Splitting the difference: Computations of the Reynolds operator in classical invariant theory

If $G$ is a linearly reductive group acting rationally on a polynomial ring $S$, then the inclusion $S^G\hookrightarrow S$ possesses a unique $G$-equivariant splitting, called the Reynolds operator. We describe algorithms for computing the Reynolds operator for the classical actions as in Weyl’s book. The groups are the general linear group, the special linear group, the orthogonal group, and the symplectic group, with their classical representations: direct sums of copies of the standard representation and copies of the dual representation.