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！ (noun) noun.

Extended data

Arrangement is one of the important concepts in mathematics. The subsets of finite sets are arranged in columns and are circular, and they are not allowed to be repeated or repeated according to some conditional sorting method. Take out m( 1≤m≤n) different elements from n different elements at a time and arrange them in a row, which is called non-repetitive arrangement or linear arrangement of taking out m elements from n elements, which is called arrangement for short. The number of all different permutations of M different elements from N different elements is called permutation number or permutation number, and it is recorded as (or).

Note: Two permutations are the same if and only if the elements are exactly the same, and the arrangement order of the elements is the same. For example, the elements of abc and abd are not exactly the same, but are in different arrangements; Another example is abc and acb. Although the elements are exactly the same, the arrangement order of the elements is different, which is also a different arrangement.

References:

Baidu encyclopedia arrangement