Geodesic
A~Chebyshev theorem for the approximation of a~function of two variables by sums of the type $\varphi(x)+\psi(y)$
Izvestiya. Mathematics , Tome 3 (1969) no. 3, pp. 617-632.

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Criteria are obtained for sums of the form$\varphi(x)+\psi(y)$ to deviate by the smallest possible amount from a given function $f(x, y)$ in various metrics. 
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S. Ya. Havinson. A~Chebyshev theorem for the approximation of a~function of two variables by sums of the type $\varphi(x)+\psi(y)$. Izvestiya. Mathematics , Tome 3 (1969) no. 3, pp. 617-632. http://geodesic.mathdoc.fr/item/IM2_1969_3_3_a6/

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