Two columns of equations, aligned and just one number per column

Question

I'm writing a two-column document, and I want to display two sets of large equations floating at the top. Each equation set should be aligned within the same column, and only the two top equations should be numbered. Something like this:

a = (b,c) , where        (1)    a = (b,c) , where        (2)
b = B                           b = B
c = C                           c = C

This is the best I could get:

\begin{figure*}
\begin{tabularx}{\textwidth}{@{}XX@{}}
    \begin{align}
        a\in(b,c) \label{eq1}, \text{where}\\
        b=B \nonumber \\
        c=C \nonumber
    \end{align} &
    \begin{align}
        a\in(b,c) \label{eq1}, \text{where}\\
        b=B \nonumber \\
        c=C \nonumber
    \end{align} 
\end{tabularx}
\end{figure*}

That should have worked fine, but as you see, the two top equations have \in instead of =, so the alignment is messed up. I could just write &\in and &= below, but for some reason it seems to take that as the column separator of tabularx, and it won't compile.

How can I make this work?

Answer 1

You haven't put any alignment points in your align so it is not that alignment is messed up, it wasn't attempted.

Use

{\begin{align}
    a&\in(b,c) \label{eq1}, \text{where}\\
    b&=B \nonumber \\
    c&=C \nonumber
\end{align} }

The outer {} would hide the alignment from tabularx although really using tabularx is very inefficient here. You could just use two minipage of half \textwidth.

\documentclass[twocolumn]{article}
\usepackage{amsmath}
\begin{document}

\begin{figure*}
\noindent\begin{minipage}[t]{.5\textwidth}
    \begin{align}
        a&\in(b,c) \label{eq1}, \text{where}\\
        b&=B \nonumber \\
        c&=C \nonumber
    \end{align}
\end{minipage}%
\begin{minipage}[t]{.5\textwidth}
    \begin{align}
        a&\in(b,c) \label{eq1}, \text{where}\\
        b&=B \nonumber \\
        c&=C \nonumber
    \end{align} 
\end{minipage}
\end{figure*}

\end{document}