Missing characters inserted when using new commands in equations

Question

I'm having a problem that I can't seem to fix. I would like to define a math symbol, and shown in the code snippet, which I use frequently throughout the text. When I want to include this same symbol inside an equation, I get a lot of "missing character inserted" errors and the formatting is wrong at the other side.

\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}

\newcommand{\WPT}{$p_{\text{T}}^W$}

\begin{document}
The result for \WPT is shown below.

\begin{equation}
  r_s^{\WPT} &= 100.00
\end{equation}

\end{document}

Thanks in advance for your help.

Answer 1

If I run the code you posted I get the error:

! Missing } inserted.
<inserted text> 
                }
l.10   r_s^{\WPT
                } = 100.00
?

this error happens because when your command expands you have (up to the point where the error happens) basically:

$$ r_s^{ $

which is not a valid TeX construct. You start display math, you start a group, then you end an inline math. Later on TeX would say that:

! Display math should end with $$.
<to be read again> 
                   p
l.13   r_s^{$p
              _{\text{T}}^W$} = 100
?

and would get even more confused trying to fix all that.

---

You can use \ensuremath which, as the name says, ensures that its contents are typeset in math-mode (notice the extra {} after \WPT in the text here):

\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}

\newcommand{\WPT}{\ensuremath{p_{\text{T}}^W}}

\begin{document}
The result for \WPT{} is shown below.

\begin{equation}
  r_s^{\WPT} = 100.00
\end{equation}

\end{document}

but, since \WPT is essentially math content, I'd go for the proper markup and wrap \WPT in $...$ in the text:

\documentclass[12pt]{article}
\usepackage{amsmath,amssymb}

\newcommand{\WPT}{p_{\text{T}}^W}

\begin{document}
The result for $\WPT$ is shown below.

\begin{equation}
  r_s^{\WPT} = 100.00
\end{equation}

\end{document}