Semi-invariant of a quiver

In mathematics, the ring of semi-invariants is a subring of the coordinate ring of a quiver that containing those functions which are invariant under the action of a certain algebraic group, up to a character of that group. Specifically, given a representation of a quiver with vertices Q0, there is a natural action of the algebraic group Πi∈Q0 GL(d(i)) by simultaneous base change.