Centered left aligned `gather` environment

amsmathmath-mode

I know that if you pass fleqn as an option, all amsmath's environments becomes left aligned, including gather, however, along with that, they become flushleft.

Actually, what I'm trying to create is a left aligned gather environment that would be horizontally centered relatively to the page. fleqn is not what I need, because I want to keep the default behavior of environments, but I found a way to temporarily turn this option on. The question is about how I can center my leftceq environment without using additional packages as etoolbox?

There is a dirty workaround with align environment that makes me put & at the beginning of each line that's drastically inconvenient with lots of math equations.

\documentclass{article}
\usepackage{mathtools}
\mathtoolsset{showonlyrefs=true}
\usepackage{showframe}

\makeatletter
\newenvironment{leftceq}{
\@fleqntrue%
\gather%
}{%
\endgather
}
\makeatother


\begin{document}
\verb|my environment|
\begin{leftceq}
y_1 = C_1e^{-4x}+C_2e^{-5x}+C_3xe^{-5x}\\
y_2 = C_1e^{6x}+C_2e^{-9x}\cos x+C_3e^{-9x}\sin x
\end{leftceq}\\

\verb|desired result, faked with align environment|
\begin{align}
&y_1 = C_1e^{-4x}+C_2e^{-5x}+C_3xe^{-5x}\\
&y_2 = C_1e^{6x}+C_2e^{-9x}\cos x+C_3e^{-9x}\sin x
\end{align}\\

\verb|default gather|
\begin{gather}
y_1 = C_1e^{-4x}+C_2e^{-5x}+C_3xe^{-5x}\\
y_2 = C_1e^{6x}+C_2e^{-9x}\cos x+C_3e^{-9x}\sin x
\end{gather}

\end{document}

Best Answer

Use the “fake” but making it automatic.

\documentclass{article}
\usepackage{amsmath}

\ExplSyntaxOn

\NewDocumentEnvironment{leftceq}{b}
 {
  \antshar_leftceq:nn { } { #1 }
 }
 {}
\NewDocumentEnvironment{leftceq*}{b}
 {
  \antshar_leftceq:nn { * } { #1 }
 }
 {}

\seq_new:N \l__antshar_leftceq_seq

\cs_new_protected:Nn \antshar_leftceq:nn
 {
  \seq_set_split:Nnn \l__antshar_leftceq_seq { \\ } { #2 }
  \begin{align#1}
  & \seq_use:Nn \l__antshar_leftceq_seq { \\ & }
  \end{align#1}
 }

\ExplSyntaxOff

\begin{document}

Numbered
\begin{leftceq}
y_1 = C_1e^{-4x}+C_2e^{-5x}+C_3xe^{-5x}\\
y_2 = C_1e^{6x}+C_2e^{-9x}\cos x+C_3e^{-9x}\sin x
\end{leftceq}

Unnumbered
\begin{leftceq*}
y_1 = C_1e^{-4x}+C_2e^{-5x}+C_3xe^{-5x}\\
y_2 = C_1e^{6x}+C_2e^{-9x}\cos x+C_3e^{-9x}\sin x
\end{leftceq*}

\end{document}

This splits the environment's content at \\ and places & in front of every line when passing the lines to align.

Bad version asked in comments

\ExplSyntaxOn
\NewDocumentEnvironment{leftceq}{b}
 {
  \seq_set_split:Nnn \l_tmpa_seq { \\ } { #1 }
  \begin{align*} & \seq_use:Nn \l_tmpa_seq { \\ & } \end{align*}
 }{}
\ExplSyntaxOff

I cannot recommend doing this, because if it turns out that the numbered version is necessary, one has to duplicate the code. And separating the user interface from the internal code is a plus, besides being the recommended way.

If you want to support nested alignments such as cases, I'm afraid you have to do it the hard way.

This is a fast (and quite likely it can be improved) modification of the amsmath code.

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

\makeatletter

\newenvironment{leftceq}{%
  \start@leftceq\@ne\st@rredtrue\m@ne
}{\endalign}

\def\start@leftceq#1#2#3{%
    \let\xatlevel@#1% always \z@, \@ne, or \tw@
    \maxfields@#3\relax
    \ifnum\maxfields@>\m@ne
        \checkat@true
        \ifnum\xatlevel@=\tw@
            \xxat@true
        \fi
        \multiply\maxfields@\tw@
    \else
        \checkat@false
    \fi
    \ifingather@
        \iffalse{\fi\ifnum0=`}\fi
        \DN@{\vcenter\bgroup\savealignstate@\leftceq@#2}%
    \else
        \ifmmode
          \if@display
             \DN@{\align@recover}%
          \else
            \nomath@env
            \DN@{\@namedef{end\@currenvir}{}\@gobble}%
          \fi
        \else
            $$%
            \let\split\insplit@
            \DN@{\leftceq@#2}%
        \fi
    \fi
    \collect@body\next@
}
\def\leftceq@#1#2{%
    \inalign@true \intertext@ \Let@ \chardef\dspbrk@context\z@
    \ifingather@\else\displ@y@\fi
    \let\math@cr@@@\math@cr@@@align
    \ifxxat@\else \let\tag\tag@in@align \fi
    \let\label\label@in@display
    #1% set st@r
    \ifst@rred\else \global\@eqnswtrue \fi
    \measure@{#2}%
    \global\row@\z@
    \tabskip\eqnshift@
    \halign\bgroup
        \span\leftceq@preamble\crcr
        #2%
}
\def\leftceq@preamble{%
   &\setboxz@h{\@lign$\m@th\displaystyle{{}##}$}%
    \ifmeasuring@\savefieldlength@\fi
    \set@field
    \hfil
    \tabskip\alignsep@
}

\makeatletter

\begin{document}

\begin{leftceq}
y_1 = C_1e^{-4x}+C_2e^{-5x}+C_3xe^{-5x}\\[2ex]
y_2 = C_1e^{6x}+C_2e^{-9x}\cos x+C_3e^{-9x}\sin x \\[4ex]
\begin{cases} a & x \\ b & y \end{cases} \\
y_1 = C_1e^{-4x}+C_2e^{-5x}+C_3xe^{-5x}\\
y_2 = C_1e^{6x}+C_2e^{-9x}\cos x+C_3e^{-9x}\sin x
\end{leftceq}

\end{document}

Related Solutions

[Tex/LaTex] Difference between (split, align) and (gather, aligned)

For the example alignment the following are equivalent; actually, the correct outer environment to use is equation, as the following code will show; using split or aligned for the inner environment in this case doesn't matter.

\documentclass{article}
\usepackage{amsmath,lipsum,geometry}
\geometry{margin=1cm}
\begin{document}
%\thispagestyle{empty}

\lipsum*[2]
%==================
% equation+aligned
\begin{equation}\tag{EA}
\begin{aligned}
x &= a + b\\
y &= c+ d
\end{aligned}
\end{equation}
\lipsum*[2]
%==================
% equation+split
\begin{equation}\tag{ES}
\begin{split}
x &= a + b\\
y &= c+ d
\end{split}
\end{equation}
\lipsum*[2]
%==================
% gather+aligned
\begin{gather}\tag{GA}
\begin{aligned}
x &= a + b\\
y &= c+ d
\end{aligned}
\end{gather}
\lipsum*[2]
%==================
% gather+split
\begin{gather}\tag{GS}
\begin{split}
x &= a + b\\
y &= c+ d
\end{split}
\end{gather}
\lipsum*[2]
%==================
% align+split
\begin{align}\tag{AS}
\begin{split}
x &= a + b\\
y &= c+ d
\end{split}
\end{align}
\lipsum*[2]
%==================
% align+aligned
\begin{align}\tag{AA}
\begin{aligned}
x &= a + b\\
y &= c+ d
\end{aligned}
\end{align}
\lipsum*[2]

\end{document}

In the output it's possible to see that the correct spacing is obtained with equation. Use align or gather only when there is really more than one equation to number.

One should also note that "equation+split" results in a different vertical spacing than "equation+aligned" (which isn't really expected).

enter image description here

[Tex/LaTex] Linebreaking gather (equation) environment

You can get the numbers by setting the equation counter to one less than the first number you want, in this case 19. You had a glitch after the {split} because you used \newline, which is a text and not a math command. Don't mix the two! I put in \\ instead, and a \notag to suppress the number that would be there (going by your example). Finally, I replaced your explicit spacing in the last few lines with an {aligned} subenvironment, which allows me to get the final equations lined up as well as automatically spread out.

  \documentclass{article}
  \usepackage[fleqn]{amsmath}

  \begin{document}

  \setcounter{equation}{19}
  \begin{gather}
  Q(CSAD_t(\tau)|\mathbf{Z})
        = \alpha(\tau) + \beta(\tau)|R_{M,t}| + \gamma(\tau)R_{M,t}^2 + \epsilon_t \\
  \begin{aligned}
  Q(CSAD_t(\tau)|\mathbf{Z})
        &= \alpha(\tau) + \beta(\tau)_0|R_{M,t}| + \gamma(\tau)_0R_{M,t}^2 \\
        &+ \beta(\tau)_1D_{ALL,t}|R_{M,t}| + \gamma(\tau)_1D_{ALL,t}R_{M,t}^2 + \epsilon_t
  \end{aligned} \notag\\
  Q(CSAD_t(\tau)|\mathbf{Z})
        = \alpha(\tau) + \mathbf{\beta(\tau)}\mathbf{D}'|R_{M,t}| + \mathbf{\gamma(\tau)}\mathbf{D}'R_{M,t}^2 + \epsilon_t \\
    \begin{aligned}
  \mathbf{\beta(\tau)} &= [\beta(\tau)_0, \beta(\tau)_1,...,\beta(\tau)_6]; &
    \mathbf{\gamma(\tau)} &= [\gamma(\tau)_0,\gamma(\tau)_1,...,\gamma(\tau)_6];\notag\\
    \mathbf{D}& = [1, D_{1,t},...,D_{6,t}] &
  \tau &= 0.05,0.1,0.5,0.9,0.95 \notag 
\end{aligned}
  \end{gather}

  \end{document}

Best Answer

Bad version asked in comments

Related Solutions

[Tex/LaTex] Difference between (split, align) and (gather, aligned)

[Tex/LaTex] Linebreaking gather (equation) environment

Related Question