# Invariant theory

My question might be an easy or could be a bit complicate and classic. Actually I am trying to understand why the discriminant of a binary quadratic form is a "the fundamental invariant" under $GL(2,\mathbb{Z})$-action i.e any other invariant is a polynomial of the discriminant. Also I am interested to know about general binary forms. More precisely I would like to know the number of fundamental invariant of binary form of degree n under $GL(2,\mathbb{Z})$-action.

• @Ryan: Well, I guessed I have explained it. I want to know how many fundamental invariants exists for the $GL(2,\mathbb{Z})$-action on binary form of degree n. For example for binary quadratic and cubic form, there exists only one invariant which is the discriminant. I hope I have made it clear.
– M.B
Commented Dec 13, 2010 at 5:49
• Dear Ryan, I don't think the question is vague, although I don't have a good answer off the top of my head. Commented Dec 13, 2010 at 6:13
• Dear Qiaochu, A fundamental invariant is a member of a generating set for the full ring of invariants. Commented Dec 13, 2010 at 6:14
• @Denis Serre: if I may, I wonder whether your comment is predicated on a conflation of two different notions of "fundamental system of invariants". You seem to be intending it in the sense "a collection of properties of an object which collectively determine it up to isomorphism", whereas the OP means something very different: "the subring of elements fixed under the action of a group on a [polynomial, classically] ring". (Along with Emerton, I don't find the question to be vague. In fact, I would be interested in reading answers to it.) Commented Dec 13, 2010 at 8:28
• You should have a look at "Classical Invariant Theory" by Olver. Commented Dec 13, 2010 at 8:47

• Perhaps one should add to this that the polynomial invariants for $\mathrm{GL}(2,\mathbb Z)$ are the same as for its Zariski closure which consists of the matrices whose determinant has a square equal to $1$. This connects up the question to classical invarant theory. Commented Dec 13, 2010 at 19:26