On the symmetric powers of cusp forms on GL (2) of icosahedral type

In this note we study the symmetric powers of strongly modular icosahedral representations $\rho$ of ${\rm Gal} (\bar{F}/F)$, $F$ a number field, and their twisted $L$--functions. We prove that for such $\rho$, there exists a cuspidal automorphic representation $\Pi = \Pi_{\infty} \otimes \Pi_{f}$ of $GL_{6} (\mathbb{A}_{F})$ such that $L (s, {\rm sym}^{5} (\rho)) = L (s, \Pi_{f})$. One sees that ${\rm sym}^{5} (\rho)$ is twist equivalent to $\rho' \otimes {\rm sym}^{2} (\rho)$ for another modular icosahedral representation $\rho'$, and our theorem is a special case of a cuspidality criterion formulated and proved in this paper, which may be of independent interest, for the Kim--Shahidi automorphic tensor product $\pi \boxtimes {\rm sym}^{2} (\pi')$, where $\pi$ and $\pi'$ are cuspidal automorphic representations of $GL (2) / F$. We also give a complete structure theory of modular icosahedral representations. As a result, we prove that $L (s, {\rm sym}^{m} (\rho) \otimes \chi)$ does not admit any Landau--Siegel zero when it is not divisible by $L$--functions of quadratic characters. In general, there is no such divisibility and and there are no Landau--Siegel zeros for such $L$--functions.