  For an algebraically closed field $K$ of characteristic zero and a
non-singular matrix $A\in \mbox{GL}_n(K)$, a semi-invariant polynomial of $A$
is defined to be a polynomial $p(x)=p(x_1,\dots,x_n)$ with coefficients in $K$
such that $p(xA)=\lambda p(x)$ for some $\lambda\in K$. In this article, we
classify all semi-invariant polynomials of $A$ in terms of a canonically
constructed basis that will be made precise in the text.
