🛠️TeX

\overset{!}{=}                    % overset
\underbrace{...}_{\text{...}}     % underbrace

result

test

$% new command \newcommand \x {\times} \newcommand \y {\colorbox{green}{1}} \left( \begin{array}{cccc} \x & \x & \x & \x \\ 0 & \x & \x & \x \\ 0 & 0 & \x & \x \\ 0 & 0 & 0 & \x \\ \y & \y & \y & \y \\ \end{array} \right)$ $% new command \newcommand \x {\times} \newcommand \y {\colorbox{green}{1}} \left( \begin{array}{cccc} \x & \x & \x & \x \\ 0 & \x & \x & \x \\ 0 & 0 & \x & \x \\ 0 & 0 & 0 & \x \\ \y & \y & \y & \y \\ \end{array} \right)$ $% new command \newcommand \x {\times} \newcommand \y {\colorbox{green}{1}} \left( \begin{array}{cccc} \x & \x & \x & \x \\ 0 & \x & \x & \x \\ 0 & 0 & \x & \x \\ 0 & 0 & 0 & \x \\ \y & \y & \y & \y \\ \end{array} \right)$ $% new command \newcommand \x {\times} \newcommand \y {\colorbox{green}{1}} \left( \begin{array}{cccc} \x & \x & \x & \x \\ 0 & \x & \x & \x \\ 0 & 0 & \x & \x \\ 0 & 0 & 0 & \x \\ \y & \y & \y & \y \\ \end{array} \right)$ <- the equation ⁉️

showed up then ~~disappeared~~ shrinked to be invisible? GitBook doesn't support \newcommand in Math & TeX ❓

% test `\newcommand`
\newcommand \x {\times}
\newcommand \y {\colorbox{green}{1}}

\left(
  \begin{array}{cccc}
    \x  & \x  & \x & \x \\
    0   & \x  & \x & \x \\
    0   & 0   & \x & \x \\
    0   & 0   & 0  & \x \\
    \y  & \y  & \y & \y \\
  \end{array}
\right)

$% new commands \newcommand \x {\times} \newcommand \y {\colorbox{green}{1}} \begin{pmatrix} \x & \x & \x & \x \\ 0 & \x & \x & \x \\ 0 & 0 & \x & \x \\ 0 & 0 & 0 & \x \\ \y & \y & \y & \y \\ \end{pmatrix}$

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