# Kolmogorov-Seliverstov theorem

If the condition

$$\sum_{n=1}^\infty(a_n^2+b_n^2)W(n)<\infty$$

holds with $W(n)=\log n$, then the Fourier series

$$\frac{a_0}{2}+\sum_{n=1}^\infty(a_n\cos nx+b_n\sin nx)$$

converges almost-everywhere. This was established by A.N. Kolmogorov and G.A. Seliverstov (see , ). In  it was actually proved that $W(n)$ can be taken to be $\log^{1+\delta}n$ for any $\delta>0$, and this statement was strengthened in , where its validity was proved for $\delta=0$ as well. This strong form was also obtained by A.I. Plessner . Prior to the Kolmogorov–Seliverstov theorem, the theorem (G.H. Hardy, 1916) was known with $W(n)=\log^2 n$. The Kolmogorov–Seliverstov theorem remained the strongest result in this direction until 1966, when the Carleson theorem was proved, according to which one can take $W(n)=1$. S. Kaczmarz  transferred the Kolmogorov–Seliverstov theorem from the trigonometric system to arbitrary orthonormal systems by proving that for the almost-everywhere convergence of series in such systems on some set, one can take for $W(n)$ a monotone majorant of the Lebesgue function on this set.

How to Cite This Entry:
Kolmogorov-Seliverstov theorem. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Kolmogorov-Seliverstov_theorem&oldid=32556
This article was adapted from an original article by S.A. Telyakovskii (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article