math-mode
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?
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\DeclareMathOperatorcommand:\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.
\documentclass{aomart}
\newcommand*{\Union}{\bigcup}
\begin{document}
\[ \Union_{i \in I} \bigcup_{i \in I} \]
\end{document}
\documentclass{article}
\usepackage{amsfonts,amsmath}
\begin{document}
\[
\int_{\substack{a\\\mathcal{P}}}^b (\nabla ...)
\]
\[
\int\limits_{\substack{a\\\mathcal{P}}}^b (\nabla ...)
\]
\end{document}
Best Answer
there is a bigcup operator for this use, or in general you can use
\mathoparound any math expression which gives it operator spacing and limits behaviour for superscripts, however doesn't make it larger in displaystyle as\sumand\bigcupare.