Sets with small l^1 Fourier norm

A famous conjecture of Littlewood states that the Fourier transform of every set of N integers has l^1 norm at least log(N), up to a constant multiplicative factor. This was proved independently by McGehee-Pigno-Smith and Konyagin in the 1980s. This lower bound is the best possible, as it is achieved by an arithmetic progression. An interesting question, especially from the perspective of additive combinatorics, is the 'inverse problem': what can we say about sets which are close to optimal, say with l^1 norm at most 100 log(N)? I will discuss an inverse result of this type, showing that (in a certain sense) such sets are approximately the union of O(1) sets with small doubling.



University of Oxford


Thomas Bloom