problem with substack command

Question

Help me please with the following problem.

I want to write the following equation

$$\phi(U)_{\substack{\cap\\ \mathbb{R}}}$$ 

and my problem is \cap and \mathbb{R} don't appear under \phi.

How can I do?

Answer 1

I can't tell if the items \cap and \mathbb{R} are supposed be typeset next to each other or one above the other. I suppose you may want one of the following two possibilities:

The \mathclap instruction lets its argument "protrude" to the left and right of \phi without introducing unnecessary whitespace on either side.

\documentclass{article}
\usepackage{mathtools} % for \mathclap and \substack macros
\usepackage{amsfonts}  % for \mathbb macro
\begin{document}
\[
\mathop{\phi}_{\mathclap{\cap\mathbb{R}}}(U) 
\quad
\mathop{\phi}_{\substack{\cap\\ \mathbb{R}}}(U) 
\]
\end{document}

Incidentally, don't use $$ in a LaTeX document; use \[ and \] instead.

Answer 2

I'm not sure about your aims. However the task is easy:

\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}
\[
\underset{\substack{\cap\\\mathbb{R}}}{\phi(U)}
\]
\end{document}

If you don't want the size to decrease, use an array:

\documentclass{article}
\usepackage{amsmath,amssymb}
\begin{document}
\[
\begin{array}[t]{@{}c@{}}
\phi(U)\\
\cap\\
\mathbb{R}
\end{array}
\]
\end{document}

Answer 3

Is this the output you want to achieve?

\documentclass{amsart}

\begin{document}

\begin{displaymath}
  \mathop{\phi(U)}_{\substack{\cap\\ \mathbb{R}}}
\end{displaymath}

\end{document}

You need to make the \phi(U) a math operator for the limits to work.

Another, maybe nicer solution is:

\begin{displaymath}
  \underset{\substack{\cap\\ \mathbb{R}}}{\phi(U)}
\end{displaymath}