[Tex/LaTex] problem with substack command


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?

Best Answer

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:

enter image description here

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

\usepackage{mathtools} % for \mathclap and \substack macros
\usepackage{amsfonts}  % for \mathbb macro
\mathop{\phi}_{\substack{\cap\\ \mathbb{R}}}(U) 

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

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