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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     author = {S. Ya. Havinson},
     title = {A~Chebyshev theorem for the approximation of a~function of two variables by sums of the type $\varphi(x)+\psi(y)$},
     journal = {Izvestiya. Mathematics },
     pages = {617--632},
     publisher = {mathdoc},
     volume = {3},
     number = {3},
     year = {1969},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1969_3_3_a6/}
}
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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/