In competitive programming sometime we can find problem that title “count the possibility”, “find the number of ways”, etc that involve combinatoric

Combinatorics is a branch of discrete math concerning the study of countable discrete structures — Felix Halim

as we know in written mathematics, combinatoric is written as follows :

_{n}^{r}\textrm{C}=\binom{n}{r}=\frac{n!}{(n-r)!\cdot r!}

$latex i\hbar\frac{\partial}{\partial t}\left|\Psi(t)\right>=H\left|\Psi(t)\right>$