# Numbering in subequation and align environments

I use the subequations and align environments as follows:

\begin{subequations}
\begin{align}
\sum_{a \in \delta^+_G(v)} f^k_a -  \sum_{a \in \delta^-_G(v)} f^k_a  &= d^k_v, & \forall v \in V, k \in K \\
\sum_{k \in K} f^k_a  &\le \sum_{m \in M}c^m x^m_a, & \forall a \in A   & \quad \texttt{Directed}\\
\max \left\{\sum_{k \in K} \{f^k_{e+},f^k_{e-}\}  \right\}&\le \sum_{m \in M}c^m x^m_a, & \forall e \in E   & \quad \texttt{Bidirected}\\
\sum_{k \in K} (f^k_{e+}+f^k_{e-})&\le \sum_{m \in M}c^m x^m_e, & \forall e \in E   & \quad \texttt{Undirected}
\end{align}
\end{subequations}

which shows the equations numbered as
1.a
1.b
1.c
1.d

However, I'd like the numbering to be displayed as
1
2.a
2.b
2.c

• Welcome to TSE. Please post a Minimal Working Example, instead of a code snippet. Oct 24 '21 at 10:28

I would like to suggest that you (a) embed all four statements in a gather environment and (b) embed the three inequalities in combined subequations and alignat environments. I would also recommend using \smashoperator directives in order to typeset the summation expressions in the first row more compactly.

\documentclass{article}
\usepackage{mathtools} % for \smashoperator macro

\begin{document}
\begin{gather}
\smashoperator{\sum_{a \in \delta^+_G(v)}} f^k_a -
\smashoperator{\sum_{a \in \delta^-_G(v)}} f^k_a  =
d^k_v, \quad \forall v \in V,\ k \in K
\\[\jot]
\begin{subequations}
\begin{alignat}{3}
\sum_{k \in K} f^k_a
&\le \sum_{m \in M}c^m x^m_a,
\max \biggl\{\sum_{k \in K} \{f^k_{e+},f^k_{e-}\}  \biggr\}
&\le \sum_{m \in M}c^m x^m_a,
&& \forall e \in E
&& \texttt{Bidirected} \\
\sum_{k \in K} (f^k_{e+}+f^k_{e-})
&\le \sum_{m \in M}c^m x^m_e,
&& \forall e \in E
&&  \texttt{Undirected}
\end{alignat}
\end{subequations}
\end{gather}
\end{document}
• + Very good suggestion to use smash for the summation symbol. Personally I would use \tag for sub equations. Nice and simple. Oct 24 '21 at 12:43

Mico just posted his answer as I was writing this, and you should consider that, but to directly answer the question with align I'd probably not use subequations and simply tag the later equations so

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align}
\sum_{a \in \delta^+_G(v)} f^k_a -  \sum_{a \in \delta^-_G(v)} f^k_a  &= d^k_v, & \forall v \in V, k \in K \\
\sum_{k \in K} f^k_a  &\le \sum_{m \in M}c^m x^m_a, & \forall a \in A   & \quad \texttt{Directed} \refstepcounter{equation}\tag{\theequation a}\\
\max \left\{\sum_{k \in K} \{f^k_{e+},f^k_{e-}\}  \right\}&\le \sum_{m \in M}c^m x^m_a, & \forall e \in E   & \quad \texttt{Bidirected} \tag{\theequation b}\\
\sum_{k \in K} (f^k_{e+}+f^k_{e-})&\le \sum_{m \in M}c^m x^m_e, & \forall e \in E   & \quad \texttt{Undirected}  \tag{\theequation c}
\end{align}
\end{document}
• The simpler the better, always! Oct 24 '21 at 12:44

You can define a new specialsubequations environment in which the first item is treated specially with the help of \specialequation.

\documentclass{article}
\usepackage{amsmath}
%\usepackage{hyperref} % not mandatory

\makeatletter
\newcounter{specialsubequations}
\newenvironment{specialsubequations}
{%
\stepcounter{specialsubequations}%
\stepcounter{equation}%
\let\@currentlabel\theequation
\label{@@@fake\thespecialsubequations @@@}%
\subequations
}
{\endsubequations}
\newcommand{\specialequation}{\tag{\ref*{@@@fake\thespecialsubequations @@@}}}%
}{%
\newcommand{\specialequation}{\tag{\ref{@@@fake\thespecialsubequations @@@}}}%
}
\makeatother

\begin{document}

\lipsum[1][1-3]
\begin{specialsubequations}
\begin{align}
\sum_{a \in \delta^+_G(v)} f^k_a -  \sum_{a \in \delta^-_G(v)} f^k_a
&= d^k_v, & \forall v \in V, k \in K \specialequation\label{1}
\\
\sum_{k \in K} f^k_a  &\le \sum_{m \in M}c^m x^m_a,
& \forall a \in A   & \quad \texttt{Directed}
\\
\max \left\{\sum_{k \in K} \{f^k_{e+},f^k_{e-}\}  \right\}
&\le \sum_{m \in M}c^m x^m_a, & \forall e \in E   & \quad \texttt{Bidirected}
\\
\sum_{k \in K} (f^k_{e+}+f^k_{e-})&\le \sum_{m \in M}c^m x^m_e,
& \forall e \in E   & \quad \texttt{Undirected}
\end{align}
\end{specialsubequations}
\lipsum[2][1-3]

\end{document}

However I'd recommend not changing the number.

\documentclass{article}
\usepackage{amsmath}
%\usepackage{hyperref} % not mandatory; use \ref* if hyperref is loaded

\usepackage{lipsum} % for mock text

\begin{document}

\lipsum[1][1-3]
\begin{subequations}\label{main}
\begin{align}
\sum_{a \in \delta^+_G(v)} f^k_a -  \sum_{a \in \delta^-_G(v)} f^k_a
&= d^k_v, & \forall v \in V, k \in K \tag{\ref{main}}
\\
\sum_{k \in K} f^k_a  &\le \sum_{m \in M}c^m x^m_a,
& \forall a \in A   & \quad \texttt{Directed}
\\
\max \left\{\sum_{k \in K} \{f^k_{e+},f^k_{e-}\}  \right\}
&\le \sum_{m \in M}c^m x^m_a, & \forall e \in E   & \quad \texttt{Bidirected}
\\
\sum_{k \in K} (f^k_{e+}+f^k_{e-})&\le \sum_{m \in M}c^m x^m_e,
& \forall e \in E   & \quad \texttt{Undirected}
\end{align}
\end{subequations}
\lipsum[2][1-3]

\end{document}