I need to write norm of sum, but the sum symbol is larger than tho norm symbol (||) and it doesn't look good. Is there any symbol for norm which will adjust its size?
\documentclass[12pt,a4paper]{article}
\begin{document}
\begin{equation}
||\left(\sum_{n=1}^N \bf P_{\rm n}\rm\right) ||^2 = \left(\sum_n \frac{E_n}{c}\right)^2 - \left(\sum_n \bf p_{\rm n}\rm \right)^2
\end{equation}
\end{document}
\mathbf{P}_{n}
and don't use\rm
or\bf
any more: they have been obsolete for more than ten years in LaTeX. – egreg Apr 6 '13 at 10:22