Let $\overline{\rho}_{\Delta,\ell}$ be the mod$\ell$ representation associated to Ramanujan's $\Delta$function. It is wellknown 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"?
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"Eisenstein" ? 

