1, 2 can form 12, 2 1, and its permutation number is 2! = 2

Permutation and combination is the most basic concept of combinatorics. The so-called arrangement refers to taking out a specified number of elements from a given number of elements for sorting. Combination refers to taking out only a specified number of elements from a given number of elements, regardless of sorting. The central problem of permutation and combination is to study the total number of possible situations in a given permutation and combination. Permutation and combination are closely related to classical probability theory.

Directory [hidden]

definition

sign

history

Parity check of combination number

Basic theory and formula of permutation and combination

Music album introduction

Album track

definition

sign

history

Parity check of combination number

Basic theory and formula of permutation and combination

Music album introduction

Album track

Permutation and combination formula

[Edit this paragraph] Definition

Formula P is an arrangement formula, and R elements are selected from N elements for arrangement (i.e. sorting).

P is an old usage, and now a is often used in textbooks, which means arranging music.

Formula C is a combination formula, which takes r from n elements and does not arrange (that is, does not sort).

[Edit this paragraph] symbol

A common topic

C combination number

P- permutation number (now the textbook is A)

Total number of n elements

R- the number of elements participating in the selection

! -factorial, like 5! =5*4*3*2* 1= 120

C combination

P- permutation (now the textbook is A- permutation)

Some combinatorial identities

Combinational identity

Common formulas for permutation and combination