\sum_{i=1}^{n} i = {n(n+1)\over 2}
\left[\matrix{1&2&3\cr4&5&6\cr7&8&9\cr}\right]
{(n_1+n_2+\cdots+n_m)!\over n_1!\,n_2!\ldots n_m!} ={n_1+n_2\choose n_2}{n_1+n_2+n_3\choose n_3} \ldots{n_1+n_2+\cdots+n_m\choose n_m}
\{\underbrace{\overbrace{\mathstrut a,\ldots,a}^{k\;a'\rm s}, \overbrace{\mathstrut b,\ldots,b}^{l\;b'\rm s}\>}_{k+1\rm\;elements}\}
\sqrt{1+\sqrt{1+\sqrt{1+\sqrt{1+\sqrt{1+\sqrt{1+\sqrt{1+x}}}}}}}
\alpha + \beta = \Gamma
\pmatrix{\pmatrix{a&b\cr c&d} & \pmatrix{e&f\cr g&h}\cr \rule 0pt 2.25em 0pt 0&\pmatrix{i&j\cr k&l}}