\centering\arraybackslash with an array environment?

Question

The following code gives me an error:

\documentclass[12pt]{article}
\usepackage[margin=2cm]{geometry}
\usepackage [english]{babel}
\usepackage{amsmath, amssymb}
\usepackage{enumitem, array}
\begin{document}
{\begin{minipage}[t]{\textwidth}
\begin{enumerate}[label=(\roman*)]
{\setlength\itemindent{10pt}\item 
\begin{equation*}
\begin{array}{>{\centering\arraybackslash}m{1.5cm}>{\centering\arraybackslash}m{1.5cm}>{\centering\arraybackslash}m{1.5cm}>{\centering\arraybackslash}m{1.5cm}>{\centering\arraybackslash}m{1.5cm}}

\hline
x & l_{[x]} & d_{[x]} & l_{x+1} & \mathring{e}_{[x]} \\
\hline
\end{array}
\end{equation*}}
\end{enumerate}
\end{minipage}}
\end{document}

Note: for various reasons, I have to keep minipage there. For some reason, this code gives me an error when it compiles. It gives the following output:

The errors either say there is a } missing or a $ missing. My eyes don't see such a thing!

Answer 1

Only l, c and r columns in array are supposed to contain just math mode material, since paragraphs in math mode don't make much sense. However, you can state math mode in p, m or b columns by adding $ to the specifications. Namely

\begin{equation*}
\begin{array}{
  >{\centering\arraybackslash$}m{1.5cm}<{$}
  >{\centering\arraybackslash$}m{1.5cm}<{$}
  >{\centering\arraybackslash$}m{1.5cm}<{$}
  >{\centering\arraybackslash$}m{1.5cm}<{$}
  >{\centering\arraybackslash$}m{1.5cm}<{$}
}
\hline
x & l_{[x]} & d_{[x]} & l_{x+1} & \mathring{e}_{[x]} \\
\hline
\end{array}
\end{equation*}

Actually, \arraybackslash is necessary only in the last column. It would be much easier adding

\newcolumntype{M}[1]{>{\centering\arraybackslash$}m{#1}<{$}}

to your preamble and specify the table as

\begin{equation*}
\begin{array}{ *{5}{M{1.5cm}} }
\hline
x & l_{[x]} & d_{[x]} & l_{x+1} & \mathring{e}_{[x]} \\
\hline
\end{array}
\end{equation*}

Answer 2

Your problem comes from the blankline and the fact the m type columns are not in math mode. While I was at it, I simplified your code:

\documentclass[12pt]{article}
\usepackage[margin=2cm]{geometry}
\usepackage [english]{babel}
\usepackage{amsmath, amssymb}
\usepackage{enumitem, array}

\begin{document}

{\begin{minipage}[t]{\textwidth}
\begin{enumerate}[label=(\roman*)]
{\setlength\itemindent{10pt}\item%
\begin{equation*}
\begin{array}{*{5}{>{\centering\arraybackslash $}m{1.5cm}<{$}}}
\hline
x & l_{[x]} & d_{[x]} & l_{x+1} & \mathring{e}_{[x]} \\
\hline
\end{array}
\end{equation*}}
\end{enumerate}
\end{minipage}}

\end{document}