Abstract:
It is proved that if the continuous periodic function $f$ has bounded $\Phi$-variation,
then the deviation of $f$ from the sum of $n$ terms of its Fourier series has the bound
$$
||f-S_n(f)||\leqslant c\int_0^{\omega(\pi n^{-1})}\log(v_\Phi(f)/\Phi(\xi))d\xi.
$$
Here $c$ is an absolute constant, $\omega$ is the modulus of continuity, $v_\Phi(f)$
is the complete $\Phi$-variation of $f$ over a period.
It is established that the Salem and Garsia–Sawyer criteria for the uniform convergence
of the Fourier series in terms of the $\Phi$-variation and the Banach indicatrix
respectively are definitive, and it is proved that the second of these variants
is a corrolary of the first.