Pointwise convergence of Fourier series with respect to multiplicative systems
Vestnik Moskovskogo universiteta. Matematika, mehanika, no. 2 (1983), pp. 37-42
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We study the pointwise convergence of Fourier series with respect to multiplicative Vilenkin systems. We derive some two-sided estimates of Dirichlet kernels. We find analogies of the Dini condition for the convergence of the Fourier series at some point $x$. In particular, we show that, whenever the condition $$ \int_G\frac{|f(x\dotplus t)+f(x\overset{.}-t)-2f(x)|}{t}\,dt<\infty $$ guarantees the convergence of the Fourier series $f(x)$ at $x$ the same is not true of the condition $$ \int_G\frac{|f(x\dotplus t)-f(x)|}{t}\,dt<\infty $$ (for unbounded systems).