# Summation operator issue

I need to write a summation operator inside a matrix. To achieve this, I'm using the code,

\begin{pmatrix}
\sum_{j+k \geq 2} \frac{1}{j! k!} a_{jk}r^j w^k \\
\sum_{j+k \geq 2} \frac{1}{j! k!} b_{jk}r^j w^k
\end{pmatrix}

which yields the following equation:

However, I expect to obtain the lower bound of the summation like in this equation:

How can I achieve this?

• \displaystyle \sum Jun 13, 2023 at 10:29
• @daleif It worked, could you make a complete answer to this question? I'd love to accept your answer! Jun 13, 2023 at 10:46

Here you have:

\begin{pmatrix}
\sum\limits_{j+k \geq 2} \frac{1}{j! k!} a_{jk}r^j w^k \\
\sum\limits_{j+k \geq 2} \frac{1}{j! k!} b_{jk}r^j w^k
\end{pmatrix}

If you want a bigger sum operator use what it is specified in the comments:

\begin{pmatrix}
\displaystyle \sum_{j+k \geq 2} \frac{1}{j! k!} a_{jk}r^j w^k \\
\displaystyle \sum_{j+k \geq 2} \frac{1}{j! k!} b_{jk}r^j w^k
\end{pmatrix}

Note that, the \displaystyle directive not only increases the size of the summation symbol but of the fraction expression as well.

• Thanks @Mico, I have just edited my answer.
– scd
Jun 13, 2023 at 16:02

You have two choices:

1. just the limits below and standard “text style”, or
2. display style, with bigger summation and fractions.

Which one depends on your preferences and the context. Personally, I'd go with the first.

In either case, some additional vertical space is needed. I also recommend a thin space in front of the summations, in order to avoid the subscript being too near to the bracket in the second row (but it's necessary also in the first row to keep the alignment).

\documentclass{article}
\usepackage{amsmath}
\usepackage{booktabs}

\begin{document}

This is not very prominent
$\begin{pmatrix} \,\sum\limits_{j+k \geq 2} \frac{1}{j! k!} a_{jk}r^j w^k \\ \addlinespace \,\sum\limits_{j+k \geq 2} \frac{1}{j! k!} b_{jk}r^j w^k \end{pmatrix}$

This is very prominent
$\begin{pmatrix} \displaystyle\,\sum_{j+k \geq 2} \frac{1}{j! k!} a_{jk}r^j w^k \\ \addlinespace \displaystyle\,\sum_{j+k \geq 2} \frac{1}{j! k!} b_{jk}r^j w^k \end{pmatrix}$

\end{document}

Looking at the output, a \, might be also used at the end, in the first case.