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How do I make the following set of equations look bigger?

The exponent in the exponential looks terribly small even when I use 'displaystyle' .

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[]{amsmath}
\usepackage[]{xcolor}
\usepackage[]{enumitem}
\usepackage[]{graphicx}
\allowdisplaybreaks

\title{something}
 \author{someone}
 \date{October 2016}

\begin{document}

\maketitle

\section{something}

\begin{enumerate}[label=(\alph*)]
\item

\begin{align*}
    \displaystyle{\langle x_{b}, t_{b}|x_{a}, t_{a}\rangle}
    &\displaystyle{= \int \mathcal{D}x\ \mathcal{D}p\ e^{iS/\hbar}}\\
    &\displaystyle{= \lim_{N\to\infty}\prod_{i=1}^{N}\bigg[\int_{-\infty}^{\infty}dx_{i}\bigg]\prod_{j=1}^{N+1}\bigg[\int_{-\infty}^{\infty}\frac{dp_{j}}{2\pi\hbar}\bigg]e^{iS_{N}/\hbar}}\\
    &\displaystyle{= \lim_{N\to\infty}\prod_{i=1}^{N}\bigg[\int_{-\infty}^{\infty}dx_{i}\bigg]\prod_{j=1}^{N+1}\bigg[\int_{-\infty}^{\infty}\frac{dp_{j}}{2\pi\hbar}\bigg]
      e^{\frac{i}{\hbar}\sum\limits_{k=1}^{N+1}[p_{k}(x_{k}-x_{k-1})-\epsilon H(p_{k},x_{k},t_{k})]}}.
\end{align*}
\end{enumerate}
\end{document}
3
  • 2
    you need to remove all the \displaystyle as they are doing nothing align is already display, and as it does not take an argument the {...} form a group and mess up the spacing, if you want it bigger you could put \large before the environment. – David Carlisle Oct 17 '16 at 19:26
  • please always post complete documents no one can run that and see the problem and test answers without filling in all the missing parts, it is more helpful if they are there to start with. – David Carlisle Oct 17 '16 at 19:28
  • Probably for legibility Zarko's "exp" notation is better, but if you really want to use a superscript you can put \textstyle or \displaystyle inside the superscript to force it to the style of inline or displayed math. (Being inside a superscript, it is by default shown in \scriptstyle.) – Emma Oct 17 '16 at 20:04
3

Superscripts and subscripts of math operators in superscript are really small. Therefore it is sensible to move math expression in the superscript in the last line to main line, i.e. replace e^{...} with \exp(...) which is actually intended for just such a case:

enter image description here

\documentclass{article}
    \usepackage{mathtools}

\begin{document}
    \begin{align*}
\langle x_{b}, t_{b}|x_{a}, t_{a}\rangle
    & = \int \mathcal{D}x\,\mathcal{D}p\,\mathrm{e}^{iS/\hbar}\\
    & = \lim_{N\to\infty}\prod_{i=1}^{N}\biggl[\int_{-\infty}^{\infty}dx_{i}\biggr]
            \prod_{j=1}^{N+1}\biggl[\int_{-\infty}^{\infty}\frac{dp_{j}}{2\pi\hbar}\biggr]
            \mathrm{e}^{iS_{N}/\hbar}\\
    & = \begin{multlined}[t][0.7\textwidth]
        \lim_{N\to\infty}\prod_{i=1}^{N}\biggl[\int_{-\infty}^{\infty}dx_{i}\biggr]
            \prod_{j=1}^{N+1}\biggl[\int_{-\infty}^{\infty}\frac{dp_{j}}{2\pi\hbar}\biggr]    \\
        \exp\biggl(\frac{i}{\hbar}\sum_{k=1}^{N+1}[p_{k}(x_{k}-x_{k-1})-\epsilon H(p_{k},x_{k},t_{k})]\biggr).
\end{multlined}
    \end{align*}
\end{document}
3
  • +1. A small point: if you replace \bigg[ and \bigg] with \biggl[ and \biggr], respectively, you'll tighter (and, IMHO, better) spacing of the square brackets. Also, in the final row, consider replacing \left( and \right) with \biggl( and \biggr), resp. The directive \limits after \sum isn't needed since display math is in effect. Finally, in the first integral expression, the amount of space inserted by "\ " seems excessive; a thinspace (``) seems more appropriate. – Mico Oct 17 '16 at 20:10
  • @Mico, in my edit of answer I consider (hopefully all) your suggestion. In the first answer version my intention was to show a possible solution of OP problem. – Zarko Oct 17 '16 at 20:28
  • I've taken the liberty of replacing a few more \bigg directives -- those in row 3 -- with \biggl and \biggr, resp., and I've deleted \limits. Feel free to revert. – Mico Oct 17 '16 at 20:33

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