## Intégration sur un cycle évanescent.(French)Zbl 0538.13007

For f a formal power series in n indeterminates $$x_ 1,...,x_ n$$ over a field $$k$$, let $$I(f)$$ be the diagonal power series $$\Sigma a_ i,...,i^{t^ i}$$. The author proves that if f is an algebraic function of $$x_ 1,...,x_ n$$ and that k is of characteristic $$p>0$$, then $$I(f)$$ is algebraic in t. Results of the same nature had been obtained by H. Furstenberg [J. Algebra 7, 271-277 (1967; Zbl 0175.039)] and J. Denef and L. Lipschitz [”Some remarks on algebraic power series” (preprint)] recently succeeded in extending Furstenberg’s method to cover the case treated here. The aim of the present paper is to offer a geometric explanation of Furstenberg’s results, in terms of what in substance should be p-adic properties of vanishing cycles. In an appendix, signs in the coherent duality formalism are discussed.

### MSC:

 13F25 Formal power series rings 14C99 Cycles and subschemes 14F30 $$p$$-adic cohomology, crystalline cohomology 13J05 Power series rings

Zbl 0175.039
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### References:

 [1] Furstenberg, H.: Algebraic functions over finite fields. J. of Algebra7, 271-277 (1967) · Zbl 0175.03903 [2] Raynaud, M.: Spécialisation du foncteur de Picard. Publ. Math. IHES38, 27-76 (1970) · Zbl 0207.51602 [3] Raynaud, M.: Anneaux locaux henséliens. Lecture Notes in Mathematics, vol. 169. Berlin-Heidelberg-New York: Springer 1970 · Zbl 0203.05102
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