“…gap> N := NumericalSemigroup(4,11,13,18);; gap> SmallElementsOfNumericalSemigroup(N); [ 0, 4,8,11,12,13,15 ] From basic results on irreducible numerical semigroups (see [6]) it follows that an irreducible numerical semigroup with 6 small elements must have Frobenius number 11 or 12. These can be computed as follows: Example 6.4. gap> n := 11;; gap> irrn := IrreducibleNumericalSemigroupsWithFrobeniusNumber(n);; gap> List(irrn,s->SmallElementsOfNumericalSemigroup(s)); [ [ 0, 5, 7, 8, 9, 10, 12 ], [ 0, 4,5,8,9,10,12 ], [ 0, 3, 6, 7, 9, 10, 12 ], [ 0, 6, 7, 8, 9, 10, 12 ], [ 0, 2, 4, 6, 8, 10, 12 ], [ 0, 4, 6, 8, 9, 10, 12 ] ] gap> n := 12;; gap> irrn := IrreducibleNumericalSemigroupsWithFrobeniusNumber(n);; gap> List(irrn,s->SmallElementsOfNumericalSemigroup(s)); [ [ 0, 7, 8, 9, 10, 11, 13 ], [ 0, 5, 8, 9, 10, 11, 13 ] ]…”