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In mathematics, the '''fundamental group scheme''' is a [[group scheme]] canonically associated to a [[Scheme (mathematics)|scheme]] over a Dedekind scheme (e.g. the spectrum of a [[field (mathematics)|field]] or the spectrum of a [[discrete valuation ring]]). It is a generalisation of the [[étale fundamental group]]. Although its existence was conjectured by [[Alexander Grothendieck]], the first construction is due to Madhav Nori,<ref>M. V. Nori ''On the Representations of the Fundamental Group'', Compositio Mathematica, Vol. 33, Fasc. 1, (1976), p. 29-42</ref><ref>T. Szamuely ''Galois Groups and Fundamental Groups.'' Cambridge Studies in Advanced Mathematics, Vol. 117 (2009)</ref> who only worked on schemes over fields. A generalisation to schemes over Dedekind schemes is due to Carlo Gasbarri.<ref>C. Gasbarri, ''Heights of Vector Bundles and the Fundamental Group Scheme of a Curve'', Duke Mathematical Journal, Vol. 117, No. 2, (2003) p. 287-311</ref> | |||
==First definition== | |||
Let <math>k</math> be a perfect field and <math>X\to \text{Spec}(k)</math> a faithfully flat and proper morphism of schemes with <math>X</math> a reduced and connected scheme. Assume the existence of a section <math>x:\text{Spec}(k)\to X</math>, then the fundamental group scheme <math>\pi_1(X,x)</math> of <math>X</math> in <math>x</math> is defined as the affine group scheme naturally associated to the neutral [[tannakian category]] (over <math>k</math>) of [[essentially finite vector bundle]]s over <math>X</math>. | |||
==Second definition== | |||
Let <math>S</math> be a Dedekind scheme, <math>X</math> any connected scheme (not necessarily reduced)<ref>M. Antei, ''The fundamental group scheme of a non reduced scheme'', Bulletin des Sciences Mathématiques, Volume 135, Issue 5, July–August 2011, Pages 531-539.</ref> and <math>X\to S</math> a faithfully flat morphism of finite type (not necessarily proper). Assume the existence of a section <math>x:S\to X</math>. Once we prove that the [[Category (mathematics)|category]] of isomorphism classes of [[torsor]]s over <math>X</math> (pointed over <math>x</math>) under the action of finite and flat <math>S</math>-[[group scheme]]s is cofiltered then we define the universal torsor (pointed over <math>x</math>) as the projective limit of all the torsors of that category. The <math>S</math>-group scheme acting on it is called the fundamental group scheme and denoted by <math>\pi_1(X,x)</math> (when <math>S</math> is the spectrum of a perfect field the two definitions coincide so that no confusion can arise). | |||
==See also== | |||
*[[Étale fundamental group]] | |||
*[[Fundamental group]] | |||
==Notes== | |||
<references/> | |||
[[Category:Scheme theory]] | |||
[[Category:Topological methods of algebraic geometry]] | |||
Revision as of 10:35, 8 November 2013
In mathematics, the fundamental group scheme is a group scheme canonically associated to a scheme over a Dedekind scheme (e.g. the spectrum of a field or the spectrum of a discrete valuation ring). It is a generalisation of the étale fundamental group. Although its existence was conjectured by Alexander Grothendieck, the first construction is due to Madhav Nori,[1][2] who only worked on schemes over fields. A generalisation to schemes over Dedekind schemes is due to Carlo Gasbarri.[3]
First definition
Let be a perfect field and a faithfully flat and proper morphism of schemes with a reduced and connected scheme. Assume the existence of a section , then the fundamental group scheme of in is defined as the affine group scheme naturally associated to the neutral tannakian category (over ) of essentially finite vector bundles over .
Second definition
Let be a Dedekind scheme, any connected scheme (not necessarily reduced)[4] and a faithfully flat morphism of finite type (not necessarily proper). Assume the existence of a section . Once we prove that the category of isomorphism classes of torsors over (pointed over ) under the action of finite and flat -group schemes is cofiltered then we define the universal torsor (pointed over ) as the projective limit of all the torsors of that category. The -group scheme acting on it is called the fundamental group scheme and denoted by (when is the spectrum of a perfect field the two definitions coincide so that no confusion can arise).
See also
Notes
- ↑ M. V. Nori On the Representations of the Fundamental Group, Compositio Mathematica, Vol. 33, Fasc. 1, (1976), p. 29-42
- ↑ T. Szamuely Galois Groups and Fundamental Groups. Cambridge Studies in Advanced Mathematics, Vol. 117 (2009)
- ↑ C. Gasbarri, Heights of Vector Bundles and the Fundamental Group Scheme of a Curve, Duke Mathematical Journal, Vol. 117, No. 2, (2003) p. 287-311
- ↑ M. Antei, The fundamental group scheme of a non reduced scheme, Bulletin des Sciences Mathématiques, Volume 135, Issue 5, July–August 2011, Pages 531-539.