Molien's formula

Let ${\displaystyle \chi _{1},\dots ,\chi _{r}}$ denote the irreducible characters of a finite group G and V, R as above. Then the character ${\displaystyle \chi _{R_{n}}}$ of ${\displaystyle R_{n}}$ can be written as:

${\displaystyle \chi _{R_{n}}=\sum _{i=1}^{r}a_{i,n}\chi _{i}.}$

Here, each ${\displaystyle a_{i,n}}$ is given by the inner product:

${\displaystyle a_{i,n}=\langle \chi _{R_{n}},\chi _{i}\rangle =(\#G)^{-1}\sum _{g\in G}{\overline {\chi _{i}}}(g)\chi _{R_{n}}(g)=(\#G)^{-1}\sum _{g\in G}{\overline {\chi _{i}}}(g)\sum _{|\alpha |=n}\lambda (g)^{\alpha }}$

where ${\textstyle \lambda (g)^{\alpha }=\prod _{i=1}^{m}\lambda _{i}(g)^{\alpha _{i}}}$ and ${\displaystyle \lambda _{1}(g),\dots ,\lambda _{m}(g)}$ are the possibly repeated eigenvalues of ${\displaystyle g:V^{*}\to V^{*}}$. Now, we compute the series:

${\displaystyle {\begin{aligned}\sum _{n=0}^{\infty }a_{i,n}t^{n}&=(\#G)^{-1}\sum _{g\in G}{\overline {\chi _{i}}}(g)\sum _{\alpha }(\lambda _{1}(g)t)^{\alpha _{1}}\cdots (\lambda _{m}(g)t)^{\alpha _{m}}\\&=(\#G)^{-1}\sum _{g\in G}{\overline {\chi _{i}}}(g)(1-\lambda _{1}(g)t)^{-1}\cdots (1-\lambda _{m}(g)t)^{-1}\\&=(\#G)^{-1}\sum _{g\in G}{\overline {\chi _{i}}}(g)\det(1-tg|V^{*})^{-1}.\end{aligned}}}$

Taking ${\displaystyle \chi _{i}}$ to be the trivial character yields Molien's formula.

Consider the symmetric group ${\displaystyle S_{3}}$ acting on R³ by permuting the coordinates. We add up the sum by group elements, as follows. Starting with the identity, we have

${\displaystyle \det {\begin{pmatrix}1-t&0&0\\0&1-t&0\\0&0&1-t\end{pmatrix}}=(1-t)^{3}}$.

There is a three-element conjugacy class of ${\displaystyle S_{3}}$, consisting of swaps of two coordinates. This gives three terms of the form

${\displaystyle \det {\begin{pmatrix}1&-t&0\\-t&1&0\\0&0&1-t\end{pmatrix}}=(1-t)(1-t^{2}).}$

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

${\displaystyle \det {\begin{pmatrix}1&-t&0\\0&1&-t\\-t&0&1\end{pmatrix}}=\left(1-t^{3}\right).}$

Notice that different elements of the same conjugacy class yield the same determinant. Thus, the Molien series is

${\displaystyle M(t)={\frac {1}{6}}\left({\frac {1}{(1-t)^{3}}}+{\frac {3}{(1-t)(1-t^{2})}}+{\frac {2}{1-t^{3}}}\right)={\frac {1}{(1-t)(1-t^{2})(1-t^{3})}}.}$

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

${\displaystyle M(t)=(1+t+t^{2}+t^{3}+\cdots )(1+t^{2}+t^{4}+\cdots )(1+t^{3}+t^{6}+\cdots )=1+t+2t^{2}+3t^{3}+4t^{4}+5t^{5}+6t^{6}+7t^{7}+9t^{8}+11t^{9}+\cdots }$

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

${\displaystyle \sigma _{1}=x+y+z}$

${\displaystyle \sigma _{2}=xy+xz+yz}$

${\displaystyle \sigma _{3}=xyz}$

we can see for example that in degree 5 there is a basis consisting of ${\displaystyle \sigma _{3}\sigma _{2}}$, ${\displaystyle \sigma _{3}\sigma _{1}^{2}}$, ${\displaystyle \sigma _{2}^{2}\sigma _{1}}$, ${\displaystyle \sigma _{1}^{3}\sigma _{2}}$, and ${\displaystyle \sigma _{1}^{5}}$.

(In fact, if you multiply the series out by hand, you can see that the ${\displaystyle t^{k}}$ term comes from combinations of ${\displaystyle t}$, ${\displaystyle t^{2}}$, and ${\displaystyle t^{3}}$ exactly corresponding to combinations of ${\displaystyle \sigma _{1}}$, ${\displaystyle \sigma _{2}}$, and ${\displaystyle \sigma _{3}}$, also corresponding to partitions of ${\displaystyle k}$ with ${\displaystyle 1}$, ${\displaystyle 2}$, and ${\displaystyle 3}$ as parts. See also Partition (number theory) and Representation theory of the symmetric group.)

• David A. Cox, John B. Little, Donal O'Shea (2005), Using Algebraic Geometry, pp. 295–8
• Molien, Th. (1897). "Uber die Invarianten der linearen Substitutionsgruppen". Sitzungber. Konig. Preuss. Akad. Wiss. (J. Berl. Ber.). 52: 1152–1156. JFM 28.0115.01.
• Mukai, S. (2002). An introduction to invariants and moduli. Cambridge Studies in Advanced Mathematics. Vol. 81. ISBN 978-0-521-80906-1.
• Stanley, Richard P. (1979). "Invariants of finite groups and their applications to combinatorics". Bull. Amer. Math. Soc. New Series. 1: 475–511. doi:10.1090/S0273-0979-1979-14597-X. MR 0526968.

• https://mathoverflow.net/questions/58283/a-question-about-an-application-of-moliens-formula-to-find-the-generators-and-r