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Introduction

The invariant theory part of OSCAR provides functionality for computing polynomial invariants of group actions, focusing on finite groups, tori, and linearly reductive groups, respectively.

The basic setting in this context consists of a group GG, a field KK, a vector space VV over KK of finite dimension n,n, and a representation ρ:G→GL(V)\rho: G \to \text{GL}(V) of GG on VV. The induced right action on the dual vector space V∗V^\ast,

V∗×G→V∗,(f,π)↦f   ⁣.   ⁣π:=f∘ρ(π),V^\ast \times G \to V^\ast, (f, \pi)\mapsto f \;\! . \;\! \pi := f\circ \rho(\pi),

extends to a right action of GG on the graded symmetric algebra

K[V]:=S(V∗)=⨁d≥0SdV∗K[V]:=S(V^*)=\bigoplus_{d\geq 0} S^d V^*

which preserves the grading.

Note

In OSCAR, group actions are by convention assumed to be right actions and we follow this convention with our definition above. Note, however, that the left action given by π   ⁣.   ⁣f:=f∘ρ(π−1)\pi \;\! . \;\! f := f \circ \rho(\pi^{-1}) is quite common in the literature.

The invariants of GG are the fixed points of the action defined above, its invariant ring is the graded subalgebra

K[V]G:={f∈K[V]∣f   ⁣.   ⁣π=f for any π∈G}⊂K[V].K[V]^G:=\{f\in K[V] \mid f \;\! . \;\! \pi =f {\text { for any }} \pi\in G\} \subset K[V].

Explicitly, fixing a basis of VV and its dual basis, say, {x1,…,xn}\{x_1, \dots, x_n\} of V∗V^*, we may identify GL⁡(V)≅GL⁡n(K)\operatorname{GL}(V) \cong \operatorname{GL}_n(K) and K[V]≅K[x1,…,xn]K[V]\cong K[x_1, \dots, x_n]. Then the action of an element π∈G\pi \in G with ρ(π)=(ai,j)\rho(\pi) = (a_{i, j}) on a polynomial f∈K[x1,…,xn]f\in K[x_1,\dots, x_n] is given as follows:

(f   ⁣.   ⁣π)(x1,…,xn)=f(∑ja1,jxj,…,∑jan,jxj).(f \;\! . \;\! \pi) (x_1, \dots, x_n) = f\bigl(\sum_j a_{1, j}x_j, \dots, \sum_j a_{n, j}x_j\bigr).

Accordingly, K[V]GK[V]^G may be regarded as a graded subalgebra of K[x1,…,xn]K[x_1, \dots, x_n]:

K[V]G≅K[x1,…,xn]G:={f∈K[x1,…,xn]∣f   ⁣.   ⁣π=f for any π∈G}.K[V]^G \cong K[x_1, \dots, x_n]^G :=\{f\in K[x_1, \dots, x_n] \mid f \;\! . \;\! \pi =f {\text { for any }} \pi\in G\}.

The main objective of invariant theory in OSCAR is the computation of KK-algebra generators for invariant rings.

Note

If K[V]GK[V]^G is finitely generated as a KK-algebra, then any minimal system of homogeneous generators is called a fundamental system of invariants for K[V]GK[V]^G. By Nakayama's lemma, the number of elements in such a system is uniquely determined as the embedding dimension of K[V]GK[V]^G. Similarly, the degrees of these elements are uniquely determined.

Note

If K[V]GK[V]^G is finitely generated as a KK-algebra, then K[V]GK[V]^G admits a graded Noether normalization, that is, a Noether normalization K[p1,…,pm]⊂K[V]GK[p_1, \dots, p_m] \subset K[V]^G with p1,…,pmp_1, \dots, p_m homogeneous. Given any such Noether normalization, p1,…,pmp_1, \dots, p_m is called a homogeneous system of parameters or a system of primary invariants for K[V]GK[V]^G, and any minimal system s0=1,s1,…,sls_0=1, s_1,\dots, s_l of homogeneous generators of K[V]GK[V]^G as a K[p1,…,pm]K[p_1, \dots, p_m]-module is called a system of secondary invariants for K[V]GK[V]^G with respect to p1,…,pmp_1, \dots, p_m. A secondary invariant si≠1s_i\neq 1 is called irreducible if it cannot be written as a polynomial expression in the primary invariants and the other secondary invariants. The irreducible secondary invariants form a minimal system of homogeneous generators for K[V]GK[V]^G as a K[p1,…,pm]K[p_1, \dots, p_m]-algebra. Somewhat abusing notation, we call every minimal system of homogeneous generators for K[V]GK[V]^G as a K[p1,…,pm]K[p_1, \dots, p_m]-algebra a system of irreducible secondary invariants.

Note

For the invariant rings handled by OSCAR, the assumption that K[V]GK[V]^G is finitely generated as a KK-algebra will be guaranteed by theoretical results. In addition, where not mentioned otherwise, the following will hold:

  • There exists a Reynolds operator R:K[V]→K[V]\mathcal R: K[V] \to K[V]. That is, R\mathcal R is a KK-linear graded map which projects K[V]K[V] onto K[V]GK[V]^G, and which is a K[V]GK[V]^G-module homomorphism.
  • The ring K[V]GK[V]^G is Cohen-Macaulay. Equivalently, K[V]GK[V]^G is a free module (of finite rank) over any of its graded Noether normalizations.

The textbook

and the survey article

provide details on theory and algorithms as well as references.

Contact

Please direct questions about this part of OSCAR to the following people:

You can ask questions in the OSCAR Slack.

Alternatively, you can raise an issue on github.