Multiline equation with LHS alone on first line?

Question

I would like my multiline equations to look like this:

Left-hand-side of my equation
    = right-hand-side number 1
    = right-hand-side number 2
    = etc.

I know there is a simple way to do this, because I found it a while ago when I was looking for something else. But now that I actually want it, I can't find it again.

I do not require equation numbers.

EDIT: The "simple way to do this" I had been half-recalling was \lefteqn{}, but as far as I understand that only works (and I use the word "works" loosely) within the evil eqnarray environment.

Answer 1

Ok, converting my comment on @egreg's solution into an answer:

\begin{align*}
  \hspace{2em}&\hspace{-2em}Any short or long LHS\\
  &= ax^3 + bx^2 + cx + d \\
  &= f(x)
\end{align*}

This puts the first LHS on a separate line, and the equation is centered correctly, independent of the size of LHS.

Additional note: I've defined a shorthand command for \hspace{2em}&\hspace{-2em} myself. I wasn't sure that putting the & into a command would work, but it does.

Answer 2

this should do it.

\documentclass{article}
\usepackage{amsmath}
\multlinegap=30pt
\begin{document}
\begin{multline*}
 \text{Left-hand-side of my equation}\\
  \begin{aligned}
    &= \text{right-hand-side number 1}\\
    &= \text{right-hand-side number 2}\\
    &= \text{etc.}
  \end{aligned}
\end{multline*}
\end{document}

Edit: \multlinegap specifies the indent from the left and right margins. the default is 10pt, but often a wider gap is desirable.

Answer 3

Simpler than Werner's but using the same idea for centering with respect to the supposed long left hand side:

\begin{align*}
  \makebox[2em][l]{\text{Here is your LHS that may be very long}} &
  \hspace{-2em}\hphantom{\text{Here is your LHS that may be very long}}\\
  &= ax^3 + bx^2 + cx + d \\
  &= f(x)
\end{align*}

If instead it's one of the right hand sides to be overlong, one can use

\begin{align*}
  \makebox[2em][l]{\text{Not so long LHS}} & \\
  &= ax^3 + bx^2 + cx + d + ay^3 + by^2 + cy + d \\
  &= f(x,y)
\end{align*}

Answer 4

You can use a regular align* (from amsmath) and then add some overlapping with box-lengths. Here's a minimal example:

\documentclass{article}
\usepackage{showframe}% http://ctan.org/pkg/showframe
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\newsavebox{\LHS}\savebox{\LHS}{\text{Here is your LHS that may be very long}}
\newsavebox{\relation}\savebox{\relation}{$\qquad\;{=}\;$}
\begin{align*}
  \rlap{\usebox{\LHS}}
  \qquad & \hspace*{\dimexpr\wd\LHS-.5\wd\relation} \\
    &= ax^3 + bx^2 + cx + d \\
    &= f(x)
\end{align*}
\end{document}

The first line (LHS) is set in whichever way you want using \rlap (to obtain a right-overlap). and stored in the box \LHS. The second line contains a blank mockup of \LHS with the appropriate correction of space allocation.

You can change the \qquad to suit your needs, to increase/decrease the horizontal displacement of the start of the =.

Answer 5

In case you want to choose the exact point of alignment, you can use plain align(*):

\documentclass{article}
\usepackage{amsmath}
\begin{document}
 \begin{align*}
  ax &+ by + cx \\
     &= 2x + 3y + 4z \\
     &= 17.
 \end{align*}
\end{document}

Answer 6

Perhaps you are looking for align* from amsmath?

\begin{align*}
  \sum X_i\\
&= \prod Y_i \\
&=\Gamma \text{foo}
\end{align*}