I'm trying to make a table containing 4 equations in which the two in the left column are labelled 1a and 1b, and the two in the right column are labelled 2a and 2b.
I've been able to construct such a table using the array environment, except my equations are labelled 1, 2, 3, 4:
\documentclass{article}
\usepackage{amsmath}
\usepackage{booktabs}
\newcommand{\dd}[2] {\frac{\partial #1}{\partial #2}}
\begin{document}
\begin{table}
\centering
$\begin{array}{ c l r l r }
& \multicolumn{1}{c}{\text{Dynamic b.c.}} & & \multicolumn{1}{c}{\text{Kinematic b.c.}} & \\
\addlinespace
O(\epsilon) &
\begin{aligned} \dd{\phi_1}{t} = 0 \end{aligned} & \quad \refstepcounter{equation}(\theequation)\label{dynamicEps} \quad &
\begin{aligned} \dd{h_1}{t} = v_1 \end{aligned} & \quad \refstepcounter{equation}(\theequation)\label{kinematicEps} \\
\addlinespace
O(\epsilon^2) &
\begin{aligned} \dd{\phi_2}{t} = -h_1 \dd{v_1}{t} - \frac{1}{2}(u_1^2 + v_1^2) \end{aligned} & \quad \refstepcounter{equation}(\theequation)\label{dynamicEpsSq} \quad &
\begin{aligned} \dd{h_2}{t} = v_2 + h_1 \dd{v_1}{y} - u_1 \dd{h_1}{x} \end{aligned} & \quad \refstepcounter{equation}(\theequation)\label{kinematicEpsSq} \\
\end{array}$
\caption{Free surface boundary conditions at $y=\beta$ resulting from domain perturbation expansion.}
\label{bcTableFreeSurface}
\end{table}
\end{document}
It seems that \tag
doesn't work inside the array environment, so I'm not sure of the best way to get this subequation numbering.