Molien series

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In mathematics, a Molien series is a generating function attached to a linear representation ρ of a group G on a finite-dimensional vector space V. It counts the homogeneous polynomials of a given total degree d that are invariants for G. It is named for Theodor Molien.


More formally, there is a vector space of such polynomials, for each given value of d = 0, 1, 2, ..., and we write nd for its vector space dimension, or in other words the number of linearly independent homogeneous invariants of a given degree. In more algebraic terms, take the d-th symmetric power of V, and the representation of G on it arising from ρ. The invariants form the subspace consisting of all vectors fixed by all elements of G, and nd is its dimension.

The Molien series is then by definition the formal power series

This can be looked at another way, by considering the representation of G on the symmetric algebra of V, and then the whole subalgebra R of G-invariants. Then nd is the dimension of the homogeneous part of R of dimension d, when we look at it as graded ring. In this way a Molien series is also a kind of Hilbert function. Without further hypotheses not a great deal can be said, but assuming some conditions of finiteness it is then possible to show that the Molien series is a rational function. The case of finite groups is most often studied.


Molien showed that

This means that the coefficient of td in this series is the dimension nd defined above. It assumes that the characteristic of the field does not divide |G| (but even without this assumption, Molien's formula in the form is valid, although it does not help with computing M(t)).


Consider the symmetric group acting on R3 by permuting the coordinates. We add up the sum by group elements, as follows. Starting with the identity, we have


There is a three-element conjugacy class of , consisting of swaps of two coordinates. This gives three terms of the form


There is a two-element conjugacy class of cyclic permutations, yielding two terms of the form


Notice that different elements of the same conjugacy class yield the same determinant. Thus

On the other hand, we can expand the geometric series and multiply out to get

The coefficients of the series tell us the number of linearly independent homogeneous polynomials in three variables which are invariant under permutations of the three variables, i.e. the number of independent symmetric polynomials in three variables. In fact, if we consider the elementary symmetric polynomials

we can see for example that in degree 5 there is a basis consisting of and .

(In fact, if you multiply the series out by hand, you can see that the term comes from combinations of and exactly corresponding to combinations of and , also corresponding to partitions of with and as parts. See also Partition (number theory) and Representation theory of the symmetric group.)