On some special classes of complex elliptic curves

In this paper we classify the complex elliptic curves $E$ for which there exist cyclic subgroups $C\leq (E,+)$ of order $n$ such that the elliptic curves $E$ and $E/C$ are isomorphic, where $n$ is a positive integer. Important examples are provided in the last section. Moreover, we answer the following question: given a complex elliptic curve E, when can one find a cyclic subgroup $C$ of order $n$ of $(E,+)$ such that $(E,C)\sim(\frac{E}{C},\frac{E[n]}{C})$, $E[n]$ being the $n$-torsion subgroup of $E$, classifying in this way the fixed points of the action of the Fricke involution on the open modular curves $Y_0(n)$