Redefinition of equation* break the \[ ... \], why?

Question

EDIT2: As @Henri commented, this would work: \def\[#1\]{\begin{equation*}#1\end{equation*}}. Thanks for all your kind helping!

EDIT: I think I have convinced myself that the current code is enough to produce my desired result. Now I just wonder:

Why does the following redefinition of equation* break the \[ ... \]? With this, the use of \[ ... \] will produce bunch of errors.

\makeatletter
\let\MYequation\equation
\let\endMYequation\endequation
\RenewEnviron{equation*}{%
\begin{minipage}{\linewidth}
   \begin{MYequation}%
      \st@rredtrue \global\@eqnswfalse%
      \BODY%
    \end{MYequation}%
\end{minipage}
}
\makeatother

Redefining the \[ ... \] as below still cannot fix this. I really want to make \[ ... \] the same as \begin{equation*} ... \end{equation}.

\makeatletter
\DeclareRobustCommand\[{%
    \begin{equation*}
}%
\DeclareRobustCommand\]{%
    \end{equation*}
}%
\makeatother

Here's a MWE:

\documentclass{article}
\usepackage{amsmath}
\usepackage{environ}

\makeatletter
\let\MYequation\equation
\let\endMYequation\endequation
\RenewEnviron{equation*}{%
\begin{minipage}{\linewidth}
   \begin{MYequation}%
      \st@rredtrue \global\@eqnswfalse%
      \BODY%
    \end{MYequation}%
\end{minipage}
}
\makeatother

\begin{document}

\begin{equation*} % this is fine
   y^2
\end{equation*}

% \[y^2\]         % this will produce an error

\end{document}

---

Original question:

I tried to redefine the math-mode double dollar $$ ... $$. Let's call this CODE1: latex \makeatletter \global\let\tikz@ensure@dollar@catcode=\relax \catcode`\$=\active \protected\def${\@ifnextchar$\@doubledollar\@singledollar} \def\@doubledollar$#1$${\begin{equation*}#1\end{equation*}} \def\@singledollar#1${\(#1\)} \makeatother (Here the \global\let\tikz@ensure@dollar@catcode=\relax thing is to prevent tikz from producing an error.)

I also redefined the \begin{equation*}...\end{equation*}, and thus to prevent (the AMS version of) \[ ... \] from making an error, I have to change the definition of \[ ... \] from the amsmath.sty version to the original ltmath.dtx version, and let's call this CODE2: latex \makeatletter \DeclareRobustCommand\[{% \relax\ifmmode \@badmath \else \ifvmode \nointerlineskip \makebox[.6\linewidth]{}% \fi $$%%$$ BRACE MATCH HACK \fi }% \DeclareRobustCommand\]{% \relax\ifmmode \ifinner \@badmath \else $$%%$$ BRACE MATCH HACK \fi \else \@badmath \fi \ignorespaces }% \makeatother Now here's the question:

If I write CODE 2 CODE 1 Then there's no error, but the effect (I mean vertical space) of \[ ... \] is not the same as $$ ... $$. And even if I change CODE2 to the simplified version:

    $$%%$$ BRACE MATCH HACK }% \DeclareRobustCommand\]{%
    $$%%$$ BRACE MATCH HACK }% \makeatother ``` They still look different. However, if I write ``` CODE 1 CODE 2 ``` Then there's an
error ``` Paragraph ended before \@doubledollar was complete. ``` Why
is this happening? Is there any way to achieve this properly?

-----

Below is a MWE: ```latex \documentclass{article} \usepackage{amsmath}

\usepackage{tikz} \usetikzlibrary{calc}

% CODE2 \makeatletter \DeclareRobustCommand\[{%    \relax\ifmmode
      \@badmath    \else
      \ifvmode
         \nointerlineskip
         \makebox[.6\linewidth]{}%
      \fi
      $$%%$$ BRACE MATCH HACK    \fi }% \DeclareRobustCommand\]{%    \relax\ifmmode
      \ifinner
         \@badmath
      \else
         $$%%$$ BRACE MATCH HACK
      \fi    \else
      \@badmath    \fi    \ignorespaces }% \makeatother

% CODE1 \makeatletter \global\let\tikz@ensure@dollar@catcode=\relax
\catcode`\$=\active
\protected\def${\@ifnextchar$\@doubledollar\@singledollar}
\def\@doubledollar$#1$${\begin{equation*}#1\end{equation*}}
\def\@singledollar#1${\(#1\)} \makeatother

\begin{document}

\$\$ : $$y^2+\int\mathrm{d} x \frac{p}{q_p^p}$$

$\backslash[ ... \backslash]$ : \[y^2+\int\mathrm{d} x
\frac{p}{q_p^p}\]

\end{document} ```

Answer 1

Why not use the etoolbox?

\documentclass{article}
\usepackage{amsmath}
\usepackage{etoolbox}

\csdef{[}{\begin{equation*}}
\csdef{]}{\end{equation*}}

\begin{document}

  \begin{equation*} % this is fine
     y^2
  \end{equation*}

  \[y^2\]         % this works too

\end{document}

Your MWE does not show any difference between \[...\] and the equation* environment but I believe that this does what you want with a minimum of technology.