1. S. V. Konyagin, “On a problem of Littlewood,” Math. USSR-Izv. 18 (2), 205–225 (1982).
2. O. C. McGehee, L. Pigno and B. Smith, “Hardy’s inequality and the L norm of exponential sums,” Ann. of Math. (2) 113 (3), 613–618 (1981).
3. V. N. Temlyakov, Multivariate Approximation, in Cambridge Monogr. Appl. Comput. Math. (Cambridge Univ. Press, Cambridge, 2018), Vol. 32.
4. B. Green and S. Konyagin, “On the Littlewood problem modulo prime,” Canad. J. Math., No. 1, 141–164 (2009).
5. T. Sanders, “The Littlewood–Gowers problem,” J. Anal. Math., No. 101, 123–162 (2007).