Subalgebras in K[x] of small codimension
We introduce the concept of subalgebra spectrum, Sp(A), for a subalgebra A of finite codimension in K[x]. The spectrum is a finite subset of the underlying field. We also introduce a tool, the characteristic polynomial of A, which has the spectrum as its set of zeroes. The characteristic polynomial can be computed from the generators of A, thus allowing us to find the spectrum of an algebra given