Left-justified equations with left tags (so it looks like an enumeration)

Left-justified equations with left tags (so it looks like an enumeration)



I want to make a math enivornment (preferably a customized align/flalign

environment) with left-justified/left-aligned equations (as it is possible

with the flalign environment) and left-aligned tags (instead of the

default right alignment) but with the same indent handling flalign offers

when using right-aligned equations with default right-aligned tags.

I've added a screenshot and a MWE for clarification.

I'd appreciate any help :-)



\documentclass[twoside]{scrbook}

\parindent0pt

\parskip6pt

\usepackage[ngerman]{babel}

\usepackage[utf8]{inputenc}

\usepackage[T1]{fontenc}

\usepackage{amsmath, amssymb, enumitem, color}

\newcommand{\red}{\color{red}}

\newcommand{\set}[1]{\left\lbrace #1 \right\rbrace}

\newcommand{\script}[1]{\mathcal{#1}}

\renewcommand{\complement}{\mathcal{C}}

\makeatletter

\newenvironment{lalign}{\tagsleft@true\flalign}{\endflalign}

\makeatother

\begin{document}

\textbf{Definition} \quad ($\sigma$-field)

\begin{align}

& \Omega \in \script{A} \tag{SF1}\label{SF1} \\

& A \in \script{A} \Rightarrow \complement A \in \script{A}

\tag{SF2}\label{SF2} \\

& A_1, A_2, \ldots \in \script{A} \Rightarrow \bigcup_{i=1}^\infty A_i \in

\script{A} \tag{SF3}\label{SF3}

\end{align}

\textbf{Exercise} \quad Is $\script{A}$ a $\sigma$-field?

\begin{enumerate}[label=(\roman*)]

\item $\Omega$ arbitrary, $\script{A} = \set{\emptyset, \Omega}$.

\item $\ldots$

\end{enumerate}

\textbf{Solution} \quad

\begin{enumerate}[label=(\roman*)]

\item Let $\Omega$ be arbitrary and $\script{A} = \set{\emptyset, \Omega}$.

{\red It should be like this, but just the other way round:}

\begin{flalign}

&& \Omega \in \script{A} & \tag{\ref{SF1}} \\

&& \emptyset \in \script{A} \Rightarrow \complement \emptyset = \Omega \in

\script{A} \tag{\ref{SF2}} \\

&& \Omega \in \script{A} \Rightarrow \complement \Omega = \emptyset \in

\script{A} \notag \\

&& \emptyset, \Omega \in \script{A} \Rightarrow \emptyset \cup \Omega =

\Omega \in \script{A} \tag{\ref{SF3}}

\end{flalign}

{\red This happens when I set \texttt{tagsleft@true} and use

\texttt{flalign} for left-aligned equations:}

\begin{lalign}

& \Omega \in \script{A} & \tag{\ref{SF1}} \\

& \emptyset \in \script{A} \Rightarrow \complement \emptyset = \Omega \in

\script{A} \tag{\ref{SF2}} \\

& \Omega \in \script{A} \Rightarrow \complement \Omega = \emptyset \in

\script{A} \notag \\

& \emptyset, \Omega \in \script{A} \Rightarrow \emptyset \cup \Omega =

\Omega \in \script{A} \tag{\ref{SF3}}

\end{lalign}

{\red I don't want centered equations because it should look like an

enumeration:}

\begin{lalign}

& \Omega \in \script{A} \tag{\ref{SF1}} \\

& \emptyset \in \script{A} \Rightarrow \complement \emptyset = \Omega \in

\script{A} \tag{\ref{SF2}} \\

& \Omega \in \script{A} \Rightarrow \complement \Omega = \emptyset \in

\script{A} \notag \\

& \emptyset, \Omega \in \script{A} \Rightarrow \emptyset \cup \Omega =

\Omega \in \script{A} \tag{\ref{SF3}}

\end{lalign}

{\red As it should look like (dirty coded, I want the code to be

universally usable):}

\begin{lalign}

& \hspace*{1.2cm} \Omega \in \script{A} & \tag{\ref{SF1}} \\

& \hspace*{1.2cm} \emptyset \in \script{A} \Rightarrow \complement

\emptyset = \Omega \in \script{A} \tag{\ref{SF2}} \\

& \hspace*{1.2cm} \Omega \in \script{A} \Rightarrow \complement \Omega =

\emptyset \in \script{A} \notag \\

& \hspace*{1.2cm} \emptyset, \Omega \in \script{A} \Rightarrow \emptyset

\cup \Omega = \Omega \in \script{A} \tag{\ref{SF3}}

\end{lalign}

\item $\ldots$

\end{enumerate}

\end{document}