On counting numerical semigroups by maximum primitive and Wilf's conjecture

Abstract: We introduce a new way of counting numerical semigroups, namely by their maximum primitive, and show its relation with the counting of numerical semigroups by their Frobenius number. For any positive integer $n$, let $A_{n}$ denote the number of numerical semigroups whose maximum primitive is $n$, and let $N_{n}$ denote the number of numerical semigroups whose Frobenius number is $n$. We show that the sequences $(A_{n})$ and $(N_{n})$ are M\"obius transforms of one another. We also establish that almost all numerical semigroups with large enough maximum primitive satisfy Wilf's conjecture. A crucial step in the proof is a result of independent interest: a numerical semigroup $S$ with multiplicity $\mathrm{m}$ such that $|S\cap (\mathrm{m},2 \mathrm{m})|\geq \sqrt{3\mathrm{m}}$ satisfies Wilf's conjecture.