We solve the sup-norm problem for spherical Hecke–Maaß newforms of square-free level for the group GL(2) over a number field, with a power saving over the local geometric bound simultaneously in the eigenvalue and the level aspect. Our bounds feature a Weyl-type exponent in the level aspect, they reproduce or improve upon all known special cases, and over totally real fields they are as strong as the best known hybrid result over the rationals.
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Valentin Blomer, Gergely Harcos, Péter Maga, Djordje Milićević, The sup-norm problem for GL(2) over number fields. J. Eur. Math. Soc. 22 (2020), no. 1, pp. 1–53DOI 10.4171/JEMS/916