant 9$, $g = 5$ and $1 \leqslant n \leqslant 12$., Comment: 13 pages, proofs much shortened, new coauthor

We prove that in a family of projective threefolds defined over an algebraically closed field, the locus of rational fibers is a countable union of closed subsets of the locus of separably rationally connected fibers. When the ground field has characteristic zero, this implies that the locus of rational fibers in a smooth family of projective threefolds is the union of at most countably many closed subfamilies., Comment: 9 pages; v2: minor changes, final version to appear in Rend. Circ. Mat. Palermo

Let k be an algebraic closure of k. In the case when P(t) has at most one root in k, the open subset of the affine variety (1) given by P(t)y~O is a principal homogeneous space under an algebraic k-torus. In this case it is well known that the Brauer Manin obstruction is the only obstruction to the Hasse principle and weak approximation on any smooth projective model of this variety (Colliot-Th~l~ne and Sansuc [CSanl]). In this paper we prove the same result when P(t) has exactly two roots in k and no other roots in k, and k is the field of rational numbers Q. An immediate change of variables then reduces (1) to the equation ta~

Let X be a non-singular projective hypersurface of degree 4, which is defined over the rational numbers. Assume that X has dimension 39 or more, and that X contains a real point and p-adic points for every prime p. Then X is shown to contain infinitely many rational points., 47 pages

Using essentially only algebra, we give a proof that a cubic rational function over $\mathbb{C}$ with real critical points is equivalent to a real rational function. We also show that the natural generalization to $\mathbb{Q}_p$ fails for all $p$., 6 pages; title changed from previous version

This paper deals with two main topics related to Diophantine approximation. Firstly, we show that if a point on an algebraic variety is approximable by rational vectors to a sufficiently large degree, the approximating vectors must lie in the topological closure of the rational points on the variety. In many interesting cases, in particular if the set of rational points on the variety is finite, this closure does not exceed the set of rational points on the variety itself. This result enables easier proofs of several known results as special cases. The proof can be generalized in some way and encourages to define a new exponent of simultaneous approximation. The second part of the paper is devoted to the study of this exponent., 14 pages. Thanks to D. Simmons for pointing out inaccuracies including typos

The approximation of a multivector by a decomposable one is a distance-optimization problem between the multivector and the Grassmann variety of lines in a projective space. When the multivector diverges from the Grassmann variety, then the approximate solution sought is the worst possible. In this paper, it is shown that the worst solution of this problem is achieved, when the eigenvalues of the matrix representation of a related two-vector are all equal. Then, all these pathological points form a projective variety. We derive the equation describing this projective variety, as well as its maximum distance from the corresponding Grassmann variety. Several geometric and algebraic properties of this extremal variety are examined, providing a new aspect for the Grassmann varieties and the respective projective spaces.

We give an upper bound on the number of rational points of an arbitrary Zariski closed subset of a projective space over a finite field F q \mathbf {F}_q . This bound depends only on the dimensions and degrees of the irreducible components and holds for very general projective varieties, even reducible and nonequidimensional. As a consequence, we prove a conjecture of Ghorpade and Lachaud on the maximal number of rational points of an equidimensional projective variety.

Using the Mori theory for threefolds, we prove that the moduli space of pairs of smooth curves of genus four and theta characteristics without global sections is a rational variety.

The investigation of rational varieties with chord length parameterization (shortly RCL varieties) was started by Farin (2006) who observed that rational quadratic circles in standard Bezier form are parametrized by chord length. Motivated by this observation, general RCL curves were studied. Later, the RCL property was extended to rational triangular Bezier surfaces of an arbitrary degree for which the distinguishing property is that the ratios of the three distances of a point to the three vertices of an arbitrary triangle inscribed to the reference circle and the ratios of the distances of the parameter point to the three vertices of the corresponding domain triangle are identical. In this paper, after discussing rational tensor-product surfaces with the RCL property, we present a general unifying approach and study the conditions under which a k-dimensional rational variety in d-dimensional Euclidean space possesses the RCL property. We analyze the entire family of RCL varieties, provide their general parameterization and thoroughly investigate their properties. Finally, the previous observations for curves and surfaces are presented as special cases of the introduced unifying approach.

In this paper we consider the scheme MQ( 2;¡1; 2; 0 ) of stable torsion free sheaves of rank 2 with Chern classes c1 = -1, c2 = 2, c3 = 0 on a smooth 3-dimensional projective quadric Q. The manifold MQ(-1; 2) of moduli bundles of rank 2 with Chern classes c1 = -1, c2 = 2 on Q was studied by Ottaviani and Szurek in 1994. In 2007 the author described the closure MQ (-1; 2) in the scheme MQ(2;¡1; 2; 0). In this paper we prove that in MQ(2;¡1; 2; 0) there exists a unique irreducible component diferent from MQ (¡1; 2) which is a rational variety of dimension 10.

Birationally invariant intersection theory is a far-reaching generalization and extension of the Bernstein-Kushnirenko theorem. This paper presents transparent proofs of Hilbert’s theorem on the degree of a projective variety and other related statements playing an important role in this theory. The paper is completely self-contained; we recall all necessary definitions and statements.

We show that for any finite field F q , any N ⩾ 0 and all sufficiently large integers g there exist curves over F q of genus g having exactly N rational points.

In this paper, we provide an algorithm to compute explicit rational solutions of a rational system of autonomous ordinary differential equations (ODEs) from its rational invariant algebraic curves. The method is based on the proper rational parametrization of these curves and the fact that by linear reparametrizations, we can find the rational solutions of the given system of ODEs. Moreover, if the system has a rational first integral, we can decide whether it has a rational general solution and compute it in the affirmative case., Highlights ► We determine the rational general solutions of a planar rational system of autonomous ODEs explicitly. ► The method is based on the proper rational parametrization of the rational invariant algebraic curves. ► We determine a rational general solution via the existence of a rational first integral.