TeX - LaTeX Stack Exchange is a question and answer site for users of TeX, LaTeX, ConTeXt, and related typesetting systems. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I have used \limits in \[\bigcup\limits_{\i \in I}\] in order to make subscripts displayed in bottom of \bigcup.

However, it looked the same as \[\bigcup_{\i \in I}\]. I.e. it didn't work.

Why? Didn't I used it in the correct way?

Update MWE:


\[\Union\limits_{i \in I}\]
share|improve this question
As you are already in displaystyle math mode (using \[ \]), \[\bigcup_{\i \in I}\] should result in subscript underneath the symbol as expected. Can you give a full MWE? – Corentin Mar 4 '13 at 18:53
As Corentin says your example should work. Can you post a complete minimal working example (MWE) so we can see which documentclass and packages your a using. – Andrew Swann Mar 4 '13 at 18:56
@AndrewSwann Okay,no problem. – Popopo Mar 4 '13 at 19:04
@Corentin Okay, I have done. – Popopo Mar 4 '13 at 19:05
Do \DeclareMathOperator*{\Union}{\bigcup} and you can use \Union without \limits? – Qrrbrbirlbel Mar 4 '13 at 19:07
up vote 9 down vote accepted

From the amsmath documentation (texdoc --view amsldoc.pdf):

If the new operator should have subscripts and superscripts placed in “limits” position above and below as with lim, sup, or max, use the * form of the \DeclareMathOperator command:


Also, the \mathop is not needed. After all, you declare a math operator with \DeclareMathOperator[*].

I refer to „What is the difference of \mathop, \operatorname and \DeclareMathOperator?“ for further reading on this topic.

\DeclareMathOperator*{\<new op>}{<stuff that should be typeset as an op with limits>}

This answers the question why the limits aren’t where you have expected them to be.

But \bicup is already an operator you should do


instead to have a more descriptive name for it (which is a good idea nonetheless).

The \DeclareMathOperator* pair of macros is to declare new operators like the already defined \lim, \sum, and so on.



\[ \Union_{i \in I} \bigcup_{i \in I} \]


enter image description here enter image description here

share|improve this answer
I see, thanks a lot. – Popopo Mar 4 '13 at 19:25
I don't think I'd recommand \DeclareMathOperator* for this. – egreg Mar 4 '13 at 20:21
@egreg I hope the updated answer differentiates better between what has been done wrong regarding the use of \DeclareMathOperator and what has been done wrong with \bigcup. – Qrrbrbirlbel Mar 4 '13 at 20:49
@Qrrbrbirlbel Yes, it seems good – egreg Mar 4 '13 at 20:51

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.