Equation 13.2.7491D0E94E07A

\begin{equation} \begin{aligned} \mathbf1_{E_ k}P^{(2k)}(u_1+u_1',u_2,\ldots ,u_ p) & =\mathbf1_{E_ k}P^{(k)}(u_1,u_2,\ldots ,u_ p)\\ & \quad +\mathbf1_{E_ k}P^{(k)}(u_1',u_2,\ldots ,u_ p). \end{aligned} \end{equation}