“…Assuming that in (29),ṽ =ṽ ,1 = 0 as x 1 = 0, x 1 = −1 (this provides ρ = (c 1 , 0), c 1 = const, due to conditions ρ 1 (0) = c 1 , ρ 2 (0) =ṽ(0), ρ 2,1 (0) = v ,1 (0)) we obtain (21) and the first relation of (30); see below. Going back to (29) the boundary conditions (24) follow, and moreover, the identity (25) is derived.…”