The subequations environment & the parentequation counter
The original subequations environment redefines \theparentequation in a way that it is independent from the counter parentequation:
\protected@edef\theparentequation{\theequation}%
But we can redefine the subequations environment so that only the value of equation is saved and \theparentequation uses the counter parentequation again:
\def\theparentequation{\arabic{parentequation}}%
But that way we would loose the setting of \theequation that might involve \thechapter in classes like book. For this, we could change it again manually to
\def\theparentequation{\thechapter.\arabic{parentequation}}%
etoolbox and \patchcmd
Another approach uses etoolbox and its \patchcmd macro.
First we let \theparentequation be the same as \theequation:
\let\theparentequation\theequation
After that we look for every ocurrence of equation and change it to parentequation:
\patchcmd{\theparentequation}{equation}{parentequation}{}{}
At this point we don't need the \definition of \theparentequation inside the subequations environment anymore:
\usepackage{etoolbox}
% let \theparentequation use the same definition as equation
\let\theparentequation\theequation
% change every occurence of "equation" to "parentequation"
\patchcmd{\theparentequation}{equation}{parentequation}{}{}
\renewenvironment{subequations}{%
\refstepcounter{equation}%
% \def\theparentequation{\arabic{parentequation}}% we patched it already :)
\setcounter{parentequation}{\value{equation}}%
\setcounter{equation}{0}%
\def\theequation{\theparentequation\alph{equation}}%
\ignorespaces
}{%
\setcounter{equation}{\value{parentequation}}%
\ignorespacesafterend
}
\nextParentEquation
Let me also define the command \nextParentEquation that increments parentequation and resets equation so that we don't have to do this manually.
\newcommand*{\nextParentEquation}{%
\stepcounter{parentequation}\setcounter{equation}{0}%
}
Workaround for parentequation labels (by OP Mobius Pizza)
While \label directly after \begin{subequations} works for referencing the first parent equation, to enable it to work after \nextParentEquation it is necessary to define a alias command \parentlabel for example, to evade sanitation performed by amsmath:
\let\parentlabel\label
Addition by Qrrbrbirlbel
I didn't think of labeling the parentequations, but with OP's addition (see above), I would even go a step further and redefine the subequations environment and the \nextParentEquation so that they take on optional argment, the label's name.
Code
This MWE uses report with one \chapter to demonstrate that \thechapter. doesn't get lost while redefining the subequations environment, this also works with article, of course.
\documentclass{report}
\usepackage{amsmath}
\usepackage{etoolbox}
% let \theparentequation use the same definition as equation
\let\theparentequation\theequation
% change every occurence of "equation" to "parentequation"
\patchcmd{\theparentequation}{equation}{parentequation}{}{}
\renewenvironment{subequations}[1][]{% optional argument: label-name for (first) parent equation
\refstepcounter{equation}%
% \def\theparentequation{\arabic{parentequation}}% we patched it already :)
\setcounter{parentequation}{\value{equation}}% parentequation = equation
\setcounter{equation}{0}% (sub)equation = 0
\def\theequation{\theparentequation\alph{equation}}%
\let\parentlabel\label% Evade sanitation performed by amsmath
\ifx\\#1\\\relax\else\label{#1}\fi% #1 given: \label{#1}, otherwise: nothing
\ignorespaces
}{%
\setcounter{equation}{\value{parentequation}}% equation = subequation
\ignorespacesafterend
}
\newcommand*{\nextParentEquation}[1][]{% optional argument: label-name for (first) parent equation
\refstepcounter{parentequation}% parentequation++
\setcounter{equation}{0}% equation = 0
\ifx\\#1\\\relax\else\parentlabel{#1}\fi% #1 given: \label{#1}, otherwise: nothing
}
\begin{document}
\chapter{Test}
\begin{equation}
0 \neq 1
\end{equation}
\begin{subequations}[eq:2]% or: \label{eq:2}
\begin{align}% or: \parentlabel{eq:2}
A & = B+1 \label{eq:2a} \\
B+1 & = C \label{eq:2b}
\intertext{therefore}\nextParentEquation[eq:3]% or: \nextParentEquation\parentlabel{eq:3}
A & = C \label{eq:3a} \\
B & = C-1 \label{eq:3b}
\end{align}
\end{subequations}
\eqref{eq:2}: \eqref{eq:2a}, \eqref{eq:2b} \\ \eqref{eq:3}: \eqref{eq:3a}, \eqref{eq:3b}
\begin{equation}
1 \neq 2
\end{equation}
\end{document}
Output
mathtools provides the multlined "subenvironment", which shifts the first line left,
the last line right, and applies only one number to each subequation.
put multiple multlined subequations together with gather:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\noindent X\hrulefill X\ignorespaces
\begin{subequations}
\label{energy13}
\begin{gather}
\begin{multlined}
u_e\frac{\partial \left(\varrho_e u_e\right)}{\partial t}
+ u_e\frac{\partial \left(\varrho_e u_e^2\right)}{\partial s}
+ u_e\frac{\partial \left(\varrho_e u_e v_e\right)}{\partial n}
-2 \varrho_e \varOmega u_e v_e \\
=- u_e\frac {\partial p_e}{\partial s}
+\qquad
+\varrho_e u_e\left(\varOmega^2R_{OPs}+\frac{\partial\varOmega}{\partial
t}(R_{OPn}+n)-\frac{\partial^2 R_{Os}}{\partial t^2}\right)
\end{multlined}
\\
\begin{multlined}
u\frac{\partial \left(\varrho u\right)}{\partial t}
+ u\frac{\partial \left(\varrho u^2 \right)}{\partial s}
+ u\frac{\partial \left(\varrho u v \right)}{\partial n}
-2 \varrho \varOmega u v \\
=- u\frac {\partial p}{\partial s}
+\mu u\frac {\partial^2 u}{\partial n^2}
+\varrho u \left(\varOmega^2R_{OPs}+\frac{\partial\varOmega}{\partial
t}(R_{OPn}+n)-\frac{\partial^2 R_{Os}}{\partial t^2}\right)
\end{multlined}
\end{gather}
\end{subequations}
\end{document}
the equation numbers are centered vertically on the subequations, rather than being
set on the last line -- that's how all the "subenvironments" work.
Best Answer
The main two problems in your code are the place of the first
&and the\textcommand in the middle of the equations. Use\intertext{...}for a text in the middle of aligned equations and to adjust the indentation use\qquads or any other\hspace{<length>}as you want in combination with\phantoms. A final note is for the11option to the\documentclass, this should be11pt.