Geodesic
Piecewise monotonic functions of several variables and a theorem of Hardy and Littlewood
Izvestiya. Mathematics , Tome 39 (1992) no. 3, pp. 1113-1128.

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The author discusses classes of periodic functions of $m$ variables that are either piecewise monotonic or piecewise monotonic in the sense of Hardy, and clarifies the connections, for such functions, between the property of belonging to $L_p$ space, $1$, and the convergence of series of their trigonometric Fourier coefficients, $$ \sum_{n_1,\dots ,\ n_m=-\infty}^{+\infty}\big|a_{n_1\dots n_m}\big|^\alpha \left(\prod_{j=1}^m(|n_j|+1)\right)^{\alpha-2}. $$ We establish the existence, when $m>1$, of certain results that differ from the one-dimensional case. 
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M. I. Dyachenko. Piecewise monotonic functions of several variables and a theorem of Hardy and Littlewood. Izvestiya. Mathematics , Tome 39 (1992) no. 3, pp. 1113-1128. http://geodesic.mathdoc.fr/item/IM2_1992_39_3_a1/

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