MUCH use is made in combinatorial problems of generating functions in the form of polynomials and infinite power series, these being obtained by the manipulation of other algebraic expressions. In ...

Let K[x,y] be the polynomial algebra in two variables over a field K of characteristic 0. A subalgebra Rof K[x,y] is called a retract if there is an idempotent homomorphism (a retraction, or ...