“…According to formula (2.1), the corresponding eigenfunction is f 1 (x, y, z) = u 0,1 (x, y), and it solves problem (2.2)-(2.5) for m = 0 and n = n 0,1 . Theorem 2.1 proved by Kulczycki & Kuznetsov (2009) where the trace u 0,1 (x, 0) attains its extremum values, say, (0, 0) and (a, 0) are the minimum and maximum points, respectively. Then, f 1 (x, 0, z) is equal to u 0,1 (0, 0) and u 0,1 (a, 0) on the whole edges…”