How to draw a border around character in a formula?

Question

So far everything I found on the matter of borders only applies to the text mode.

I want to be able to do something like

$x_i = y$

so the result will be like the ordinary "x_i = z" formula with the border around "i"?

The common answers seems to be the fbox or boxed commands. Yet they render their argument in the standart mode, unless specified to do otherwise with the specific mode. For example, the following code

\[ x_i, \boxed{x}_i, x_{\boxed{i}}, \boxed{x}_{\boxed{i}} \]

produces two normal x_i figures, and another two with an oversized i.

Answer 1

You can combine the advantages of the command \text (also provided by amsmath) and \boxed

\renewcommand{\boxed}[1]{\text{\fboxsep=.2em\fbox{\m@th$\displaystyle#1$}}}

With this redefinition and the relativ \fboxsep you get the following:

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\renewcommand{\boxed}[1]{\text{\fboxsep=.2em\fbox{\m@th$\displaystyle#1$}}}
\makeatother

\begin{document}
  \[ x_i, \boxed{x}_i, x_{\boxed{i}}, \boxed{x}_{\boxed{i}} \]
\end{document}

Answer 2

if you use amsmath, the command \boxed{...} puts a box around its argument.

to see an example, texdoc testmath. section 9.7 (p.18) deals with boxed formulas. although the entire formula is boxed in that example, the technique works as well with smaller elements.

\boxed is equivalent to \fbox except that its contents are in math mode.

Answer 3

The simplest is probably \fbox{$y$}