The fundamental group at infinity of affine surfaces.
The fundamental group of a Galois cover of .
The fundamental group of a locally finite graph with ends-a hyperfinite approach
The end compactification |Γ| of a locally finite graph Γis the union of the graph and its ends, endowed with a suitable topology. We show that π₁(|Γ|) embeds into a nonstandard free group with hyperfinitely many generators, i.e. an ultraproduct of finitely generated free groups, and that the embedding we construct factors through an embedding into an inverse limit of free groups. We also show how to recover the standard description of π₁(|Γ|) given by Diestel and Sprüssel (2011). Finally, we give...
The Fundamental Group of the Complement of a Union of Complex Hyperplanes.
The fundamental group of the complement of a union of complex hyperplanes: correction.
The Fundamental Group of the Complement of an Algebraic Curve.
The Fundamental Group-Scheme of an Abelian Variety.
The Hodge de Rham theory of relative Malcev completion
The intrinsic Hodge theory of -adic hyperbolic curves.
The Lefshetz Vanishing Cycles on a Projective Nonsingular Plane Curve.
The Mumford conjecture
The Mumford Conjecture asserts that the rational cohomology of the stable moduli space of Riemann surfaces is a polynomial algebra on the Mumford-Morita-Miller characteristic classes; this can be reformulated in terms of the classifying space derived from the mapping class groups. The conjecture admits a topological generalization, inspired by Tillmann’s theorem that admits an infinite loop space structure after applying Quillen’s plus construction. The text presents the proof by Madsen and...
The rational homotopy theory of smooth, complex projective varieties
The tame fundamental group of an abelian variety and integral points
The topological Hawaiian earring group does not embed in the inverse limit of free groups.
Théorème de Van Kampen pour les champs algébriques
Théorèmes de Lefschetz en cohomologie des faisceaux cohérents et en cohomologie étale. Application au groupe fondamental
Three-manifolds and Kähler groups
We give a simple proof of a result originally due to Dimca and Suciu: a group that is both Kähler and the fundamental group of a closed three-manifold is finite. We also prove that a group that is both the fundamental group of a closed three-manifold and of a non-Kähler compact complex surface is or .
Un théorème du type de Lefschetz
Le complémentaire d’un ensemble algébrique dans un espace projectif complexe de dimension et la trace de ce complémentaire sur un hyperplan assez général ont mêmes groupes d’homotopie jusqu’à l’ordre . Cela généralise un théorème de H. Hamm et Lê Dũng Tráng sur le complémentaire d’une hypersurface projective.
Une démonstration du théorème de Zariski sur les sections hyperplanes d'une hypersurface projective et du théorème de Van Kampen sur le groupe fondamental du complémentaire d'une courbe projective plane
Universal covering spaces and fundamental groups in algebraic geometry as schemes
In topology, the notions of the fundamental group and the universal cover are closely intertwined. By importing usual notions from topology into the algebraic and arithmetic setting, we construct a fundamental group family from a universal cover, both of which are schemes. A geometric fiber of the fundamental group family (as a topological group) is canonically the étale fundamental group. The constructions apply to all connected quasicompact quasiseparated schemes. With different methods and hypotheses,...