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I noticed that the commands \left\| ... \right\| don't always generate brackets of the same size, even when the content is the same. Example:

The code \frac{\left\|A^{-1}\right\|}{\left\|A^{-1}\right\|} yields this:

enter image description here

Clearly, the bars are larger in the numerator, although the code is identical. Is there a nice way to prevent this?

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    I recommend you to use \bigl \bigr or \Bigl \Bigr instead. – Sigur Jul 31 '19 at 13:01
  • But this is slightly annoying. \left and \right exist precisely so I don't have to think about size all the time. Isn't there any way to automate this such that same content gives same size brackets? – Frank Jul 31 '19 at 13:05
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    I'd even drop left/right here, there is no need for them – daleif Jul 31 '19 at 13:06
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    @Frank that is not correct, autoscaling should be left for certain situations, for example this looks bad: \left(\sum_j \right), you'll also often run into problems when used in text mode, they grow too large and disturb line spacing. – daleif Jul 31 '19 at 13:07
  • I don't obtain this result. Could you please post a full code reproducing the problem? – Bernard Jul 31 '19 at 13:18
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the denominator is set in "cramped" style in which the exponent is lower, which has an effect on the left-right size, it is often best to use manual size \left\right has an adverse effect on horizontal spacing, even when it does not stretch the delimiters. here you could use \bigl\| or simply \|

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    Much better is \lVert and \rVert – egreg Jul 31 '19 at 14:48
  • @egreg I wondered about mentioning that but separate issue and while they are better in general, not going to make any difference here is it, with just a single term – David Carlisle Jul 31 '19 at 15:11
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I simply add a response to generate brackets of the same size. I hope my code is clear. If there are some comments I am available.

enter image description here

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools} 
\DeclarePairedDelimiter{\norm}{\lVert}{\rVert}
\begin{document}
\[\frac{\norm{A^{-1}}}{\norm{A^{-1}}}\]
\end{document}
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