Camille Plenat
This paper has been withdrawn. The result anyway is still true for rational double points Dn. (the cases of E6,E7,E8 are not solved yet).
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The Nash problem on arcs for normal surface singularities states that there are as many arc families on a germ (S,O) of a singular surface as there are essential divisors over (S,O). It is known that this problem can be reduced to the study of quasi-rational singularities. In this paper we give a positive answer to the Nash problem for a family of non-rational quasi-rational hypersurfaces. The same method is applied to answer positively to this problem in the case of E_6 and E_7 type singularities, and to provide new proof in the case of D_n, n> =4, type singularities.
A Steiner bundle over the projective 3-space is the kernel in a trivial bundle of a morphism defined by a matrix of linear forms. We produce various Steiner bundles E of rank n such that E(1) has n-1 sections, the dependency locus of which is a smooth curve.
We deal with the Brill-Noether problem for stable vector bundles of slope between one and two.
Consider weak approximation for 0-cycles on a smooth proper variety defined over a number field, it is conjectured to be controlled by its Brauer group. Let $X$ be a Châtelet surface or a smooth compactification of a homogeneous space of a connected linear algebraic group with connected stabilizer. Let $Y$ be a rationally connected variety. We prove that weak approximation for 0-cycles on the product $X\times Y$ is controlled by its Brauer group if it is the case for $Y$ after every finite extension of the base field. We do not suppose the existence of 0-cycles of degree $1$ neither on $X$ nor…
We obtain a lower bound for the normalised height of a non-torsion hypersurface $V$ of a C.M. abelian variety $A$ which is a refinement of a precedent result. This lower bound is optimal in terms of the geometric degree of $V$, up to an absolute power of a ``log'' (independant of the dimension of $A$). We thus extend the results of F. Amoroso and S. David on the same problem on a multiplicative group $\mathbb{G}_m^n$. When $A$ is an elliptic curve and $V=\bar{P}$ is the set of conjugates of a non torsion $\bar{k}$-point, we reobtain the result of M. Laurent on the elliptic Lehmer's problem.
Soit l un entier et E_{c,l} la famille de Kubert des courbes elliptiques definies sur Q munies d'un point rationnel A d'ordre l. On note F_{c,l} la courbe elliptique quotient de E_{c,l} par le groupe engendre' par A, et f_l l'isogenie de E_{c,l} sur F_{c,l}. Pour l = 3, 4, 5 et 6, nous construisons explicitement, pour des parametrisations convenables de c, des elements non-triviaux de F_{c,l}(Q) / (f_l(E_{c,l}(Q))), autrement dit, des points explicites de F_{c,l}(Q) qui ne sont l'image par f_l d'aucun element de E_{c,l}(Q). Ces points sont en general d'ordre infini. Nous donnons des applicatio…
Let X be a smooth compactification of a homogeneous space of a linear algebraic group G over a number field k. We establish the conjecture of Colliot-Thélène, Sansuc, Kato and Saito on the image of the Chow group of zero-cycles of X in the product of the same groups over all the completions of k. When G is semisimple and simply connected and the geometric stabiliser is finite and supersolvable, we show that rational points of X are dense in the Brauer-Manin set. For finite supersolvable groups, in particular for finite nilpotent groups, this yields a new proof of Shafarevich's theorem on the i…
For each integer d=2,3,4, there exists a field F with cohomological dimension 1 and a del Pezzo surface of degree d over F having no rational point. Proofs use the theorem of Merkur'ev and Suslin, the Riemann-Roch theorem on a surface and Rost's degree formula. ----- Pour chaque entier d=2,3,4, il existe un corps F de dimension cohomologique 1 et une surface de del Pezzo de degre d sur F sans point rationnel. Les demonstrations utilisent le theoreme de Merkur'ev et Suslin, le theoreme de Riemann-Roch sur une surface et la formule du degre de Rost.
In 1974, D. Coray showed that on a smooth cubic surface with a closed point of degree prime to 3 there exists such a point of degree 1, 4 or 10. We first show how a combination of generisation, specialisation, Bertini theorems and large fields avoids considerations of special cases in his argument. For smooth cubic surfaces with a rational point, we show that any zero-cycle of degree at least 10 is rationally equivalent to an effective cycle. We establish analogues of these results for del Pezzo surfaces of degree 2 and of degree 1. For smooth cubic surfaces without a rational point, we relate…
Soit $f:X\to S$ un morphisme surjectif et de présentation finie de schémas intègres. Lorsque $S$ est de type fini sur un corps ou sur $\mathbb{Z}$, et de dimension $>0$, Poonen a montré qu'il existe un point $x\in X$ dont le corps résiduel $κ(x)$ est une extension radicielle de $κ(f(x))$. Dans ce travail, on prolonge ce résultat de plusieurs façons. D'abord, on peut choisir $x$ tel que $f(x)$ soit un point de codimension $1$ de $S$; si $S$ est lisse sur un corps $k$, on peut exiger que $κ(f(x))$ soit séparable sur $k$. Dans une autre direction, on montre des résultats analogues pour d'autres c…
Let K be a global field, T a K-torus and S a finite set of places of K. Let K_v be the completion at a place v. Denote by T(O_v) the maximal compact subgroup of the group T(K_v) of K_v-points of T. We show that the diagonal map from T(K) to the product for v in S of the T(K_v)/T(O_v) need not be onto. As a corollary, for suitable v, the group T(O_v) does not cover all R-equivalence classes in T(K_v). While studying the height zeta functions of an algebraic torus over a function field in one variable over a finite field, D. Bourqui showed that Peyre's constant from the number field case should …
Let $X$ be a smooth projective variety over a finite field $\F$. We discuss the unramified cohomology group $H^3_\nr(X,\Q/\Z(2))$. Several conjectures put together imply that this group is finite. For certain classes of threefolds, $H^3_\nr(X,\Q/\Z(2))$ actually vanishes. It is an open question whether this holds true for arbitrary threefolds. For a threefold $X$ equipped with a fibration onto a curve $C$, the generic fibre of which is a smooth projective surface $V$ over the global field $\F(C)$, the vanishing of $H^3_\nr(X,\Q/\Z(2))$ together with the Tate conjecture for divisors on $X$ impl…
Let F be a finite field of characteristic p. We consider smooth surfaces over F(t) defined by an equation f+tg=0, where f and g are forms of degree d in 4 variables with coefficients in F, with d prime to p. We prove : For such surfaces over F(t), the Brauer-Manin obstruction to the existence of a zero-cycle of degree one is the only obstruction. For d=3 (cubic surfaces), this leads to the same result for rational points. -- Soit F un corps fini de caractéristique p. Pour une surface lisse sur F(t) définie par une équation f+tg=0, où f et g sont deux formes de degré d sur F en 4 variables, ave…
Let X be a smooth and proper variety over a number field k. Conjectures on the image of the Chow group of zero-cycles of X in the product of the corresponding groups over all completions of k were put forward by Colliot-Thélène, Kato and Saito. We prove these conjectures for the total space of fibrations, over curves with finite Tate-Shafarevich group, into rationally connected varieties which satisfy weak approximation, under an abelianness assumption on the singular fibers. ---- Soit X une variété propre et lisse sur un corps de nombres k. Des conjectures sur l'image du groupe de Chow des zé…
On rational quadratic cocycles
本文研究了有理二次空间的算术子群上同调中的循环类，并证明其生成函数为Siegel模形式。
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