A(m, n)m is superscript and n is subscript, so I understand, hehe ~

For example, c (1, n) = c (n- 1, n) = nc (n, n) = 1 a (n, n) = n!

Simply put, A(k, n) = n! / (n-k)！

C(k，n) = n！ / [(n-k)！ * k！ ]

namely

A(k，n) = k！ * C(k，n)

Simple skills are hard to find.