Change the default size of square brackets in latex math mode

Question

Personally I think the default size of square brackets in latex math mode is too small. For example, for an interval [a,b], I would prefer \big[\big] as opposed to plain []. My question is, what can I do to change the default size of [], so that when I input [] in math mode, they will show up automatically the same size as \big[\big]?

Right now I'm defining a new command called \b{} which takes one argument and inserts it between \big[ and \big], but the code looks ugly. Apparently \newcommand (or \def or \let) does not allow [ to be refined.

Any help is much appreciated. Thanks.

Answer 1

You can load a scaled version of the relevant font and substitute the standard definitions for [ and ]. Below I have chosen a scaling of 1.2 over the standard size for some exaggeration, probably you only want a factor of 1.1

\documentclass{article}

\DeclareFontFamily{OT1}{cmrlop}{}

\newcommand{\mybscale}{1.2}
\DeclareFontShape{OT1}{cmrlop}{m}{n}%
     {<5><6><7><8><9><10><12>gen*[\mybscale]cmr%
      <10.95>*[\mybscale]cmr10%
      <14.4>*[\mybscale]cmr12%
      <17.28><20.74><24.88>*[\mybscale]cmr17}{}

\DeclareSymbolFont{loperators}{OT1}{cmrlop}{m}{n}
\DeclareMathSymbol{[}{\mathopen}{loperators}{"5B}
\DeclareMathSymbol{]}{\mathclose}{loperators}{"5D}
\DeclareMathDelimiter{[}{\mathopen}{loperators}{"5B}{largesymbols}{"02}
\DeclareMathDelimiter{]}{\mathclose}{loperators}{"5D}{largesymbols}{"03}

\begin{document}

\( [a,b] \)

\begin{displaymath}
\left[a,c\right] \bigl[a,b\bigr] \Bigl[a,b\Bigr] \biggl[a,b\biggr] \Biggl[a,b\Biggr]
\end{displaymath}

\( ([()]) \)

\end{document}

The code adjusts the normal size of the brackets, but not the large scaled versions. You would have to apply similar techniques to the largesymbols family to change those.

Answer 2

You can define your own command (not \b because it has been already defined) to achieve your objective.

\documentclass{article}
\newcommand{\brakets}[1]{\ensuremath{\bigg[#1\bigg]}}
\begin{document}
    \noindent
    $[31]$ original\\
    \brakets{31} something something something...\\
    \brakets{xyz} something something something...\\
    $\brakets{xyz} + x = \cdots$ in mathmode as well\\
\end{document}

Answer 3

You could define \[ as \big[ and \] as \big], the problem is that \[\] are already defined to open and close the displaystyle math mode environment so one has to undefine them.

\documentclass{article}

\makeatletter
    \let\[\@undefined
    \let\]\@undefined
\makeatother

\def\[{\big[}
\def\]{\big]}

\begin{document}
\begin{equation}
    \[a,b\]
\end{equation}
\end{document}