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A generalization of the concept of an [[Algebraic group|algebraic group]]. Let <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g0452901.png" /> be the category of schemes over a ground scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g0452902.png" />; a [[Group object|group object]] of this category is known as a group scheme over the scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g0452903.png" /> (or a group <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g0452905.png" />-scheme, or an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g0452907.png" />-scheme group). For a group scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g0452908.png" /> over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g0452909.png" /> the functor of points <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529010.png" /> is a contravariant functor from the category <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529011.png" /> into the category of groups <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529012.png" />. The category <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529013.png" /> of group schemes over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529014.png" /> is defined as the complete subcategory of the category of such functors formed by the representable functors (cf. [[Representable functor|Representable functor]]).
+
A generalization of the concept of an
 +
[[Algebraic group|algebraic group]]. Let ${\rm Sch}/S$ be the category of
 +
schemes over a ground scheme $S$; a
 +
[[Group object|group object]] of this category is known as a group
 +
scheme over the scheme $S$ (or a group $S$-scheme, or an $S$-scheme
 +
group). For a group scheme $G$ over $S$ the functor of points $h_G:X\to{\rm Hom}_{\rm Sch/S}(X,G)=G(X)$ is a
 +
contravariant functor from the category ${\rm Sch}/S$ into the category of
 +
groups ${\rm Gr}$. The category $S-{\rm Gr}$ of group schemes over $S$ is defined as
 +
the complete subcategory of the category of such functors formed by
 +
the representable functors (cf.
 +
[[Representable functor|Representable functor]]).
  
 
===Examples.===
 
===Examples.===
  
  
1) An algebraic group over a field <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529015.png" /> is a reduced group scheme of finite type over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529016.png" />. (A reduced group scheme of finite type over a field is sometimes referred to as an algebraic group.)
+
1) An algebraic group over a field $k$ is a reduced group scheme of
 +
finite type over $k$. (A reduced group scheme of finite type over a
 +
field is sometimes referred to as an algebraic group.)
  
2) A functor which assigns to an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529017.png" />-scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529018.png" /> the additive (or multiplicative) group of the ring of sections of the structure sheaf <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529019.png" /> is representable. The corresponding group scheme over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529020.png" /> is said to be the additive (or multiplicative) group scheme, and is denoted by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529021.png" /> (or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529022.png" />). For any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529023.png" />-scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529024.png" /> one has
+
2) A functor which assigns to an $S$-scheme $X$ the additive (or
 +
multiplicative) group of the ring of sections of the structure sheaf
 +
$\Gamma(X,{\mathcal O}_X)$ is representable. The corresponding group scheme over $S$ is said
 +
to be the additive (or multiplicative) group scheme, and is denoted by
 +
$G_{\alpha,S}$ (or $G_{m,S}$). For any $S$-scheme $S_1$ one has
 +
$$G_{\alpha,S}\times_S S_1 \simeq G_{\alpha,S_1},\qquad G_{m,S}\times_S S_1 \simeq G_{m,S_1}$$
 +
3) Each abstract
 +
group $\Gamma$ defines a group scheme $(\Gamma)_S$, which is the direct sum of a
 +
family of schemes $(S_g)_{g\in G}$, each one of which is isomorphic to $S$. The
 +
corresponding functor maps an $S$-scheme $X$ to the direct sum $\Gamma^{\pi_0(X)}$,
 +
where $\pi_0(X)$ is the set of connected components of $X$.
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529025.png" /></td> </tr></table>
+
If $G$ is a group scheme over $S$ then, for any point $s\in S$, the fibre
 +
$G_s = G\otimes_S k(s)$ is a group scheme over the residue field $k(s)$ of this point. In
 +
particular, any group scheme of finite type over $S$ can be regarded
 +
as a family of algebraic groups parametrized by the base $S$. The
 +
terminology of the theory of schemes is extended to group schemes;
 +
thus, one speaks of smooth, flat, finite, and singular group schemes.
  
3) Each abstract group <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529026.png" /> defines a group scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529027.png" />, which is the direct sum of a family of schemes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529028.png" />, each one of which is isomorphic to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529029.png" />. The corresponding functor maps an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529030.png" />-scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529031.png" /> to the direct sum <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529032.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529033.png" /> is the set of connected components of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529034.png" />.
+
For any group scheme $G$ the corresponding reduced scheme $G{\rm red}$ is also
 +
a group scheme; the canonical closed imbedding $G{\rm red} \to G$ is a morphism of
 +
group schemes. Each reduced group scheme of locally finite type over a
 +
perfect field is smooth. Each reduced group scheme of locally finite
 +
type over a field of characteristic zero is reduced (Cartier's
 +
theorem).
  
If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529035.png" /> is a group scheme over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529036.png" /> then, for any point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529037.png" />, the fibre <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529038.png" /> is a group scheme over the residue field <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529039.png" /> of this point. In particular, any group scheme of finite type over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529040.png" /> can be regarded as a family of algebraic groups parametrized by the base <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529041.png" />. The terminology of the theory of schemes is extended to group schemes; thus, one speaks of smooth, flat, finite, and singular group schemes.
+
Many concepts and results in the theory of algebraic groups have their
 +
analogues for group schemes. Thus, there exists an analogue of the
 +
structure theory of Borel–Chevalley for affine algebraic groups
 +
[[#References|[5]]], and a cohomology theory of extensions of group
 +
schemes and homogeneous spaces over group schemes has been developed
 +
[[#References|[2]]],
 +
[[#References|[5]]]. On the other hand, many problems and results
 +
specific to the theory of group schemes are connected with the
 +
presence of nilpotent elements in the structure sheaf of both the
 +
ground scheme and the group scheme itself. Thus, infinitesimal and
 +
formal deformations of group schemes
 +
[[#References|[4]]], problems of lifting into zero characteristic, and
 +
formal completion of group schemes (cf.
 +
[[Formal group|Formal group]]) have all been studied. Group schemes
 +
arise in a natural manner in the study of algebraic groups over a
 +
field of positive characteristic (cf.
 +
[[P-divisible group|$p$-divisible group]]).
  
For any group scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529042.png" /> the corresponding reduced scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529043.png" /> is also a group scheme; the canonical closed imbedding <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529044.png" /> is a morphism of group schemes. Each reduced group scheme of locally finite type over a perfect field is smooth. Each reduced group scheme of locally finite type over a field of characteristic zero is reduced (Cartier's theorem).
+
The concept of an affine group scheme over an affine ground scheme $S={\rm Spec}\;(B)$
 +
is dual to the concept of a commutative
 +
[[Hopf algebra|Hopf algebra]]; this is the case if $G={\rm Spec}\;(A)$ is a group
 +
scheme for which $A$ is a commutative Hopf algebra.
  
Many concepts and results in the theory of algebraic groups have their analogues for group schemes. Thus, there exists an analogue of the structure theory of Borel–Chevalley for affine algebraic groups [[#References|[5]]], and a cohomology theory of extensions of group schemes and homogeneous spaces over group schemes has been developed [[#References|[2]]], [[#References|[5]]]. On the other hand, many problems and results specific to the theory of group schemes are connected with the presence of nilpotent elements in the structure sheaf of both the ground scheme and the group scheme itself. Thus, infinitesimal and formal deformations of group schemes [[#References|[4]]], problems of lifting into zero characteristic, and formal completion of group schemes (cf. [[Formal group|Formal group]]) have all been studied. Group schemes arise in a natural manner in the study of algebraic groups over a field of positive characteristic (cf. [[P-divisible group|<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529045.png" />-divisible group]]).
+
See also
 
+
[[Commutative group scheme|Commutative group scheme]];
The concept of an affine group scheme over an affine ground scheme <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529046.png" /> is dual to the concept of a commutative [[Hopf algebra|Hopf algebra]]; this is the case if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529047.png" /> is a group scheme for which <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/g/g045/g045290/g04529048.png" /> is a commutative Hopf algebra.
+
[[Finite group scheme|Finite group scheme]].
 
 
See also [[Commutative group scheme|Commutative group scheme]]; [[Finite group scheme|Finite group scheme]].
 
  
 
====References====
 
====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top"> J. Tate,   F. Oort,   "Group schemes of prime order" ''Ann. Sc. Ecole Norm. Sup.'' , '''3''' (1970) pp. 1–21</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top"> M. Demazure,   P. Gabriel,   "Groupes algébriques" , '''1''' , Masson (1970)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top"> F. Oort,   "Commutative group schemes" , ''Lect. notes in math.'' , '''15''' , Springer (1966)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top"> F. Oort,   "Finite group schemes, local moduli for abelian varieties and lifting problems" ''Compos. Math.'' , '''23''' (1971) pp. 256–296</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top"> M. Demazure,   A. Grothendieck,   "Schémas en groupes I-III" , ''Lect. notes in math.'' , '''151–153''' , Springer (1970)</TD></TR></table>
+
<table><TR><TD valign="top">[1]</TD> <TD
 +
valign="top"> J. Tate, F. Oort, "Group schemes of prime order"
 +
''Ann. Sc. Ecole Norm. Sup.'' , '''3''' (1970)
 +
pp. 1–21</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">
 +
M. Demazure, P. Gabriel, "Groupes algébriques" , '''1''' , Masson
 +
(1970)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">
 +
F. Oort, "Commutative group schemes" , ''Lect. notes in math.'' ,
 +
'''15''' , Springer (1966)</TD></TR><TR><TD valign="top">[4]</TD> <TD
 +
valign="top"> F. Oort, "Finite group schemes, local moduli for abelian
 +
varieties and lifting problems" ''Compos. Math.'' , '''23''' (1971)
 +
pp. 256–296</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">
 +
M. Demazure, A. Grothendieck, "Schémas en groupes I-III" ,
 +
''Lect. notes in math.'' , '''151–153''' , Springer
 +
(1970)</TD></TR></table>
  
  
  
 
====Comments====
 
====Comments====
Other examples of group schemes are Abelian (group) varieties.
+
Other examples of group schemes are Abelian (group)
 +
varieties.
  
 
====References====
 
====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top"> D. Mumford,   "Abelian varieties" , Oxford Univ. Press (1974)</TD></TR></table>
+
<table><TR><TD valign="top">[a1]</TD> <TD
 +
valign="top"> D. Mumford, "Abelian varieties" , Oxford Univ. Press
 +
(1974)</TD></TR></table>

Revision as of 10:16, 12 September 2011

A generalization of the concept of an algebraic group. Let ${\rm Sch}/S$ be the category of schemes over a ground scheme $S$; a group object of this category is known as a group scheme over the scheme $S$ (or a group $S$-scheme, or an $S$-scheme group). For a group scheme $G$ over $S$ the functor of points $h_G:X\to{\rm Hom}_{\rm Sch/S}(X,G)=G(X)$ is a contravariant functor from the category ${\rm Sch}/S$ into the category of groups ${\rm Gr}$. The category $S-{\rm Gr}$ of group schemes over $S$ is defined as the complete subcategory of the category of such functors formed by the representable functors (cf. Representable functor).

Examples.

1) An algebraic group over a field $k$ is a reduced group scheme of finite type over $k$. (A reduced group scheme of finite type over a field is sometimes referred to as an algebraic group.)

2) A functor which assigns to an $S$-scheme $X$ the additive (or multiplicative) group of the ring of sections of the structure sheaf $\Gamma(X,{\mathcal O}_X)$ is representable. The corresponding group scheme over $S$ is said to be the additive (or multiplicative) group scheme, and is denoted by $G_{\alpha,S}$ (or $G_{m,S}$). For any $S$-scheme $S_1$ one has $$G_{\alpha,S}\times_S S_1 \simeq G_{\alpha,S_1},\qquad G_{m,S}\times_S S_1 \simeq G_{m,S_1}$$ 3) Each abstract group $\Gamma$ defines a group scheme $(\Gamma)_S$, which is the direct sum of a family of schemes $(S_g)_{g\in G}$, each one of which is isomorphic to $S$. The corresponding functor maps an $S$-scheme $X$ to the direct sum $\Gamma^{\pi_0(X)}$, where $\pi_0(X)$ is the set of connected components of $X$.

If $G$ is a group scheme over $S$ then, for any point $s\in S$, the fibre $G_s = G\otimes_S k(s)$ is a group scheme over the residue field $k(s)$ of this point. In particular, any group scheme of finite type over $S$ can be regarded as a family of algebraic groups parametrized by the base $S$. The terminology of the theory of schemes is extended to group schemes; thus, one speaks of smooth, flat, finite, and singular group schemes.

For any group scheme $G$ the corresponding reduced scheme $G{\rm red}$ is also a group scheme; the canonical closed imbedding $G{\rm red} \to G$ is a morphism of group schemes. Each reduced group scheme of locally finite type over a perfect field is smooth. Each reduced group scheme of locally finite type over a field of characteristic zero is reduced (Cartier's theorem).

Many concepts and results in the theory of algebraic groups have their analogues for group schemes. Thus, there exists an analogue of the structure theory of Borel–Chevalley for affine algebraic groups [5], and a cohomology theory of extensions of group schemes and homogeneous spaces over group schemes has been developed [2], [5]. On the other hand, many problems and results specific to the theory of group schemes are connected with the presence of nilpotent elements in the structure sheaf of both the ground scheme and the group scheme itself. Thus, infinitesimal and formal deformations of group schemes [4], problems of lifting into zero characteristic, and formal completion of group schemes (cf. Formal group) have all been studied. Group schemes arise in a natural manner in the study of algebraic groups over a field of positive characteristic (cf. $p$-divisible group).

The concept of an affine group scheme over an affine ground scheme $S={\rm Spec}\;(B)$ is dual to the concept of a commutative Hopf algebra; this is the case if $G={\rm Spec}\;(A)$ is a group scheme for which $A$ is a commutative Hopf algebra.

See also Commutative group scheme; Finite group scheme.

References

[1] J. Tate, F. Oort, "Group schemes of prime order"

Ann. Sc. Ecole Norm. Sup. , 3 (1970)

pp. 1–21
[2]

M. Demazure, P. Gabriel, "Groupes algébriques" , 1 , Masson

(1970)
[3]

F. Oort, "Commutative group schemes" , Lect. notes in math. ,

15 , Springer (1966)
[4] F. Oort, "Finite group schemes, local moduli for abelian

varieties and lifting problems" Compos. Math. , 23 (1971)

pp. 256–296
[5]

M. Demazure, A. Grothendieck, "Schémas en groupes I-III" , Lect. notes in math. , 151–153 , Springer

(1970)


Comments

Other examples of group schemes are Abelian (group) varieties.

References

[a1] D. Mumford, "Abelian varieties" , Oxford Univ. Press (1974)
How to Cite This Entry:
Group scheme. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Group_scheme&oldid=19577
This article was adapted from an original article by I.V. Dolgachev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article