On holomorphic Artin L-functions

Let $K/\mathbb Q$ be a finite Galois extension, $s_0\in \mathbb C\setminus \{1\}$, ${\it Hol}(s_0)$ the semigroup of Artin L-functions holomorphic at $s_0$. If the Galois group is almost monomial then Artin's L-functions are holomorphic at $s_0$ if and only if ${\it Hol}(s_0)$ is factorial. This holds also if $s_0$ is a zero of an irreducible L-function of dimension $\leq 2$, without any condition on the Galois group.