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Autori: A. Dimca, S. Papadima, A.I. Suciu
Editorial: Duke Mathematical Journal, 148 (3), p.405-457, 2009.
We elucidate the key role played by formality in the theory of characteristic and resonance varieties. We define relative characteristic and resonance varieties, V_k and R_k, related to twisted group cohomology with coefficients of arbitrary rank. We show that the germs at the origin of V_k and R_k are analytically isomorphic if the group is 1-formal; in particular, the tangent cone to V_k at 1 equals R_k. These new obstructions to 1-formality lead to a striking rationality property of the usual resonance varieties. A detailed analysis of the irreducible components of the tangent cone at 1 to the first characteristic variety yields powerful obstructions to realizing a finitely presented group as the fundamental group of a smooth, complex quasi-projective algebraic variety. This sheds new light on a classical problem of J.-P. Serre. Applications to arrangements, configuration spaces, coproducts of groups, and Artin groups are given
Cuvinte cheie: 1-formal group, holonomy Lie algebra, Malcev completion, Alexander invariant, cohomology support loci, resonance variety, tangent cone, smooth quasi-projective variety, arrangement, configuration space, Artin group