Formula_14269

\[v\left(x, x^{\prime}\right)=\sum_{j=1}^{N} \lambda_{j} \varphi_{j}(x) \bar{\varphi}_{j}\left(x^{\prime}\right), \quad \varphi_{j}(x) \in L_{2}\left(\mathbb{R}^{3}\right) .\]