Let $W(N)$ be the group of Atkin-Lehner involutions on the modular curve $X_0(N)$. The purpose of this article is to give complementary result to [7, 8, 9]; namely, we determine trigonal curves of the form $X_0(N)/W'$, where $W'$ is a subgroup of $W(N)$ such that $2< |W'| < |W(N)|$.

We consider a holomorphic curve from the complex plane into the complex projective space of odd dimension and give some results on truncated defects when the truncated defect relation is extremal.