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A Cantor-Lebesgue theorem with variable ``coefficients'
Authors:J. Marshall Ash   Gang Wang   David Weinberg
Affiliation:Department of Mathematics, DePaul University, Chicago, Illinois 60614-3504 ; Department of Mathematics, DePaul University, Chicago, Illinois 60614-3504 ; Department of Mathematics, Texas Tech University, Lubbock, Texas 79409-1042
Abstract:If ${phi _n}$ is a lacunary sequence of integers, and if for each $n$, $% c_n(x)$ and $c_{-n}(x)$ are trigonometric polynomials of degree $n,$ then $% {c_n(x)}$ must tend to zero for almost every $x$ whenever $% {c_n(x)e^{iphi _nx}+c_{-n}(-x)e^{-iphi _nx}}$ does. We conjecture that a similar result ought to hold even when the sequence ${phi _n}$ has much slower growth. However, there is a sequence of integers ${n_j}$ and trigonometric polynomials ${P_j}$ such that ${e^{in_jx}-P_j(x)}$ tends to zero everywhere, even though the degree of $P_j$ does not exceed $n_j-j$ for each $j$. The sequence of trigonometric polynomials ${sqrt {n}sin ^{2n}frac x2}$ tends to zero for almost every $x$, although explicit formulas are developed to show that the sequence of corresponding conjugate functions does not. Among trigonometric polynomials of degree $n$ with largest Fourier coefficient equal to $1$, the smallest one ``at' $x=0$ is $% 4^nbinom {2n}n^{-1}sin ^{2n}left ( frac x2right ) ,$ while the smallest one ``near' $x=0$ is unknown.

Keywords:Cantor Lebesgue theorem   conjugate trigonometric series   lacunary trigonometric series   Plessner's theorem   trigonometric polynomials
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