Indentation of left-aligned equations (flalign) without fleqn

Question

I would like to have some math not centered but left aligned. So I found on this site various answers that pointed me to this:

\documentclass{article}
\usepackage{showframe}
\usepackage{amsmath}
\begin{document}
\begin{flalign*}
1+1=2  &&
\end{flalign*}
\end{document}

However I also would like the left aligned math to be indented by \parindent, how could I do that without loading fleqn as a general option?

@Ch'enMeng because only some math should be left aligned, not all. — BeeGirl, Dec 07 '13 at 13:37
@BeeGirl Well, try $ ... $ with \displaystyle (if really need)? — Ch'en Meng, Dec 07 '13 at 13:45
@Ch'enMeng but that doesn't work for multiline math? Despite my example I need this for multiline equations. — BeeGirl, Dec 07 '13 at 13:48
@BeeGirl aligned environment, provided by amsmath, works well in inline-math. And its syntax is familiar with align and split who are used to typeset multiline equations. — Ch'en Meng, Dec 07 '13 at 13:51
possible duplicate of problem using option fleqn in amsmath package — Henri Menke, Dec 07 '13 at 13:51
@HenriMenke that is not the same issue since the OP is using fleqn, while I said I don't want to — BeeGirl, Dec 07 '13 at 13:54
@Ch'enMeng thanks, aligned seems to be almost it. Is it possible to control the indent amount of an aligned env and set it to \parindent? — BeeGirl, Dec 07 '13 at 13:56
@BeeGirl The environment aligned is often used as a part of a certain equaion (or maybe the whole part), say block-env. Hence, the answer is no. :( — Ch'en Meng, Dec 07 '13 at 14:05
@Closers this question is different from the one linked as a duplicate since here the OP wants both centered and left-aligned equations. — karlkoeller, Dec 07 '13 at 15:41
Just to be picky: flalign *doesn't* mean “left aligned equations”, but “full length align”. Having some display centered and others left aligned doesn't make sense. — egreg, Jun 25 '14 at 09:42

score 12 · Answer 1 · answered Dec 07 '13 at 15:30

A very simple method.

We define a new command

\newcommand{\mathleft}{\@fleqntrue\@mathmargin\parindent}

to simulate the option fleqn with \parindent indenting, and a new command to restore the normal behavior

\newcommand{\mathcenter}{\@fleqnfalse}

So, add the following lines in the preamble

\makeatletter
\newcommand{\mathleft}{\@fleqntrue\@mathmargin\parindent}
\newcommand{\mathcenter}{\@fleqnfalse}
\makeatother

and issue \mathleft before and \mathcenter after the math stuff you want left aligned with \parindent.

You can use this solution with all math environments without redefining any of them.

MWE

\documentclass{article}
\usepackage{showframe}
\usepackage{amsmath}

\makeatletter
\newcommand{\mathleft}{\@fleqntrue\@mathmargin\parindent}
\newcommand{\mathcenter}{\@fleqnfalse}
\makeatother

\begin{document}

\mathleft
\begin{equation}
1+1=2
\end{equation}
\mathcenter

Indented paragraph

\begin{equation}
1+1=2
\end{equation}

Indented paragraph

\end{document}

Output

enter image description here

this is not really safe, the fleqn option is a mixture of load-time and run-time changes and changing the value of the boolean flag after the package is loaded leaves amsmath in a broken state, see for example https://tex.stackexchange.com/a/462885 — David Carlisle, Dec 03 '18 at 00:14

score 7 · Accepted Answer · answered Dec 07 '13 at 14:16

Remarks

You can indent your flalign by using the definition of flalignfrom amsmath.sty and modifiying it to your liking, e.g. inserting \hskip\parindent.

Implementation

\documentclass{article}
\pagestyle{empty} % for cropping
\newcommand\shortlipsum{
    Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit,
    vestibulum ut, placerat ac, adipiscing vitae, felis. Curabitur dictum gravida
    mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna.\par
} % Filltext
\usepackage{amsmath}
\makeatletter
\newenvironment{shiftedflalign}{%
    \start@align\tw@\st@rredfalse\m@ne%
    \hskip\parindent
}{%
    \endalign
}
\newenvironment{shiftedflalign*}{%
    \start@align\tw@\st@rredtrue\m@ne
    \hskip\parindent
}{%
    \endalign
}
\makeatother
\begin{document}

\shortlipsum
\begin{flalign}
    1+1=2  &&
\end{flalign}
\shortlipsum
\moveright\parindent\vbox{%
\begin{flalign}
    1+1=2  &&
\end{flalign}
}
\shortlipsum
\begin{shiftedflalign}
    1+1=2  &&
\end{shiftedflalign}
\shortlipsum

\begin{flalign*}
    1+1=2  &&
\end{flalign*}
\shortlipsum
\begin{shiftedflalign*}
    1+1=2  &&
\end{shiftedflalign*}
\shortlipsum

\end{document}

Output

ok, this is it. Could you point out to a question that describes the differences between the various math environments? Also a resource online, not necessarily on this site. Thanks — BeeGirl, Dec 07 '13 at 15:14
@BeeGirl The mathmode manual by Herbert Voß describes all the important things. — Henri Menke, Dec 07 '13 at 16:08
@HenriMenke Can you remove the superfluous % in \newenvironment{shiftedflalign}? It caused me to question why that was not in \newenvironment{shiftedflalign*}. — Sean Leather, Jun 25 '14 at 09:23
As an aside, you can use \mathindent (see LaTeX/Advanced Mathematics) instead of \parindent to get the normal indentation used by fleqn. — Sean Leather, Jun 25 '14 at 09:26

Indentation of left-aligned equations (flalign) without fleqn

2 Answers2

Remarks

Implementation

Output

Linked