Formula A, which represents all permutation numbers of m(m≤n) elements from n different elements, is called permutation numbers of m elements from n different elements, and is represented by symbol A(n, m).

A(n，n)=n！ A(n，m)=n！ (m-n)！ ? 0! = 1

Formula C means that m(m≤n) elements are randomly selected from N different elements and combined together, which is called the combination of M elements from N different elements; The number of all combinations of m(m≤n) elements from n different elements is called the number of combinations of m elements from n different elements. Represented by the symbol C(n, m).

C(n，n)= 1？ C(n，m)=A(n，m)÷m！

References:

Baidu encyclopedia-permutation and combination