“…Here we consider the case where bj(x 9 t) and c(x, t) are complex-valued functions. Put iy = 6j 1) + J6J 2) , where 6J 1) and 6J 2) are real-valued. We assume the following condition: for any (x 9 t)^R n x\J), oo) where £ is an arbitrary small positive constant and C € is a constant dependent on s. The above condition (3.13) is, in some sense, close to being necessary.…”