Tool to calculate the rank of a permutation of a set. The permutation's rank is the number associated with it in the order of generation of the permutations.
Rank of a Permutation - dCode
Tag(s) : Combinatorics
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Since it seems difficult to list all permutations when there are many items. There is a mathematical method to perform this calculation.
Take a permutation $ P $ in the set $ E $ of size $ t $.
Example: The permutation B,A,C from the initial set A,B,C of size $ t = 3 $
For each letter, calculate the position $ p $ in the set $ E $, calculate $ s = p \times (t-1)! $ and remove the letter from the set $ E $ (size $ t $ decreases). The sum of $ s $ is the rank of the permutation.
Example: B is in position $ 1 $ in ABC, $ s_B = 1 \times 2! = 2 $
A is in position $ 0 $ in AC, $ s_A = 0 \times 1! = 0 $
C is in position $ 0 $ in C, $ s_C = 0 \times 0! = 0 $
BAC is at permutation rank $ s_B + s_A + s_C = 2 + 0 + 0 = 2 $