A series with respect to a lacunary system of functions. Examples are lacunary trigonometric series; lacunary power series; Rademacher series (cf. Rademacher system); series of independent functions with zero mathematical expectations; Dirichlet series with exponents that are independent over the field of rational numbers; etc.
Lacunary series. V.F. Emel'yanov (originator), Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Lacunary_series&oldid=15249