3

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:

\documentclass{aomart}

\DeclareMathOperator{\Union}{\mathop{\bigcup}}
\begin{document}
\[\Union\limits_{i \in I}\]
\end{document}
  • 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
10

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:

\DeclareMathOperator*{\Lim}{Lim}

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

\newcommand*{\Union}{\bigcup}

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.

Code

\documentclass{aomart}
\newcommand*{\Union}{\bigcup}

\begin{document}
\[ \Union_{i \in I} \bigcup_{i \in I} \]
\end{document}

Output

enter image description here enter image description here

  • 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 clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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