# Words under sum that's in a denominator

if I try this;

Substituting the expression for $$\Psi$$ in to the above and rearranging we derive an expression for the vacuum field $$\Psi'$$ as a function of $$\mu$$,

\begin{equation}
\Psi' = \frac{\Phi_t}{\mu ab\left[1+ \frac{4}{\pi^2}\sum_{m,n \mbox{ odd }} \frac{\alpha_{mn}}{mn}\right]} \\[1ex]
\end{equation}

with the sum being over odd $$m,n$$. The total magnetic flux is defined as just the sum of the flu


Then it's weird because the {m,n \mbox{ odd }} is forced to the right of the summation instead of under it. This is because it's on the denominator. How do I fix this?

• Perhaps this: \begin{equation} \Psi' = \frac{\Phi_t}{\mu ab\biggl[\displaystyle 1+ \frac{4}{\pi^2}\sum_{m,n \mbox{ odd }} \frac{\alpha_{mn}}{mn}\biggr]} \end{equation}. – Gonzalo Medina May 7 '14 at 18:32
• ...or \begin{equation} \Psi' = \frac{\Phi_t}{\mu ab\left[1+ \dfrac{4}{\pi^2}\displaystyle\sum_{\smash{m,n \mbox{\scriptsize (odd) }}} \dfrac{\alpha_{mn}}{mn}\right]} \\[1ex] \end{equation} with the amsmath package loaded. – Steven B. Segletes May 7 '14 at 18:41
• @GonzaloMedina -- rather than \mbox which would force the type to be the main body size, \text or \textrm would produce a more appropriate size. – barbara beeton May 7 '14 at 19:04
• @barbarabeeton yes, you're right; \text (if amsmath is loaded) or \textrm are better. – Gonzalo Medina May 7 '14 at 19:08
• Or, if you don't want "odd" sticking off to the side, \documentclass{article} \usepackage{amsmath} \usepackage{stackengine} \begin{document} \begin{equation} \Psi' = \frac{\Phi_t}{\mu ab\left[1+ \dfrac{4}{\pi^2}\displaystyle\sum_{\smash{\stackunder[1pt]{$m,n$}{\tiny(odd)% }}} \dfrac{\alpha_{mn}}{mn}\right]} \\[2ex] \end{equation} \end{document} – Steven B. Segletes May 7 '14 at 19:28

Summarizing the recommendations: (1) Use \displaystyle to increase generally the size of the \sum and fractions in the denominator, as well as change from subscript position to a limit under the sum. (2) Use \textrm or \text from the amsmath package instead of \mbox. \mbox uses the surrounding text font which is the wrong size for a subscript or limit and potentially the wrong font. (3) Also, it is useful to control the sizes of the brackets somehow and the vertical space taken up by the limit below the sum. This can be done by selecting an explicit size command for the brackets (\biggl[ and \biggr]), and using \smash on the limit.

\documentclass{article}
\begin{document}
Substituting the expression for $$\Psi$$ in to the above and rearranging
we derive an expression for the vacuum field $$\Psi'$$ as a function of
$$\mu$$,
\begin{equation}
\Psi' = \frac{\Phi_t}{\mu ab \biggl[ \displaystyle 1 + \frac{4}{\pi^2}
\sum_{\smash{m,n\ \textrm{odd}}} \frac{\alpha_{mn}}{mn}\biggr]}
\end{equation}
with the sum being over odd $$m,n$$. The total magnetic flux is defined
as just the sum of the flu
\end{document}


If one only wants to change the position of the limits, one can just use \sum\limits in place of \sum (but also replace the \mbox).