For any trigonometric series with coefficients tending to zero, the convergence or divergence of the series at some point depends on the behaviour of the so-called Riemann function in a neighbourhood of this point.
The Riemann function
is the result of integrating it twice, that is,
There is a generalization of the localization principle for series with coefficients that do not tend to zero (see [2]).
[1] | N.K. [N.K. Bari] Bary, "A treatise on trigonometric series" , Pergamon (1964) (Translated from Russian) |
[2] | A. Zygmund, "Trigonometric series" , 1 , Cambridge Univ. Press (1988) |