1. E. Wachinicki, ?Rozwiqzane pewnych zagadne? brzegowych dla równania ?u?c2u=0,? Rocznin Nauk.-Dudakt. WSP w Krakowie (Prace Matematyczne),41, 165?174 (1970).
2. G. M. Polozhii, Equations of Mathematical Physics [in Ukrainian], Radyans'ka Shokola, Kiev (1959).
3. B. M. Budak, A. A. Samarskii, and A. N. Tikhonovk Collection of Problems on Mathematical Physics [in Russian], Gostekhizdat, Moscow (1956).
4. I. P. Natanson, ?On the order of approximation of a continuous 2?-periodic function by means of its Poisson integral,? Dokl. Akad. Nauk SSSR,52, No. 1, 11?14 (1950).
5. Ya. L. Geronimus, ?On the order of approximation by means of Poisson integral,? Dokl. Akad. Nauk SSSR,129, No. 4, 726?728 (1959).