Increasing the parenthesis size of binomial coefficients

Question

I'm a student of mathematics and I recently discovered LaTeX and I've ran into a problem. I used \displaystyle in front of \sum and the parenthesis of the binomial coefficient did not change to encompass the the now larger sum and it looks ugly. How do I fix this?

$$\prod_{i=1}^{\delta}\binom{\displaystyle\sum_{j=1}^{i}n_j}{n_i}$$

Answer 1

You have a few choices. The alignat* environment is just for labeling them and the labels should be quite clear about the matter at hand.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{alignat*}{2}
&\text{Good} &\qquad&
\prod_{i=1}^{\delta}\binom{\,\sum_{j=1}^{i}n_j}{n_i}
\\
&\text{Bad} &&
\prod_{i=1}^{\delta}\binom{\sum\limits_{j=1}^{i}n_j}{n_i}
\\
&\text{Ugly} &&
  \prod_{i=1}^{\delta}\binom{\displaystyle\sum_{j=1}^{i}n_j}{n_i}
\\
&\text{?} &&
  \prod_{i=1}^{\delta}\begin{pmatrix}\displaystyle\sum_{j=1}^{i}n_j \\ n_i \end{pmatrix}
\end{alignat*}

\end{document}

Newcomers are usually fond of the possibilities of \displaystyle, but, as you see, it makes humongous formulas that add nothing to clarity and disrupt typography.