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}
```

## Best Answer

Either of the following methods should work:

You could load the

`amsmath`

package and define a`\norm`

macro as follows:and then write

in the equation of interest. Note that the round parentheses will be too big if you write

`\left(`

and`\right)`

; I recommend you write`\biggl(`

and`\biggr)`

instead.Your example code may therefore be written as

Some might say that the resulting norm "fences" in the example above are a bit too large and thus threaten to dominate visually the rest of the math stuff. (This happens, of course, because the macro uses

`\left`

and`\right`

directives to size the fences.) To fix this issue, you could load the`mathtools`

package and insert the following instruction in the preamble:and use the

`\norm`

macro with an explicit size instruction (here:`\bigg`

, i.e., the same size as for the parentheses):