Bases of subalgebras of K[[x]] and K[x]

Let $f\_1,\ldots, f\_s$ be formal power series (respectively polynomials) in thevariable $x$. We study the semigroup of orders of the formal series inthe algebra $K[[ f1,\ldots, f\_s]] \subseteq K[[ x ]]$ (respectively the semigroup of degrees of polynomials in$K[f\_1,\ldots,f\_s]\subseteq K[x]$). We give procedures to compute thesesemigroups and several applications.