Compter (rapidement) le nombre de solutions d'\'equations dans les corps finis

The number of solutions in finite fields of a system of polynomial equations obeys a very strong regularity, reflected for example by the rationality of the zeta function of an algebraic variety defined over a finite field, or the modularity of Hasse-Weil's $L$-function of an elliptic curve over $\Q$. Since two decades, efficient methods have been invented to compute effectively this number of solutions, notably in view of cryptographic applications. This expos\'e presents some of these methods, generally relying on the use of Lefshetz's trace formula in an adequate cohomology theory and discusses their respective advantages. ----- Le nombre de solutions dans les corps finis d'un syst\`eme d'\'equations polynomiales ob\'eit \`a une tr\`es forte r\'egularit\'e, refl\'et\'ee par exemple par la rationalit\'e de la fonction z\^eta d'une vari\'et\'e alg\'ebrique sur un corps fini, ou la modularit\'e de la fonction $L$ de Hasse-Weil d'une courbe elliptique sur $\Q$. Depuis une vingtaine d'ann\'ees des m\'ethodes efficaces ont \'et\'e invent\'ees pour calculer effectivement ce nombre de solutions, notamment en vue d'applications \`a la cryptographie. L'expos\'e en pr\'esentera quelques-unes, g\'en\'eralement fond\'ees l'utilisation de la formule des traces de Lefschetz dans une th\'eorie cohomologique convenable, et expliquera leurs avantages respectifs.

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