On \(\gamma\)-hyperelliptic numerical semigroups
Let \(H\) be a numerical semigroup. The genus of \(H\) is the number of elements of the complement of \(H\) in the natural numbers. \(H\) is called \(\gamma\)-hyperelliptic if \(H\) has \(\gamma\) even elements in \([2,4\gamma]\), and the \((\gamma+1)\)-th element is \(4\gamma+2\). For a point \(P\) of projective, irreducible, non-singular algebraic curve \(X\) over an algebraically closed field \(k\), let \(H(P)\) be the set of orders \(m_i\) at \(P\) of meromorphic functions on \(X\) with only pole at \(P\), called the Weierstrass semigroup of \(X\) at \(P\), and \(w(P)\) be the weight of \(P\). There are numerical semigroups which are not Weierstrass semigroups. The author extends the following equivalent results on Weierstrass semigroups \(H(P)\) at ramified points of a double covering \(\pi:X\) (of genus \(g)\to\widehat X\) (of genus \(\gamma)\) to any numerical semigroup \(H\) of genus large enough: 1. \(H(P)\) is \(\gamma\)-hyperelliptic if \(\text{char} (k)\neq 2\) and \(g\geq 4\gamma+1\), and \(g\geq 6\gamma-3\) otherwise. 2. if \(g\geq 5\gamma+1\), then \(m_{2\gamma+1} =6\gamma+ 2\). 3. \(m_{g/2- \gamma-1}=g-2\). 4. \({g-2\gamma \choose 2}\leq w(P)< {g-2\gamma+2 \choose 2}\) to: 2'. if \(g\geq 6\gamma +4\), then \(m_{2\gamma+1}(H)=6\gamma+2\) or 2''. In the case \(g=6\gamma +5\) or \(g\geq 6\gamma+7\), if \(g\) is even then \(m_r(H)=g-2\), and if \(g\) is odd, then \(m_r(H)=g-1\), where \(r=[(g+1)/2]- \gamma-1\). 3''. \(m_r(H) \leq g-1 <m_{r+1} (H)\). The author also improves results on weights of (Weierstrass) semigroups.
\(\gamma\)-hyperelliptic numerical semigroup
scientific article; zbMATH DE number 1106540