[Tex/LaTex] How to use \limits correctly

math-mode

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}

Best Answer

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

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