Let F/F_q be an algebraic function field of genus g defined over a finite field F_q. We obtain new results on the existence, the number and the density of dimension zero divisors of degree g-k in F where k is a positive integer. In particular, for q=2,3 we prove that there always exists a dimension zero divisor of degree gamma-1 where gamma is the q-rank of F. We also give a necessary and sufficient condition for the existence of a dimension zero divisor of degree g-k for a hyperelliptic field F in terms of its Zeta function.
We study the existence of non-special divisors of degree $g$ and $g-1$ for algebraic function fields of genus $ggeq 1$ defined over a finite field $F_q$. In particular, we prove that there always exists an effective non-special divisor of degree $ggeq 2$ if $qgeq 3$ and that there always exists a non-special divisor of degree $g-1geq 1$ if $qgeq 4$. We use our results to improve upper and upper asymptotic bounds on the bilinear complexity of the multiplication in any extension $F_{q^n}$ of $F_q$, when $q=2^rgeq 16$.
In previous papers we define certain Lagrangian shadows of ample divisors in algebraic varieties. In the present brief note an existence condition is discussed for these Lagrangian shadows.
We provide in this paper an upper bound for the number of rational points on a curve defined over a one variable function field over a finite field. The bound only depends on the curve and the field, but not on the Jacobian variety of the curve.
Let $C$ be a smooth projective curve over $mathbb{F}_q$ with function field $K$, $E/K$ a nonconstant elliptic curve and $phi:mathcal{E}to C$ its minimal regular model. For each $Pin C$ such that $E$ has good reduction at $P$, i.e., the fiber $mathcal{E}_P=phi^{-1}(P)$ is smooth, the eigenvalues of the zeta-function of $mathcal{E}_P$ over the residue field $kappa_P$ of $P$ are of the form $q_P^{1/2}e^{itheta_P},q_{P}e^{-itheta_P}$, where $q_P=q^{deg(P)}$ and $0letheta_Plepi$. The goal of this note is to determine given an integer $Bge 1$, $alpha,betain[0,pi]$ the number of $Pin C$ where the reduction of $E$ is good and such that $deg(P)le B$ and $alphaletheta_Plebeta$.
We prove two theorems concerning isogenies of elliptic curves over function fields. The first one describes the variation of the height of the $j$-invariant in an isogeny class. The second one is an isogeny estimate, providing an explicit bound on the degree of a minimal isogeny between two isogenous elliptic curves. We also give several corollaries of these two results.
Stephane Ballet
,Christophe Ritzenthaler
,Robert Rolland
.
(2009)
.
"On the existence of dimension zero divisors in algebraic function fields defined over F_q"
.
Christophe Ritzenthaler
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