Counting the Ideals with a Given Genus of a Numerical Semigroup with Multiplicity Two

Abstract

Let S and T be two numerical semigroups. We say that T is an I(S)-semigroup if T∖{0} is an ideal of S. Given k a positive integer, we denote by Δ(k) the symmetric numerical semigroup generated by {2,2k+1}. In this paper we present a formula which calculates the number of I(S)-semigroups with genus g(Δ(k))+h for some nonnegative integer h and which we will denote by i(Δ(k),h). As a consequence, we obtain that the sequence {i(Δ(k),h)}h∈N is never decreasing. Besides, it becomes stationary from a certain term.