Combination is one of the important concepts in 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

Extended data

In repeated combination, m elements can be repeatedly selected from n different elements. Synthesizing a group of m elements called n elements regardless of their order is a repeatable combination. Two repeated combinations are the same if and only if the elements are the same and the same elements are taken the same times.

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