[Tex/LaTex] When substack and array doesn’t work

amsmatharraysmath-modemath-operatorssubscripts

I am trying to write a formula. But both substack and array cannot fulfil my needs (as far as I know how to use them).

formulae

The first is how I want it to look like. But I need an i'\neq i below the summation part.

The second is tried with \substack and the third is tried with array and both are unsatisfactory to me. Firstly, the i'\neq i is too big. Secondly, the a_{ii'} is misaligned.

Most preferable would be to maintain the size of the \sum. But if that is not possible, any compromise is okay.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{equation}
\sum\limits_{i=1}^{I}\sum\limits_{i'=1}^{I}a_{ii'} + \sum\limits_{i=1}^{I}b_{i}
\end{equation}
\begin{equation}
\substack{\sum\limits_{i=1}^{I}\sum\limits_{i'=1}^{I}\\i'\neq i}a_{ii'} + \sum\limits_{i=1}^{I}b_{i}
\end{equation}
\begin{equation}
\begin{array}{@{}c@{}}\sum\limits_{i=1}^{I}\sum\limits_{i'=1}^{I}\\i'\neq i \end{array}a_{ii'}+ \sum\limits_{i=1}^{I}b_{i}
\end{equation}

\end{document}

Best Answer

You can use \mathop to "define" a math operator that allows stacking of super-/subscripts inside a display:

enter image description here

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}

\begin{equation}
\sum\limits_{i=1}^{I}\sum\limits_{i'=1}^{I}a_{ii'} + \sum\limits_{i=1}^{I}b_{i}
\end{equation}
\begin{equation}
\substack{\sum\limits_{i=1}^{I}\sum\limits_{i'=1}^{I}\\i'\neq i}a_{ii'} + \sum\limits_{i=1}^{I}b_{i}
\end{equation}
\begin{equation}
\begin{array}{@{}c@{}}\sum\limits_{i=1}^{I}\sum\limits_{i'=1}^{I}\\i'\neq i \end{array}a_{ii'}+ \sum\limits_{i=1}^{I}b_{i}
\end{equation}
\begin{equation}
\mathop{\sum_{i = 1}^I \sum_{i' = 1}^I}_{i' \neq i} a_{ii'} + \sum_{i = 1}^I b_{i}
\end{equation}

\end{document}
Related Question