We prove strong hybrid subconvex bounds simultaneously in the $q$ and $t$ aspects for $L$-functions of selfdual $\mathrm{ GL}_3$ cusp forms twisted by primitive Dirichlet characters. We additionally prove analogous hybrid subconvex bounds for central values of certain $\mathrm{ GL}_3 \times \mathrm{ GL}_2$ Rankin-Selberg $L$-functions. The subconvex bounds that we obtain are strong in the sense that, modulo current knowledge on estimates for the second moment of $\mathrm{ GL}_3$ $L$-functions, they are the natural limit of the first moment method pioneered by Li and by Blomer. The method of proof relies on an explicit $\mathrm{ GL}_3 \times \mathrm{ GL}_2 \leftrightsquigarrow \mathrm{ GL}_4 \times \mathrm{ GL}_1$ spectral reciprocity formula, which relates a $\mathrm{ GL}_2$ moment of $\mathrm{ GL}_3 \times \mathrm{ GL}_2$ Rankin-Selberg $L$-functions to a $\mathrm{ GL}_1$ moment of $\mathrm{ GL}_4 \times \mathrm{ GL}_1$ Rankin-Selberg $L$-functions. A key additional input is a Lindel\"of-on-average upper bound for the second moment of Dirichlet $L$-functions restricted to a coset, which is of independent interest.
Categories: math.NT