# How can I write the below part of \bigcup like this? [duplicate]

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb,cases}
\usepackage[margin=2cm]{geometry}
\usepackage{mathrsfs}
\begin{document}
\noindent
$\displaystyle \mathcal{M}'= \bigcup_{ \mathcal{F} \subset \mathcal{E} ,\, \mathcal{F} \text{is countable}}\mathcal{F}(\mathcal{F}).$
\end{document}


Its output is:

But I want:

Use \substack{}

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb,cases}
\usepackage[margin=2cm]{geometry}
\usepackage{mathrsfs}
\begin{document}
\noindent
$\displaystyle \mathcal{M}'= \bigcup_{\substack{\mathcal{F} \subset \mathcal{E} \\ \mathcal{F} \text{ is countable}}}\mathcal{F}(\mathcal{F}).$
\end{document}


• Note \substack comes from amsmath, not mathtools. Is it made better with mathtools? Apr 22 '20 at 2:43
• Oh, sorry, I didn't know that. I thought it was mathtools. Gonna edit
– user193767
Apr 22 '20 at 2:49
• @DonaldArseneau -- mathtools loads amsmath and extends some of its facilities, mostly in response to legitimate requests from users. (One example is matrices in which the horizontal alignment of the cells can be specified, rather than always being flush left, which looks dreadful when some entries are negative and others aren't.) So if the extended facilities are needed, one can load mathtools and get both packages together. Apr 22 '20 at 4:24

A spacing improvement with \smashoperator from mathtools (neddless to load amsmath):

\documentclass[a4paper,12pt]{article}
\usepackage{mathtools,amssymb,cases}
\usepackage[margin=2cm]{geometry}
\usepackage{mathrsfs}

\begin{document}

\noindent
$\displaystyle \mathcal{M}'=\smashoperator{ \bigcup_{\substack{\mathcal{F} \subset \mathcal{E} \\ \mathcal{F} \text{ countable}}}}\mathcal{M}(\mathcal{F}).$

\end{document}