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:


\[\Union\limits_{i \in I}\]
  • 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
  • 1
    Do \DeclareMathOperator*{\Union}{\bigcup} and you can use \Union without \limits? – Qrrbrbirlbel Mar 4 '13 at 19:07

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.



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


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  • 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

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