Adamov, A. N. “Inequality of the Turan Type for Trigonometric Polynomials and Conjugate Trigonometric Polynomials in $L_0$”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 61, no. 7, July 2009, pp. 986-95,
https://umj.imath.kiev.ua/index.php/umj/article/view/3073.