How can the \textmu symbol be used in math mode and resize correctly?

Question

I wish to use an upright mu character in various places in my document, and so I've put the following in my preamble:

% textcomp, for its upright mu
\usepackage{textcomp}
% some user macros
\newcommand{\murm}{\hbox{\textmu}}

This is fine in both text and math mode, except when I try to place the mu in sub/superscript positions, and its size remains unchanged. Is there any way to resize it to fit its environment within math mode?

Answer 1

You can use \mathchoice to give four different definitions of the macro dependent if it is in display, in-text, script or scriptscript mode. (See also \mathpalette)

\newcommand{\murm}{%
    \mathchoice
        {\hbox{\normalsize\textmu}}
        {\hbox{\normalsize\textmu}}
        {\hbox{\scriptsize\textmu}}
        {\hbox{\tiny\textmu}}%
}

The whole expression should actually be wrapped into \ensuremath (amsmath package) or use \ifmmode manually to avoid errors if used in text-mode:

\newcommand{\murm}{%
  \ifmmode
    \mathchoice
        {\hbox{\normalsize\textmu}}
        {\hbox{\normalsize\textmu}}
        {\hbox{\scriptsize\textmu}}
        {\hbox{\tiny\textmu}}%
  \else
    \textmu
  \fi
}

Answer 2

Why don't you use \upmu from the upgreek package? It will resize automatically in math mode.