[Tex/LaTex] Environment bmatrix undefined. \begin{bmatrix} in Latex

amsmathmathtoolsmatrices

I've been using this thisspringer format latex document and wanted to add a matrix in the paper and I've been bumping into this error for a while now.

\documentclass[a4paper]{article}
\usepackage{pdfpages}
\usepackage{graphicx}
\graphicspath{ {./imgs/} }
\usepackage{changepage}% http://ctan.org/pkg/changepage
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{booktabs}
\makeatletter
\renewcommand*\env@matrix[1][*\c@MaxMatrixCols c]{%
    \hskip -\arraycolsep
    \let\@ifnextchar\new@ifnextchar
    \array{#1}}

\renewenvironment{bmatrix}
{{\ifnum`}=0 \fi\left[\env@matrix}
{\endmatrix\right]\ifnum`{=0 \fi}}
\makeatother
\usepackage{amsmath, xparse}
\usepackage{mathtools}
\newlength{\extratrim}
\setlength{\extratrim}{10mm}
\newcommand*{\trimexpr}[1]{\dimexpr(#1)+\extratrim\relax}


gsave
newpath
  20 20 moveto
  20 220 lineto
  220 220 lineto
  220 20 lineto
closepath
2 setlinewidth
gsave
  .4 setgray fill
grestore
stroke
grestore
\end{filecontents*}
%
\RequirePackage{fix-cm}
%
%\documentclass{svjour3}                     % onecolumn (standard format)
%\documentclass[smallcondensed]{svjour3}     % onecolumn (ditto)
\documentclass[smallextended]{svjour3}       % onecolumn (second format)
%\documentclass[twocolumn]{svjour3}          % twocolumn
%
\smartqed  % flush right qed marks, e.g. at end of proof
%
\usepackage{graphicx}
%
 \usepackage{mathptmx}      % use Times fonts if available on your TeX system
%
% insert here the call for the packages your document requires
%\usepackage{latexsym}
% etc.
%
% please place your own definitions here and don't use \def but
% \newcommand{}{}
%
% Insert the name of "your journal" with
% \journalname{myjournal}
%


\begin{document}


\title{Random titl}



\author{me   \&     him }


\institute{x x of x, \at
                Institute of x Technology and Electronics \\
             {}           %  \\
%            
           \and
           x x 
              x, yy
              \and  \\
              x x 
              x, yy
}

\date{May 15th 2020}
% The correct dates will be entered by the editor


\maketitle

\[
    \begin{bmatrix}
    1  2 \\ 
    2  3
    \end{bmatrix}
\]




\section{Summary}
\section{Appendix}

%\begin{acknowledgements}
%If you'd like to thank anyone, place your comments here
%and remove the percent signs.
%\end{acknowledgements}


% Authors must disclose all relationships or interests that 
% could have direct or potential influence or impart bias on 
% the work: 
%
% \section*{Conflict of interest}
%
% The authors declare that they have no conflict of interest.


% BibTeX users please use one of
%\bibliographystyle{spbasic}      % basic style, author-year citations
%\bibliographystyle{spmpsci}      % mathematics and physical sciences
%\bibliographystyle{spphys}       % APS-like style for physics
%\bibliography{}   % name your BibTeX data base

% Non-BibTeX users please use
\begin{thebibliography}{}
%
% and use \bibitem to create references. Consult the Instructions
% for authors for reference list style.
%
\bibitem{RefJ}
% Format for Journal Reference
Author, Article title, Journal, Volume, page numbers (year)
% Format for books
\bibitem{RefB}
Author, Book title, page numbers. Publisher, place (year)
% etc
\end{thebibliography}

\end{document}
% end of file template.tex

I get an error Environment bmatrix undefined. \begin{bmatrix}. I also added amsmath package but that didn't seem to help. What could be the problem?

Best Answer

I also added amsmath package but that didn't seem to help. What could be the problem?

The instruction

\usepackage{amsmath, xparse}

has to earlier in the preamble. It should be executed immediately after \usepackage{booktabs} at the latest.

Why? Because the preamble then redefines the macro \env@matrix and the environment bmatrix. Both are defined in the amsmath package. Unsurprisingly, LaTeX crashes if it's told to redefine something that hasn't been defined yet.

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