We give a short proof that for each multiplicative subgroup of finite index in , the set of integers with is an IP-set. This generalizes a theorem of Hildebrand concerning completely multiplicative functions taking values in the -th roots of unity.
"On a theorem of Hildebrand." Mosc. J. Comb. Number Theory 8 (2) 189 - 191, 2019. https://doi.org/10.2140/moscow.2019.8.189