How to get sub-numbering on equations?

Question

How can I get a numbering of my equations like this:

(1)  a. ...
     b. ...

I know that I get

(1a) ...
(1b) ... 

with

\begin{subequations}
\begin{flalign}
    & ...\\
    & ...
\end{flalign}
\end{subequations}

but this is not exactly what I want.

I also want to be able to reference (1) as well as (1a).

Answer 1

You can do it. But don't abuse flalign.

\documentclass[fleqn,leqno]{article}
\usepackage{amsmath}
\usepackage{lipsum} % for context
\makeatletter
% from https://tex.stackexchange.com/a/261647/4427
% detach \eqref and \tag making
\renewcommand{\eqref}[1]{\textup{\eqreftagform@{\ref{#1}}}}
\let\eqreftagform@\tagform@
% specific code for the question
% 1. define a new tag form for subequations
\newcommand\subtagform@[1]{%
  \maketag@@@{%
    \ifx#1\theequation
      \ifnum\value{equation}=1
        (\theparentequation)%
      \else
        \phantom{(\theparentequation)}%
      \fi
      \ \alph{equation}.%
    \else
      \ignorespaces(#1\unskip@@italiccorr)%
    \fi
  }%
}
% 2. tell LaTeX to use the new tag form
\AtBeginEnvironment{subequations}{\let\tagform@\subtagform@}
\makeatother
% you need bigger left margin
\setlength{\mathindent}{4em}
\begin{document}
\begin{subequations}\label{test}
\lipsum[1][1-4]
\begin{align}
1&=1 \label{test-a}\
2&=2 \label{test-b}
\end{align}
\lipsum[1][1-4]
\end{subequations}
\eqref{test}, \eqref{test-a}, \eqref{test-b}
Now a standard equation
\begin{equation}
3=3
\end{equation}
\end{document}