\[\delta\left(x-x^{\prime}\right)=\sum_{\boldsymbol{\sigma}_{p}}\phi_{\lambda_{n%
}}(x)\overline{\phi_{\lambda_{n}}(x^{\prime})}+\int_{\boldsymbol{\sigma}_{c}}%
\phi_{\lambda}(x)\overline{\phi_{\lambda}(x^{\prime})}\,\mathrm{d}\lambda,\]