[Tex/LaTex] Root within square root symbol

math-mode

how can I avoid that the following expression 

\documentclass[a4paper,fontsize=14pt]{scrbook}  
\usepackage{mathtools}
\usepackage{lmodern}
\begin{document}
\begin{align}
    a = \sqrt[m]{\sum_{i=1}^{n}b_{i}^m}.
\end{align}
\end{document}

is displayed in this form ($m$ is cut by the root symbol):

enter image description here

Thanks for you help.

Best Answer

It's the usual problem due to the setting of the math extension font done by lmodern.sty, which defines OMX/lmex/m/n for it with this completely wrong font definition file:

\ProvidesFile{omxlmex.fd}[2009/10/30 v1.6 Font defs for Latin Modern]

\DeclareFontFamily{OMX}{lmex}{}
\DeclareFontShape{OMX}{lmex}{m}{n}{%
   <->sfixed*lmex10%
   }{}
\endinput

This means that lmex10 is used no matter what the surrounding font size is. This might seem to work at standard sizes (it doesn't, however, try with \sum in a subscript).

I usually suggest to reinstate the Computer Modern math extension font, but applying the fact that nowadays arbitrary scaling is possible.

\documentclass[a4paper,fontsize=14pt]{scrbook}
\usepackage{mathtools}
\usepackage{lmodern}

\DeclareFontShape{OMX}{cmex}{m}{n}{%
  <-7.5>cmex7
  <7.5-8.5>cmex8
  <8.5-9.5>cmex9
  <9.5->cmex10
}{}
\DeclareSymbolFont{largesymbols}{OMX}{cmex}{m}{n}


\begin{document}
\[
a = \sqrt[m]{\sum_{i=1}^{n}b_{i}^m}.
\]
\end{document}

enter image description here

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