Defining `$$$ $$$` to align

Question

I would like to define

$$$ \zeta(s) = \dfrac1{1^s} + \dfrac1{2^s} + \cdots $$$

as a short hand for

\begin{align}
\zeta(s) = \dfrac1{1^s} + \dfrac1{2^s} + \cdots
\end{align}`

How should I go about doing this?

PS: I am aware of using newcommand to define something like

\newcommand{\ba}[1]{\begin{align}#1 \end{align}}

Answer 1

You can. But I strongly discourage you to use the following code that implements your idea. The resulting document code is obscure and error prone. If you forget a $ somewhere, you'll probably get weird error messages when TeX is very far from the point where the missing $ should be.

Disclaimer. Using this code can cause kittens die; it can also cause the computer to rebel against you and create a computer domination over the world. You've been advised.

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

\let\normaldollar=$
\catcode`$=\active
\makeatletter
\protected\def${\new@ifnextchar${\check@two}{\(\close@single}}
\def\close@single#1${#1\)}
\def\check@two#1{\@ifstar{\do@equation@star}{\check@three}}
\def\check@three{\new@ifnextchar${\check@three@star}{\do@equation}}
\def\check@three@star#1{\@ifstar{\do@align@star}{\do@align}}
\def\do@equation@star#1$${\[#1\]}
\def\do@equation#1$${\begin{equation}#1\end{equation}}
\def\do@align#1$$${\begin{align}#1\end{align}}
\def\do@align@star#1$$${\begin{align*}#1\end{align*}}
\makeatother

\begin{document}
In line math $abc$, followed by a numbered equation
$$
1+1=\left\lbrace\begin{array}{@{}l>{\normaldollar}l<{\normaldollar}@{}}
2 & if it rains\\
3 & otherwise
\end{array}\right.
$$
followed by an unnumbered equation
$$*
2+2=4
$$
followed by a numbered align
$$$
a&=b\\
c&=d
$$$
followed by an unnumbered align
$$$*
f&=g\\
&=h
$$$
\end{document}

For example, forgetting the $ after abc, you get an error at line 34, which reads

! Misplaced alignment tab character &.

Try it. Then forget about this idea. Using \newcommand{\ba}[1]{\begin{align}#1\end{align}} is even worse.

Answer 2

You shouldn't. You actually shouldn't use $$...$$ either in LaTeX: Why is \[ … \] preferable to $$?

It is probably doable, but would be very difficult to do so that it doesn't break things like these two (\usepackage{array} needed for the 2nd one):

$$$ x_n = 0 \quad\text{for $n=1,2,3,\dotsm$} $$$
\begin{tabular}{>$l<$} 3x^2 \\ $Hello, World!$ \end{tabular}

Please, please, write a legible code and don't use cryptic shorthands like proposed $$$...$$$ or (maybe even worse) \ba{...}.

---

As giordano points out, if you want to type the code faster, ask a question on how to type the code faster, and not how to shorten the code. For typing the code faster, you can find some good editors that support various kinds of auto-completion: LaTeX Editors/IDEs

Answer 3

If we are using plain TeX then our life is simpler. We are typing:

$$\eqalign{
   a &= b + c \cr
   c &= d + e
}$$

when we need to typeset aligned equations. And your idea about $$$ ... $$$ can be implemented simply by \everydisplay:

\everydisplay={\futurelet\next\eqalignQ}
\def\eqalignQ{\ifx\next$\expandafter\eqalignX\fi}
\def\eqalignX$#1$$${\eqalign{#1}$$}

$$$
   a &= b + c \cr
   c &= d + e
$$$

\end

And we have no LaTeX-syntactical purists in our own ranks.

Answer 4

The earlier answers have already provided several solutions, all the while warning you against pursuing your objective. Here's a LuaLaTeX-based solution that does what you want. Importantly, the earlier warnings do not apply to this solution, as it doesn't actually redefine $$$ in any way. Instead, it sets up a function that acts as a pre-processor that sweeps over all input lines before TeX starts any of its regular processing, converting all instances of $$$ to either \begin{align} or \end{align}. Thus, TeX's "eyes" never actually see any instances of $$$.

Two types of input formats are handled by this setup:

• Single-line input: "$$$ (body of equation) $$$" all on a single line: The input is converted to \begin{equation} (body of equation) \end{equation}. Note that because the entire equation consists of a single line, there's nothing to "align" the material to. Hence, this case employs the simpler equation environment.

• Multi-line input: $$$ on separate lines, with the math material between these two lines: The opening instance of $$$ is converted to \begin{align}, and the closing instance of $$$ is converted to \end{align}.

Both "un-starred" and "starred" variants of $$$ are handled, with the latter producing un-numbered equation and align environments.

Examples of both single-line and multi-line input formats, both unstarred and starred ($$$*) are shown in the example below.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{amsmath} % for 'align' environment

\usepackage{luacode}
\begin{luacode}

not_in_align = true -- Initially, *not* in an "align" env.
function dollars2align ( line )
   line = string.gsub ( line, "%$%$%$(%*?)(.+)%$%$%$(%*?)", 
                        "\\begin{equation%1}%2\\end{equation%3}" )
   if string.find ( line, "%$%$%$" ) then
      if not_in_align then
          line = string.gsub ( line , "%$%$%$(%*?)", "\\begin{align%1}" )
          not_in_align = false
      else
          line = string.gsub ( line , "%$%$%$(%*?)", "\\end{align%1}" )
          not_in_align = true
      end
   end
   return line
end

\end{luacode}
\AtBeginDocument{\directlua{luatexbase.add_to_callback (
   "process_input_buffer", dollars2align, "dollars2align" )}}


\begin{document}
$$$ \zeta(s) = \dfrac1{1^s} + \dfrac1{2^s} + \cdots $$$

$$$ 
a^2+b^2&=c^2\\
e^{i\pi}-1 &=0
$$$

$$$* 1+1=2 $$$*

$$$*
2+2&=4\\
4+4&=8
$$$*
\end{document}

Answer 5

\documentclass{scrartcl}
\usepackage{fixltx2e}
\usepackage{mathtools}

\makeatletter
\catcode`\$=\active
\protected\def${\new@ifnextchar$\adhvaitha@checkthird\adhvaitha@math}
\protected\def\adhvaitha@checkthird${\new@ifnextchar$\adhvaitha@align\adhvaitha@dmath}
\protected\def\adhvaitha@math#1${\(#1\)}
\protected\def\adhvaitha@dmath#1$${\[#1\]}
\protected\def\adhvaitha@align$#1$$${\begin{align}#1\end{align}}
\makeatother

\begin{document}
Some math $x^2$

and a little more
$$ x^2 $$$x^2$$24$

\begin{figure}[h]
    \rule{2cm}{2cm}
    \caption{$x^2 +1$}
\end{figure}

$$$ \text{aligned} &= M \cdot A \cdot T \cdot H $$$
\end{document}

ADDITION: Same disclaimer as in egreg's answer applies here.

Answer 6

I understood that you need not to use align environment because of aligning more equations but because you need numbering of the single equation. IMHO it is bad idea because the vertical space between short paragraph followed by the equation is constructed from \abovedisplayskip, no from \abovedisplayshortskip.

This is a reason why I'm suggesting to use your $$$ ... $$$ markup only for numbered single equations. My following implementation puts the right vertical space above such display. There are only three lines of code.

\documentclass{article}
\usepackage{amsmath}

\everydisplay={\futurelet\next\numeqA}
\def\numeqA{\ifx\next$\expandafter\numeqB\fi} %$
\def\numeqB$#1$$${#1\eqno \csname make@display@tag\endcsname$$}

\begin{document}

The text:
$$$
   a=b 
$$$

The text:
\begin{align}
c=d
\end{align}

\end{document}

Please, compare the vertical space between the text and the equation in the first paragraph (it is OK) and in the second paragraph (it is bad).