A numerical semigroup is a subset of N containing 0, closed under addition and with finite complement in N. An important example of numerical semigroup is given by the Weierstrass semigroup at one point of a curve. In the theory of algebraic geometry codes, Weierstrass semigroups are crucial for defining bounds on the minimum distance as well as for defining improvements on the dimension of codes. We present these applications and some theoretical problems related to classification, characterization and counting of numerical semigroups.Notice that a symmetric numerical semigroup can not be pseudo-symmetric. Next lemma as well as its proof is analogous to Lemma 10.
Lemma 12.A numerical semigroup Λ with odd conductor c is pseudo-symmetric if and only if for any non-negative integer i different from (c − 1)/2, if i is a gap, then c − 1 − i is a non-gap. 9