# Fixed-point subring

In algebra, the **fixed-point subring**
of an automorphism *f* of a ring *R* is the subring of the fixed points of *f*:

More generally, if *G* is a group acting on *R*, then the subring of *R*:

is called the **fixed subring** or, more traditionally, the **ring of invariants**. In Galois theory, when *R* is a field and *G* is a group of field automorphisms, the fixed ring is a subfield called the **fixed field** of the automorphism group; see Fundamental theorem of Galois theory.

Along with a module of covariants, the **ring of invariants** is a central object of study in invariant theory. Geometrically, the rings of invariants are the coordinate rings of (affine or projective) GIT quotients and they play fundamental roles in the constructions in geometric invariant theory.

**Example**: Let
be a polynomial ring in *n* variables. The symmetric group *S*_{n} acts on *R* by permutating the variables. Then the ring of invariants *R*^{G} is the ring of symmetric polynomials. If a reductive algebraic group *G* acts on *R*, then the fundamental theorem of invariant theory describes the generators of *R*^{G}.

Hilbert's fourteenth problem asks whether the ring of invariants is finitely generated or not (the answer is affirmative if *G* is a reductive algebraic group by Nagata's theorem.) The finite generation is easily seen for a finite group *G* acting on a finitely generated algebra *R*: since *R* is integral over *R*^{G},[1] the Artin–Tate lemma implies *R*^{G} is a finitely generated algebra. The answer is negative for some unipotent group.

Let *G* be a finite group. Let *S* be the symmetric algebra of a finite-dimensional *G*-module. Then *G* is a reflection group if and only if
is a free module (of finite rank) over *S*^{G} (Chevalley's theorem).

In differential geometry, if *G* is a Lie group and
its Lie algebra, then each principal *G*-bundle on a manifold *M* determines a graded algebra homomorphism (called the Chern–Weil homomorphism)

where
is the ring of polynomial functions on
and *G* acts on
by adjoint representation.

## See also

## Notes

- Given
*r*in*R*, the polynomial is a monic polynomial over*R*^{G}and has*r*as one of its roots.

## References

- Mukai, Shigeru; Oxbury, W. M. (8 September 2003) [1998],
*An Introduction to Invariants and Moduli*, Cambridge Studies in Advanced Mathematics,**81**, Cambridge University Press, ISBN 978-0-521-80906-1, MR 2004218 - Springer, Tonny A. (1977),
*Invariant theory*, Lecture Notes in Mathematics,**585**, Springer