# Use 'Limits' on arbitrary math symbol

In set theory, the union of an arbitrary number of sets can be taken:

$A_1 \cup A_2 \cup A_3 \cup \dots \cup A_k = \cup\limits_{i=1}^{k} A_i$


I want to set the limits as they appear in $\sum_{i=1}^{k}$, however even in block equations, they do not show as limits.

Is this possible for an arbitrary math-mode symbol?

• to show tex output make a screenshot locally and upload an image, we do not have mathjax running as is used on the math site. – David Carlisle Jan 10 '17 at 20:36
• there is a \bigcup designed for this use – David Carlisle Jan 10 '17 at 20:37
• Thanks, didn't realise the difference. Is there a way to do it arbitrarily though? Or are there any cases where it would be needed that it isn't already available? – Brydon Gibson Jan 10 '17 at 20:39
• Possible duplicate: How do I do a one-shot MathOperator? – Werner Jan 10 '17 at 20:44

there is a bigcup operator for this use, or in general you can use \mathop around any math expression which gives it operator spacing and limits behaviour for superscripts, however doesn't make it larger in displaystyle as \sum and \bigcup are.

\documentclass{article}

\usepackage{amssymb}

\begin{document}

$A_1 \cup A_2 \cup A_3 \cup \dots \cup A_k = \bigcup_{i=1}^{k} A_i$

$A_1 \cup A_2 \cup A_3 \cup \dots \cup A_k = \mathop{\cup}_{i=1}^{k} A_i$
\end{document}

• It could be worthwhile to remark, for the benefit of user who could read this answer in the future, that the authors of LaTeX followed this naming scheme consistently: \cup->\bigcup, \cap->\bigcap, \oplus->\bigoplus, \otimes->\bigotimes, etc. etc. There are exceptions, however, as in +->\sum ;-) – GuM Jan 10 '17 at 23:57
• @GustavoMezzetti actually all those names come from plain TeX so Donald Knuth – David Carlisle Jan 10 '17 at 23:59
• Well, yes, I knew! :-) Edit: It was not Don Knuth who wrote this answer, though! ;-) – GuM Jan 11 '17 at 0:03