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Molien Series


Molien Series






Let R=K[V]^G be the invariant ring of the group G over the field K.
If G is a matrix group in the non-modular case or
a permutation group (in either the modular or non-modular case)
then the Molien series of G yields the Hilbert Series of R.



MolienSeries(G) : GrpMat -> FldFunUElt


MolienSeries(G) : GrpPerm -> FldFunUElt



The Molien series of G, returned as an element of the rational
function field Z(t).  If G is a permutation group, the Molien
series always exists and equals the Hilbert series of the invariant
ring of G for any field.  If G is a matrix group, the characteristic
of the coefficient field of G must be coprime with the order of G.





Example RngInvar_MolienSeries (H30E5)

We compute the Molien series of a matrix G and verify that the coefficients
of the corresponding power series match the number of independent invariants
for each degree.





> K<z> := CyclotomicField(5);
> w := -z^3 - z^2;
> G := MatrixGroup<3,K |
>    [1,0,-w, 0,0,-1, 0,1,-w],
>    [-1,-1,w, -w,0,w, -w,0,1]>;
> M<t> := MolienSeries(G);
> M;
(-t^8 - t^7 + t^5 + t^4 + t^3 - t - 1)/(t^11 + t^10 - 
    t^9 - 2*t^8 - t^7 + t^4 + 2*t^3 + t^2 - t - 1)
> P<u> := PowerSeriesRing(IntegerRing());
> P ! M;
1 + u^2 + u^4 + 2*u^6 + 2*u^8 + 3*u^10 + 4*u^12 + 
    4*u^14 + u^15 + 5*u^16 + u^17 + 6*u^18 + u^19 + 
    O(u^20)
> Coefficients(P ! M);
[ 1, 0, 1, 0, 1, 0, 2, 0, 2, 0, 3, 0, 4, 0, 4, 1, 5, 1,
6, 1 ]
> time [#InvariantsOfDegree(G, i): i in [0 .. 19]];
[ 1, 0, 1, 0, 1, 0, 2, 0, 2, 0, 3, 0, 4, 0, 4, 1, 5, 1,
6, 1 ]





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