# Formatting the union of sets

In the example

$\cup^{i=1}_{\infty}F_i$


How do you get the i=1 directly below the union symbol and the infinity directly above?

• You should use $\bigcup\limits_{i=1}^{\infty} F_{i}$, but in inline math mode it's better to set the limits on the side (just don't use \limits, which is implicit in display math mode). Note that MathJax is not used on this site, because we don't want the code to become an approximation of the result, but want to talk about the code itself. – egreg Oct 8 '14 at 11:13

The correct command to use in this case is \bigcup; you could go with

$\bigcup\limits_{i=1}^{\infty} F_{i}$


but this will irremediably spoil your document. When a formula is inline, it's better if limits are set to the side of the symbol. In display math mode the \limits is implicit. See the following example and judge for yourself.

\documentclass{article}
\begin{document}

The correct command to use in this case is \verb|\bigcup|;
if you use \verb|\limits| your formula will appear inline
like $\bigcup\limits_{i=1}^{\infty} F_{i}$, but this will
irremediably spoil your document. When a formula is inline,
it's better if limits are set to the side of the symbol. In
display math mode the \verb|\limits| is implicit. Here is the
same formula without it $\bigcup_{i=1}^{\infty} F_{i}$ and you
can clearly see that the vertical spacing is much better.
In a display there is no problem
$\bigcup_{i=1}^{\infty} F_{i}$
and the limits can be set above and below without spoiling
the spacing, which is an important factor when readability
is considered.

\end{document}


• “irremediably” seems a bit harsh; after all, you can change the limits to super/subscripts and it's all better. :-) – Matthew Leingang Oct 8 '14 at 17:21
• @internet the vertically inverted version \bigcup is \bigcap – Carter Pape Aug 21 '16 at 21:06
• What should we do to put the limits in side even for the display mode? – Hosein Rahnama Mar 19 '19 at 22:26
• @H.R. \bigcup\nolimits does it. You can do it globally for all big operators by passing the nosumlimits option to amsmath. – egreg Mar 19 '19 at 22:50
• @GennaroArguzzi \bigcup is used for “indexed union”; for the binary operation, use \cup. – egreg Mar 6 '20 at 17:06