Equation numbers for boxed multiline equations

Question

I am attempting to set a box around multiline equations is LaTeX using the following code:

\documentclass{article}

\usepackage{empheq}
\newcommand*\widefbox[1]{\fbox{\hspace{2em}#1\hspace{2em}}}

\begin{document}

\begin{subequations}
\begin{empheq}[box=\widefbox]{align}
z &= a + b + c + d + e + f + g + h + i + j + k \\
&\qquad\qquad + l + m + n + o + p + q + r + s \\
&\qquad\qquad + t + u + v + w + x + y
\end{empheq}
\end{subequations}

\end{document}

The problem is that the equations are automatically labelled (1a), (1b), and (1c). Since this is only one equation, how can I label the whole box simply as (1)?

Answer 1

You could omit the subequations environment specifiers, change align to equation (in the supplemental argument to empheq), and use a split environment inside empheq to achieve the alignment objectives.

\documentclass{article}
\usepackage{empheq}
\newcommand*\widefbox[1]{\fbox{\hspace{2em}#1\hspace{2em}}}
\begin{document}

\begin{empheq}[box=\widefbox]{equation}
\begin{split}
z &= a + b + c + d + e + f + g + h + i + j + k \\
&\qquad\qquad + l + m + n + o + p + q + r + s \\
&\qquad\qquad + t + u + v + w + x + y
\end{split}
\end{empheq}
\end{document}