An algebraic analogue of the concept of a local Lie group (cf. Lie group, local). The theory of formal groups has numerous applications in algebraic geometry, class field theory and cobordism theory.
A formal group over a field is a group object in the category of connected affine formal schemes over (see , , , ). Let be the functor that associates with an algebra the set of algebra homomorphism from some Noetherian commutative local -algebra with maximal ideal and field of residues , complete in the -adic topology, such that the homomorphisms map into the set of nilpotent elements of . Then a connected affine formal scheme is a covariant functor from the category of finite-dimensional commutative -algebras into the category of sets that is isomorphic to an . That is a group object means that there is a group structure given on all the sets such that for every -algebra homomorphism the corresponding mapping is a group homomorphism. If all the groups are commutative, then the formal group is said to be commutative. Every connected group scheme over defines a formal group . Here one can take as the completion of the local ring of at the unit element.
If is the ring of formal power series in variables over , then is called an -dimensional formal Lie group. For a connected algebraic group over , is a formal Lie group. A formal Lie group is isomorphic, as a functor in the category of sets, to the functor that associates with an algebra the -fold Cartesian product of its nil radical with itself. The group structure on the sets is given by a formal group law — a collection of formal power series in variables :
satisfying the following conditions:
Here , , . This group law on the sets is given by the formulas
where ; because and are nilpotent, all except a finite number of terms of the series are zero. Every formal group law gives group structures on by means of
and converts the functor into a formal Lie group. The concept of a formal group law, and thus of a formal Lie group, can be generalized to the case of arbitrary commutative ground rings (see , ). Sometimes by a formal group one means just a formal Lie group or even a formal group law.
Just as for local Lie groups (cf. Lie group, local) one can define the Lie algebra of a formal Lie group. Over fields of characteristic 0 the correspondence between a formal Lie group and its Lie algebra defines an equivalence of the respective categories. In characteristic the situation is more complicated. Thus, over an algebraically closed field (for ) there is a countable number of pairwise non-isomorphic one-dimensional commutative formal Lie groups , while all one-dimensional Lie algebras are isomorphic . Over perfect fields of finite characteristic, commutative formal Lie groups are classified by means of Dieudonné modules (see , ).
The theory of formal groups over fields can be generalized to the case of arbitrary formal ground schemes .
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A universal formal group law (for -dimensional formal group laws) is an -dimensional formal group law , , such that for every -dimensional formal group law over a ring there is a unique homomorphism of rings such that . Here denotes the result of applying to the coefficients of the power series . Universal formal group laws exist and are unique in the sense that if over is another one, then there exists a ring isomorphism such that .
For commutative formal group laws explicit formulas are available for the construction of universal formal group laws, cf. [a3]. The underlying ring is a ring of polynomials in infinitely many indeterminates (Lazard's theorem).
A homomorphism of formal group laws , , , is an -tuple of power series in -variables , , such that . The homomorphism is an isomorphism if there exists an inverse homomorphism such that , and it is a strict isomorphism of formal group laws if (higher order terms).
Let be a ring of characteristic zero, i.e. the homomorphism of rings which sends to the unit element in is injective. Then is injective. Over all commutative formal group laws are strictly isomorphic and hence isomorphic to the additive formal group law
It follows that for every commutative formal group law over there exists a unique -tuple of power series , with coefficients in such that
where is the "inverse function" to , i.e. . This is called the logarithm of the group law .
The formal group law of complex cobordism is a universal one-dimensional formal group law (Quillen's theorem) and its logarithm is given by Mishchenko's formula
Combined with the explicit construction of a one-dimensional universal group law these facts yield useful information on the generators of the complex cobordism ring . Cf. Cobordism for more details.
Let be an -dimensional group law over . A curve over in is an -tuple of power series in one variable such that . Two curves can be added by . The set of curves is given the natural power series topology and there results a commutative topological group . The group admits a number of operators , , , , defined as follows:
where is a primitive -th root of unity. There are a number of relations between these operators and they combine to define a (non-commutative) ring , which generalizes the Dieudonné ring, cf. Witt vector for the latter. Cartier's second and third theorems on formal group laws say that the modules classify formal groups and they characterize which groups occur as 's. This is the covariant classification of commutative formal groups in contrast with the earlier contravariant classification of commutative formal groups over perfect fields by Dieudonné modules.
Let be the functor of Witt vectors (cf. Witt vector). Let be the (one-dimensional) multiplicative formal group law over . Then . Cartier's first theorem for formal group laws says that the functor is representable. More precisely, let be the (infinite-dimensional) formal group law given by the addition formulas of the Witt vectors and let be the curve . Then for every formal group law and curve there is unique homomorphism of formal group laws such that .
There exists a Pontryagin-type duality between commutative formal groups and commutative affine (algebraic) groups over a field , called Cartier duality. Cf. [a3], [a4] for more details. Correspondingly, Dieudonné modules are also important in the classification of commutative affine (algebraic) groups. Essentially, Cartier duality comes from the "duality" between algebras and co-algebras; cf. Co-algebra.
Let be a discrete valuation ring with finite residue field and maximal ideal . The Lubin–Tate formal group law associated to is defined by the logarithm
Then has its coefficients in . These formal group laws are in a sense formal -adic analogues of elliptic curves with complex multiplication in that they have maximally large endomorphism rings. They are also analogues in the role they play vis à vis the class field theory of , the quotient field of . Indeed, let be the set with the addition . Here is the maximal ideal of the ring of integers of an algebraic closure of . Then a maximal Abelian totally ramified extension of is generated by the torsion elements of ; cf. [a3], [a5] for more details.
Formal groups also are an important tool in algebraic geometry, especially in the theory of Abelian varieties. This holds even more so for a generalization: -divisible groups; cf. -divisible group.
Lazard's theorem on one-dimensional formal group laws says that all one-dimensional formal group laws over a ring without nilpotents are commutative.
Let be a one-dimensional formal group law. Define inductively , , . Let be defined over a field of characteristic . Then is necessarily of the form (higher degree terms) or is equal to zero. The positive integer is called the height of ; if , the height of is taken to be . Over an algebraically closed field of characteristic the one-dimensional formal group laws are classified by their heights, and all heights occur.
Let be a one-dimensional formal group law over a ring in which every prime number except is invertible, e.g. is the ring of integers of a local field of residue characteristic or is a field of characteristic . Assume for the moment that is of characteristic zero and let
be the logarithm of . Then is strictly isomorphic over to the formal group law whose logarithm is equal to
The result extends to the case that is not of characteristic zero and to more-dimensional commutative formal group laws. is called the -typification of .
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Formal group. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Formal_group&oldid=21866