# numbering a system of equations [duplicate]

How can I add a number to the following system of equations so that the first equation take the number (1.a) and the second (1.b)?

\begin{equation}
\left\{
\begin{array}{l}
\begin{aligned}
x_k & = f_d(x_{k-1},u_{k-1},\theta) + q_{k-1}\\
y_k & = h(x_k,\theta,k) + r_k
\end{aligned}
\end{array}
\right.
\label{eq:State}
\end{equation}

• See the empheq package and combine that with the the subequations env from amsmath – daleif Apr 25 '17 at 18:25
• – Werner Apr 25 '17 at 18:26

Use empheq to style your equation set, together with subequations to achieve your numbering objective: \documentclass{article}

\usepackage{empheq,etoolbox}
% Update display of subequation numbering (Xy) > (X.y)
\patchcmd{\subequations}% <cmd>
{\theparentequation\alph{equation}}% <search>
{\theparentequation.\alph{equation}}% <replace>
{}{}% <success><failure>

\begin{document}

\begin{subequations}
\begin{empheq}[left=\empheqlbrace]{align}
x_k &= f_d(x_{k-1},u_{k-1},\theta) + q_{k-1} \\
y_k &= h(x_k,\theta,k) + r_k
\end{empheq}
\end{subequations}

\end{document}


The default numbering within subequations has the form (Xy). The etoolbox patch is applied in the preamble to change this to (X.y) for all subequations.