\[\int_{X}{\left|f(x)\right|}^{2}\,\mathrm{d}x=\int_{\boldsymbol{\sigma}_{c}}{%
\left|\widehat{f}(\lambda)\right|}^{2}\,\mathrm{d}\lambda+\sum_{\boldsymbol{%
\sigma}_{p}}{\left|\widehat{f}(\lambda_{n})\right|}^{2},\]