Equation (13) should be changed into
$$I\lpar z\comma \; t\rpar =I_I \lpar z\comma \; t\rpar +\sum\limits_{n=1}^{+\infty } I_{II\comma n} \lpar z\comma \; t\rpar \cdot \cos \lsqb n \omega \lpar t - \lpar z - z_1 \rpar /\upsilon \lpar \tau \rpar \rpar +n {\rm \varphi} _0 \rsqb$$That is to say the “n = −∞” should be changed into “n = 1”.
