Cohomologie non ramifiée et conjecture de Hodge entière

Details Cohomologie non ramifiée et conjecture de Hodge entière Cohomologie non ramifiée et conjecture de Hodge entière

2010-05-16

arxiv.org/abs/1005.2778v2

Mathematics

Algebraic Geometry

Final version, 48 pages, to appear in Duke Mathematical Journal

Scientific paper

Building upon the Bloch-Kato conjecture in Milnor K-theory, we relate the third unramified cohomology group with Q/Z coefficients with a group which measures the failure of the integral Hodge conjecture in degree 4. As a first consequence, a geometric theorem of the second-named author implies that the third unramified cohomology group with Q/Z coefficients vanishes on all uniruled threefolds. As a second consequence, a 1989 example by Ojanguren and the first named author implies that the integral Hodge conjecture in degree 4 fails for unirational varieties of dimension at least 6. For certain classes of threefolds fibered over a curve, we establish a relation between the integral Hodge conjecture and the computation of the index of the generic fibre. En nous appuyant sur la conjecture de Bloch-Kato en K-th\'eorie de Milnor, nous \'etablissons un lien g\'en\'eral entre le d\'efaut de la conjecture de Hodge enti\`ere pour la cohomologie de degr\'e 4 et le troisi\`eme groupe de cohomologie non ramifi\'e \`a coefficients Q/Z. Ceci permet de montrer que sur un solide unir\'egl\'e le troisi\`eme groupe de cohomologie non ramifi\'e \`a coefficients Q/Z s'annule, ce que la K-th\'eorie alg\'ebrique ne permet d'obtenir que dans certains cas. Ceci permet \`a l'inverse de d\'eduire d'exemples ayant leur source en K-th\'eorie que la conjecture de Hodge enti\`ere pour la cohomologie de degr\'e 4 peut \^etre en d\'efaut pour les vari\'et\'es rationnellement connexes. Pour certaines familles \`a un param\`etre de surfaces, on \'etablit un lien entre la conjecture de Hodge enti\`ere et l'indice de la fibre g\'en\'erique.

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