Geodesic
Sharp estimates for the convergence rate of “hyperbolic” partial sums of double fourier series in orthogonal polynomials
Žurnal vyčislitelʹnoj matematiki i matematičeskoj fiziki, Tome 52 (2012) no. 11, pp. 1952-1958 
V. A. Abilov; M. K. Kerimov. Sharp estimates for the convergence rate of “hyperbolic” partial sums of double fourier series in orthogonal polynomials. Žurnal vyčislitelʹnoj matematiki i matematičeskoj fiziki, Tome 52 (2012) no. 11, pp. 1952-1958. http://geodesic.mathdoc.fr/item/ZVMMF_2012_52_11_a3/
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     title = {Sharp estimates for the convergence rate of {\textquotedblleft}hyperbolic{\textquotedblright} partial sums of double fourier series in orthogonal polynomials},
     journal = {\v{Z}urnal vy\v{c}islitelʹnoj matematiki i matemati\v{c}eskoj fiziki},
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Voir la notice de l'article provenant de la source Math-Net.Ru

Two-variable functions $f(x,y)$ from the class $L_2=L_2((a,b)\times(c,d);p(x)q(y))$ with the weight $p(x)q(y)$ and the norm $$ ||f||=\sqrt{\int_a^b\int_c^dp(x)q(x)f^2(x,y)dx\,dy} $$ are approximated by an orthonormal system of orthogonal $P_n(x)Q_n(y)$, $n, m=0, 1,\dots$, with weights $p(x)$ and $q(y)$. Let $$ E_N(f)=\inf_{P_N}||f-P_N|| $$ denote the best approximation of $f\in L_2$ algebraic polynomials of the form \begin{gather*} P_N(x,y)=\sum_{0<n,m<N}a_{m,n}x^ny^m,\\ P_1(x,y)=\mathrm{const}. \end{gather*} Consider a double Fourier series of $f\in L_2$ in the polynomials $P_n(x)Q_m(y)$, $n, m=0, 1,\dots$, and its “hyperbolic” partial sums \begin{gather*} S_1(f; x,y)=c_{0,0}(f)P_0(x)Q_0(y),\\ S_N(f; x,y)=\sum_{0<n,m<N}c_{n,m}(f)P_n(x)Q_m(y),\qquad N=2,3,\dots. \end{gather*} A generalized shift operator $F_h$ and a $k$th-order generalized modulus of continuity $\Omega_k(A,h)$ of a function $f\in L_2$ are used to prove the following sharp estimate for the convergence rate of the approximation: \begin{gather*} E_N(f)\leqslant(1-(1-h)^{2\sqrt{N}})^{-k}\,\Omega_k(f; h), \qquad h\in(0,1),\\ N=4,5,\dots;\qquad k=1,2,\dots. \end{gather*} Moreover, for every fixed $N=4,9,16,\dots$, the constant on the right-hand side of this inequality is cannot be reduced.

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