Let $\overline{\rho}\_{\Delta,\ell}$ be the mod-$\ell$ representation associated to Ramanujan's $\Delta$-function. It is well-known that (the semisimplification of) this representation is reducible if, say, $\ell=5$ or $\ell=691$. Is there a general name for primes like this? Serre calls them (in a more general context) "exceptional primes," but the word exceptional always strikes me as vague. "Primes of residual reducibility"?



A name for primes where residual Galois representations are reducible
"Eisenstein" ? 


