The fundamental combinatorial structure of a net in $ℂP^2$ is its associated set of mutually orthogonal Latin squares. We define equivalence classes of sets of orthogonal Latin squares by label equivalences of the lines of the corresponding net in $ℂP^2$. Then we count these equivalence classes for small cases. Finally we prove that the realization spaces of these classes in $ℂP^2$ are empty to show some non-existence results for 4-nets in $ℂP^2$.

The webs have been studied mainly locally, near regular points (see a short list of references on the topic in the bibliography). Let d be an integer ≥ 1. A d-web on an open set U of ℂ² is a differential equation F(x,y,y’) = 0 with $F(x,y,y^{\text{'}})={\sum }_{i=0}^d{a}_i(x,y){\left(y^{\text{'}}\right)}^{d-i}$, where the coefficients $a_i$ are holomorphic functions, a₀ being not identically zero. A regular point is a point (x,y) where the d roots in y’ are distinct (near such a point, we have locally d foliations mutually transverse to each other, and caustics appear through...

Beaucoup de concepts sur les tissus n’ont été étudiés que localement. Il apparaît que certains d’entre eux se laissent globaliser, mais pas toujours de façon immédiate. Le premier objectif de cet article est de préciser à chaque fois ce qu’il en est, et de mettre en place les outils utiles à une étude globale des tissus sur une surface holomorphe $M$ arbitraire, et en particulier sur le plan projectif complexe ${\mathbb{P}}_2$. Certains concepts nouveaux vont alors apparaître, tels le type (ou le degré si $M={\mathbb{P}}_2$), la...

Let $Y$ be an $(m+1)$-dimensional irreducible smooth complex projective variety embedded in a projective space. Let $Z$ be a closed subscheme of $Y$, and $\delta$ be a positive integer such that ${ℐ}_{Z,Y}\left(\delta \right)$ is generated by global sections. Fix an integer $d\ge \delta +1$, and assume the general divisor $X\in |H^0(Y,{ℐ}_{Z,Y}\left(d\right))|$ is smooth. Denote by $H^m{(X;\mathbb{Q})}_{\perp Z}^{\mathrm{van}}$ the quotient of $H^m(X;\mathbb{Q})$ by the cohomology of $Y$ and also by the cycle classes of the irreducible components of dimension $m$ of $Z$. In the present paper we prove that the monodromy representation on $H^m{(X;\mathbb{Q})}_{\perp Z}^{\mathrm{van}}$ for the family of smooth...

In this article, we introduce a special class of non complete webs, the NN-webs. We also study the algebraic and geometric properties of these webs.

Sia $X\subset \mathbb{P}\left(H^0{\left(X,L\right)}^{*}\right)$ una curva proeittiva e lissa, generali nel senso di Brill-Noether, indichiamo con ${\mathcal{R}}_4\left(X\right)$ l'insieme algebrico di quadrici di rango $4$ contenendo a $X$. In questo lavoro noi descriviamo birazionalmente i componenti irriducibile di ${\mathcal{R}}_4\left(X\right)$.