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Abstract: This article consists of two independent, but related, parts. The first one proves the vanishing of the Chow group of classes of zero-cycles of degree zero modulo rational equivalence for a cubic hypersurface of dimension ⩾10 on a p-adic or field (and, in fact, the R-triviality of such a hypersurface). This is done without the assumption of good reduction (or even smoothness). The second part goes in the other direction and gives an explicit example of a smooth cubic hypersurface of dimension 3 (necessarily of bad reduction) on a field such as (or ) whose Chow group of classes of zero-cycles of degree zero modulo rational equivalence does not vanish. [Copyright &y& Elsevier]

Abstract: Let Dumont has conjectured the identity , which generalizes, in particular, the classical results of Lagrange, Gauß, Jacobi and Kronecker on the sums of two, three and four squares. We give a combinatorial proof of Dumont''s conjecture. To cite this article: B. Lass, C. R. Acad. Sci. Paris, Ser. I 341 (2005). [Copyright &y& Elsevier]

Abstract: In this Note, we give a geometric proof of Kolchin''s Theorem, using resolution of singularities. We give a counter-example in positive characteristic. To cite this article: J. Nicaise, J. Sebag, C. R. Acad. Sci. Paris, Ser. I 341 (2005). [Copyright &y& Elsevier]

Thanks to results of A. Beauville (Acta Math. 164 (1990) 211–235), we give an explicit description of translation-invariant vector fields on affine Jacobians of spectral curves, which gives a mathematical support for the work of F.A. Smirnov and V. Zeitlin (preprint math-ph/0203037). In the hyperelliptic case, this description is due to D. Mumford (Tata Lectures on Theta, Vol. II, Birkha¨user, Boston, 1983). To cite this article: B. Fu, C. R. Acad. Sci. Paris, Ser. I 337 (2003). [Copyright &y& Elsevier]