In algebra, the Hochster–Roberts theorem, introduced by Melvin Hochster and Joel L. Roberts in 1974,[1] states that rings of invariants of linearly reductive groups acting on regular rings are Cohen–Macaulay.
In other words,[2] if V is a rational representation of a linearly reductive group G over a field k, then there exist algebraically independent invariant homogeneous polynomials such that is a free finite graded module over .
![{\displaystyle k[V]^{G}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/46cff3c2e34ea58c55ce70a7ddfc7ed4c1ad390d)
![{\displaystyle k[f_{1},\cdots ,f_{d}]}](https://wikimedia.org/api/rest_v1/media/math/render/svg/3211271992cc493961e3e71ce3c4941bc6f9b0cc)
In 1987, Jean-François Boutot proved[3] that if a variety over a field of characteristic 0 has rational singularities then so does its quotient by the action of a reductive group; this implies the Hochster–Roberts theorem in characteristic 0 as rational singularities are Cohen–Macaulay.
In characteristic p>0, there are examples of groups that are reductive (or even finite) acting on polynomial rings whose rings of invariants are not Cohen–Macaulay.
References edit
- ^ Hochster, Melvin; Roberts, Joel L. (1974). "Rings of invariants of reductive groups acting on regular rings are Cohen-Macaulay". Advances in Mathematics. 13 (2): 115–175. doi:10.1016/0001-8708(74)90067-X. ISSN 0001-8708. MR 0347810.
- ^ Mumford, David; Fogarty, John; Kirwan, Frances (1994), Geometric invariant theory. Third edition., Ergebnisse der Mathematik und ihrer Grenzgebiete 2. Folge (Results in Mathematics and Related Areas (2)), vol. 34, Springer-Verlag, Berlin, ISBN 3-540-56963-4, MR 1304906 p. 199
- ^ Boutot, Jean-François (1987). "Singularités rationnelles et quotients par les groupes réductifs". Inventiones Mathematicae. 88 (1): 65–68. doi:10.1007/BF01405091. ISSN 0020-9910. MR 0877006.
 | This algebra-related article is a stub. You can help Wikipedia by expanding it. |