“…The solvability of Equation 3.5 was proved in with the help of the following Lemma which can be formulated in the following form:Lemma Let be periodic with the bounded second derivative (i.e ., ) and let be periodic with respect to both variables. Assume that there exist numbers and a constant such that Then, the approximate solution of the uniquely resolvable Fredholm integral equation of the second kind can be reduced to solution of finite linear system with error estimated by where is the finite linear system rank .…”