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fixed permutation

Calculates the number of permutations of \e n objects with \e m fixed.
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C++
Excel

Fixed Permutation

 intfixed_permutation( int n int m )
A permutation of n objects with m fixed is a permutation in which exactly m of the objects retain their original positions. In more formal terms, consider the following permutation



then  has exactly m fixed objects if



If , the permutation is a derangement, while if , the permutation is the identity.

The number of permutations of n objects with m fixed is given by



or



where  is the number of derangements of  objects.

This function calculates the value of  based on the above formula.

References:

SUBSET, a C++ library of combinatorial routines, http://www.csit.fsu.edu/~burkardt/cpp_src/subset/subset.html

Example 1

#include <codecogs/maths/combinatorics/sequences/fixed_permutation.h>
#include <iostream>
int main()
{
std::cout << "The number of permutations of 10 elements with 5 fixed is: ";
std::cout << Maths::Combinatorics::Sequences::fixed_permutation(10, 5) << std::endl;
return 0;
}

Output

The number of permutations of 10 elements with 5 fixed is: 11088

Parameters

 n the size of the permutation m the number of fixed objects

Returns

the number of permutations of n items with m fixed

Authors

Lucian Bentea (August 2005)
Source Code

Source code is available when you agree to a GP Licence or buy a Commercial Licence.

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