The second boundary condition will be satisfied if 144 t 3 ( 1 + β β t λ λ ) n = 144 t 3 β β n t λ λ n ( 1 + β β − − 1 t − − λ λ ) n ∼ ∼ 1 as t → → ∞ ∞ . This condition is satisfied if λ λ n + 3 = 0 , 144 β β n = 1 and since λ λ 1 λ λ 2 = − − 6 , Sommerfeld found the approximation as λ λ = λ λ 1 , n = − − 3 / λ λ 1 = λ λ 2 / 2 . Therefore, the approximate solution is