This working example:
\documentclass[a4paper]{report}
\usepackage{mleftright}
\usepackage{braket}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathrsfs}
\newcommand{\diff}{\mathop{}\!\mathrm{d}}
\begin{document}
they are related by Fourier transformation:
\begin{subequations}
\begin{subequations}
\label{Position-MomentumSpaceWaveFunctionsDiscr}
\begin{align}
\psi \mleft( x \mright) = \braket{x \vert \psi} = \braket{x \vert \hat{\mathbb{I}} \vert \psi} & = \braket{x \vert \mleft( \sum_{p' \in \mathscr{P}} \ket{p'} \bra{p'} \mright) \vert \psi} \notag \\
& = \sum_{p' \in \mathscr{P}} \braket { x \vert p' } \braket{ p' \vert \psi } = \sum_{p' \in \mathscr{P}} \braket { x \vert p' } \phi \mleft( p' \mright) \, , \label{PositionSpaceWaveFunctionsDiscr} \\
\phi \mleft( p \mright) = \braket{p \vert \psi} = \braket{p \vert \hat{\mathbb{I}} \vert \psi} & = \braket{p \vert \mleft( \sum_{x' \in \mathscr{X}} \ket{x'} \bra{x'} \mright) \vert \psi} \notag \\
& = \sum_{x' \in \mathscr{X}} \braket { p \vert x' } \braket{ x' \vert \psi } = \sum_{x' \in \mathscr{X}} \braket { p \vert x' } \psi \mleft( x' \mright) \label{MomentumSpaceWaveFunctionsDiscr}
\end{align}
\end{subequations}
or
\begin{subequations}
\label{Position-MomentumSpaceWaveFunctionsCont}
\begin{align}
\psi \mleft( x \mright) = \braket{x \vert \psi} = \braket{x \vert \hat{\mathbb{I}} \vert \psi} & = \braket{x \vert \mleft( \int\limits_{\mathscr{P}} \! \diff p' \, \ket{p'} \bra{p'} \mright) \vert \psi} \notag \\
& = \int\limits_{\mathscr{P}}\! \diff p' \, \braket { x \vert p' } \braket{ p' \vert \psi } = \int\limits_{\mathscr{P}}\! \diff p' \, \braket { x \vert p' } \phi \mleft( p' \mright) \, , \label{PositionSpaceWaveFunctionsCont} \\
\phi \mleft( p \mright) = \braket{p \vert \psi} = \braket{p \vert \hat{\mathbb{I}} \vert \psi} & = \braket{p \vert \mleft( \int\limits_{\mathscr{X}} \! \diff x' \, \ket{x'} \bra{x'} \mright) \vert \psi} \notag \\
& = \int\limits_{\mathscr{X}}\! \diff x' \, \braket { p \vert x' } \braket{ x' \vert \psi } = \int\limits_{\mathscr{X}}\! \diff x' \, \braket { p \vert x' } \psi \mleft( x' \mright) \, , \label{MomentumSpaceWaveFunctionsCont}
\end{align}
\end{subequations}
\end{subequations}
respectively, with $ \braket{p' \vert x'} = \braket{x' \vert p'}^* $ as outlined in \eqref{InnerProduct} and $ \mathscr{X} $ and $ \mathscr{P} $ denoting the position and momentum eigenspectra. Orthonormality requires:
\newline
\begin{subequations}
\begin{minipage}[t]{0.5\textwidth}
\begin{equation}
\label{OrthoNormPositionKetsDiscr}
\text{and }\braket{x_{1} \vert x} = \delta_{x_{1},x}
\end{equation}
\end{minipage}
\begin{minipage}[t]{0.5\textwidth}
\begin{equation}
\label{OrthoNormPositionKetsCont}
\braket{x_{1} \vert x} = \delta(x_{1}-x)
\end{equation}
\end{minipage}\hfill
\end{subequations}
\newline
\newline
\newline
for position kets and also momentum eigenkets:
\end{document}
produces the following output:
whereas naturally the numbering should continue at 2.
In my actual document, the nested subequations are 1.30aa, 1.30ab, 1.30ba, and 1.30bb, respectively and the numbering 'resets' to 1.3a & 1.3b in the following subequations in one line.
How to fix this?
\tag
and label by hand to just use normal equations and locally specify the equation counter formatting to get what you need – David Carlisle Jun 1 '20 at 15:53