# Root within square root symbol

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):

Thanks for you help.

-
Try to use \sqrt[m\,]{\sum...}. –  wipet Jun 24 '14 at 10:59
please make a complete document that produces that display and post, the input shown would set the summation in inline style not with the limits above and below. –  David Carlisle Jun 24 '14 at 10:59
That is not possible with standard LaTeX math mode. So do what David said! –  Herbert Jun 24 '14 at 11:02
Sorry for not posting the full code. Now I think, the problem might be the lmodern package in combination with the 14pt fontsize. –  Ankaios Argo Jun 24 '14 at 11:13
And the m\, does help! –  Ankaios Argo Jun 24 '14 at 11:14

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}


-
Thanks, that solves the problem! If you have some additinal time, would be nice, if you could explain, how you came up with the numbers. –  Ankaios Argo Jun 24 '14 at 12:56
@AnkaiosArgo The figures in the \DeclareFontShape command are based on the available optical sizes for cmex; if the requested size is around 7pt (or below, for second level sub/superscripts), use cmex7; if it is around 8pt, use cmex8 and so on; cmex10 is the largest size available, so from 9.5pt up there's no other choice. –  egreg Jun 24 '14 at 13:00

Something strange in the lmodern font metrics it appears.

You can shift the index with \leftroot

\documentclass[a4paper,fontsize=14pt]{scrbook}
\usepackage{mathtools}
\usepackage{lmodern}
\begin{document}
\begin{align}
a = \sqrt[\leftroot{2}m]{\sum_{i=1}^{n}b_{i}^m}.
\end{align}
\end{document}


The input you show would use an inline style unlike the image shown, and display style should not over print the index Please always post a complete document that shows the problem:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

$a = \sqrt[m]{\sum_{i=1}^{n}b_{i}^m}$

$a = \sqrt[m]{\sum_{i=1}^{n}b_{i}^m}$

\end{document}

-
Thanks a lot David! The leftroot is a nice workaround. However, it seems to be better to solve the problem at the "root". Thus I think egreg's solutions is the way to go. –  Ankaios Argo Jun 24 '14 at 12:54
@AnkaiosArgo you made an old man very happy: chat.stackexchange.com/transcript/message/16249651#16249651 –  David Carlisle Jun 24 '14 at 12:56