This talk could interest both analysts and number theorists. I will first present 8 variants of Hilbert transforms, with a focus on the techniques used in the study of these operators. Then I will show how to use Fourier analysis tools to reduce a number theory problem (Roth theorem) to an algebraic geometry problem: this joint work Li and Sawin fully answers a question of Bourgain and Chang about the number of three-term polynomial progressions in subsets of finite fields.