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On the properties of sums of trigonometric series with monotone coefficients. (English. Russian original) Zbl 0881.42004

Mosc. Univ. Math. Bull. 50, No. 3, 19-27 (1995); translation from Vestn. Mosk. Univ., Ser. I 1995, No. 3, 22-32 (1995).
The authors, among others, extend a well-known theorem of Hardy-Littlewood to the case \(0<p\leq 1\). Let \(f(x)\) and \(g(x)\) denote the sums of the series \({a_0\over 2}+ \sum^\infty_{n=1} a_n\cos nx\) and \(\sum^\infty_{n=1} a_n\sin nx\), respectively, where \(a_n\to 0\) and \(\Delta_ka_n\geq 0\) for some \(k\geq 1\) and any \(n\). The new theorem reads as follows:
a) If \(\Delta_2a_n\geq 0\), then for any \(p\in(0,\infty)\), \[ C_1\left(\sum^\infty_{n=0}(\Delta_1a_n)^p(n+ 1)^{2p-2}\right)^{1/p}\leq|f|_p\leq C_2\Biggl(\sum^\infty_{n=0}(\Delta_1 a_n)^p(n+ 1)^{2p-2}\Biggr)^{1/p}. \] b) If \(\Delta_1a_n\geq 0\), then for any \(p\in(0,\infty)\), \[ C_3\Biggl(\sum^\infty_{n=1} a^p_n n^{p-2}\Biggr)^{1/p}\leq|g|_p\leq C_4\Biggl(\sum^\infty_{n=1} a^p_n n^{p-2}\Biggr)^{1/p}, \] where the positive constants \(C_i\) do not depend on \(\{a_n\}\). Unfortunately in the English translation there are several upsetting misprints.

MSC:

42A32 Trigonometric series of special types (positive coefficients, monotonic coefficients, etc.)
42A05 Trigonometric polynomials, inequalities, extremal problems
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