“…Systems 12- (14) and 18, (19), (16), (14) are equivalent. So 12- 14is being reduced to two separate systems of lower order: one system is (18), (19), (14) and the other system is (16). In system (18), (19), (14) only the unknowns z ij , i, j = 1, N -1, in the internal points of the domain D are presented.…”