Let H be a numerical semigroup. We consider the problem that whether H is the semigroup of a ramification point on a double covering of a cuve or not. We denote by d2(H) the semigroup generated by the elements h with 2h ∈ H. We solve the problem, when d2(H) is either N0 or a 2-semigroup. In the case d2(H) =〈3, 4, 5〉we give the answer to this problem for any H except three semigroups.
Jiryo Komeda (Fri,) studied this question.