Projective image (reviewed)

The projective image of a weight one newform is the image of its associated projective Galois representation $\rho\colon \Gal(\overline\Q/\Q)\to \PGL_2(\C)$. It is a finite subgroup of $\PGL_2(\C)$ that can be classified as one of four types: It is either isomorphic to a dihedral group $D_n$ for some integer $n\ge 2$ (where $D_2:=C_2\times C_2$ is the Klein group), or to one of $A_4, S_4, A_5$, where $A_n$ and $S_n$ respectively denote the alternating and symmetric groups on $n$ letters.