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Is it possible to use the align environment without the display math? For example, I would like \sum to look as if it were inline. The solution given here does not work, insofar as the summation symbols are still written in the big, display style. The alignat environment also produces this behaviour.

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3 Answers 3

As a workaround you can use \textstyle inside the aligned environment:

\documentclass{standalone}
\usepackage{amsmath}
\begin{document}
text
$\begin{aligned}\textstyle
\sum_{i=1}^\infty&=1\\ \textstyle
\sum_{i=1}^\infty&=1
\end{aligned}$
more text
\end{document}

But it would be better to find better solution. It's more like a hack.

enter image description here

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This seems to break the moment \left( is used. e.g. \begin{aligned}\textstyle \left(\sum_{i=1}^\infty&=\right) \\ \textstyle \sum_{i=1}^\infty&=1 \end{aligned} –  goblin Aug 7 at 10:01
    
Put the \right. before the &. And paired \left. after. –  m0nhawk Aug 7 at 10:02
    
@goblin related for \left and \right. –  m0nhawk Aug 7 at 10:05
    
Thanks, but I'm still getting odd behaviours. Why doesn't the following code work, for example? \begin{aligned}\textstyle 2 &= \sum_{i \in k} i + \sum_{i \in k}i \\ \textstyle 2 &= \sum_{i \in k} i + \sum_{i \in k}i \end{aligned} –  goblin Aug 7 at 10:08
    
@goblin You need to put \textstyle right before the \sum symbols. Actually, @Bernard solution are much better with a custom \sum symbol. –  m0nhawk Aug 7 at 10:10

You can use the fact that amsmath defines big operators in a uniform way; for instance \sum is redefined to use \sum@ surrounded by suitable macros.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\makesmaller}{m}
 {
  \goblin_makesmaller:n { #1 }
 }

\cs_new_protected:Npn \goblin_makesmaller:n #1
 {
  \clist_map_inline:nn { #1 } { \goblin_makesmaller_sym:n { ##1 } }
 }

\cs_new_protected:Npn \goblin_makesmaller_sym:n #1
 {
  \cs_gset_eq:cc { #1@saved@ } { #1@ }
  \cs_set_protected:cpn { #1@ }
   {
    \mathop
     {
      \mathchoice { \textstyle \use:c { #1@saved@ } }
                  { \use:c { #1@saved@ } }
                  { \use:c { #1@saved@ } }
                  { \use:c { #1@saved@ } }
     }
   }
 }
\ExplSyntaxOff

\makesmaller{sum,bigoplus,bigcup}

\begin{document}

Here's an inline formula $\sum_{k=1}^n k^2=\frac{1}{6}n(n+1)(2n+1)$
and the same displayed, with some nonsense
\[
\bigoplus_{i\in I}\bigcup_{j=0}^{\infty}\sum_{k=1}^n k^2=\frac{1}{6}n(n+1)(2n+1)
\]

\end{document}

The argument to \makesmaller is the list of operators you want to be treated as \sum.

When you'll change your mind, hopefully soon ;-), just remove the call to \makesmaller.

enter image description here

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You can define a \tsum command, just like there exists a \frac command. And also an \msum command (medium-sized sum), based on the nccmath package.

Another solution would be to use the `tabstackenginepackage and its\alignCenterstack` command. Here is a demonstration of all theses possibilities, with different vertical alignments:

\documentclass{article}
\usepackage{amsmath}
\usepackage{nccmath}
\newcommand\tsum{\textstyle\sum\nolimits}
\newcommand\msum{\medop\sum}

\usepackage{tabstackengine} 

\begin{document}

\noindent Text
$\begin{aligned}[t]
 \tsum_{i = 1}^{n}x_{i}&=z\\
u+v&=w
\end{aligned}$
 \enspace more text \enspace $ \sum_{i = 1}^{n}x_{i} $
\enspace more text\enspace
$\begin{aligned}[b]
 \msum_{i = 1}^{n}x_{i}&=z\\
u+v&=w
\end{aligned}$
\enspace  more text\enspace
$\begin{aligned}
 \sum_{i = 1}^{n}x_{i}&=z\\
u+v&=w
\end{aligned}$
\enspace more text. 

Some other text\enspace\stackMath\setstackgap{L}{3.5ex}$ \left[\alignCenterstack{
\sum_{i = 1}^{n}x_{i}&=z\\
\sum_{i = 1}^{n}u_{i} &=w }\right. $
\enspace  more text. 

\end{document} 

enter image description here

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