Shintani relation for base change: unitary and elliptic representations

Let $E/F$ be a cyclic extension of $p$-adic fields and $n$ a positive integer. Arthur and Clozel constructed a base change process $π\mapsto π_E$ which associates to a smooth irreducible representation of $GL_n(F)$ a smooth irreducible representation of $GL_n(E)$, invariant under $Gal(E/F)$. When $π$ is tempered, $π_E$ is tempered and is characterized by an identity (the Shintani character relation) relating the character of $π$ to the character of $π_E$ twisted by the action of $Gal(E/F)$. In this paper we show that the Shintani relation also holds when $π$ is unitary or elliptic. We prove similar results for the extension $C/R$. As a corollary we show that for a cyclic extension $E/F$ of number fields the base change for automorphic residual representations of the adèle group $GL_n(A_F)$ respects the Shintani relation at each place of $F$.

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