Double trigonometric series with lacunar sign-constant coefficients / A. Zh. Ydyrys. //Vestnik Moskovskogo Universiteta. Seriya 1. Matematika. Mekhanika. 2012. № 6. P. 33-35 [Moscow Univ. Math. Bulletin. Vol. 67, N 6, 2012.].
The article presents an analogue of Sidon's theorem for series of the form ∑∞k=1 ∑∞n=0ak,n cosmkx cosny, where the coefficients ak,n have a constant sign for any fixed k.
Key words: double trigonometric series, lacunary series, Sidon's theorem.