One of the important concepts of combinatorial mathematics. Taking out M different elements (0≤m≤n) from N different elements at a time and synthesizing a group regardless of their order is called selecting the combination of M elements from N elements without repetition. The total number of all such combinations is called the combination number, and the calculation formula of this combination number is

or

The combination obtained by repeatedly extracting M elements from N-ary set A is essentially an M-ary subset of A. ..

The calculation method of permutation and combination is as follows:

The arrangement a (n, m) = n× (n- 1). (n-m+ 1) = n！ /(n-m)！ (n is subscript and m is superscript, the same below)

Combination C(n, m)=P(n, m)/P(m, m) =n! /m！ (n-m)！ ;

For example:

A(4，2)=4！ /2! =4*3= 12

C(4，2)=4！ /(2! *2! )=4*3/(2* 1)=6