This reminded me of the principle of Inclusion/Exclusion which is basically about subtracting over counted elements. It's deceiving because the k! is actually DIVIDING the entire permutation equation. The part that confused me about the combinations formula is what the multiplication of k! in the denominator is doing to the formula. the number of permutations is equal to n!/(n-k)! so the number of combinations is equal to (n!/(n-k)!)/k! which is the same thing as n!/(k!*(n-k)!). So the formula for calculating the number of combinations is the number of permutations/k!. The group size can be calculated by permuting over the number of chairs which is equal to the factorial of the number of chairs(k!). So the number of combinations is equal to the number of permutations divided by the size of the groups(which in this case is 6). If we didn't care about these specific orders and only cared that they were on the chairs then we could group these people as one combination. So some of the permutations would be ABC, ACB, BAC, BCA, CAB and CBA. In our example, let the 5 people be A, B, C, D, and E. The number of combinations is the number of ways to arrange the people on the chairs when the order does not matter. So the formula for the number of permutations is n!/((n-k)!. For n people sitting on k chairs, the number of possibilities is equal to n*(n-1)*(n-2)*.1 divided by the number of extra ways if we had enough people per chair. We can make a general formula based on this logic. So the total number of permutations of people that can sit on the chair is 5*(5-1)*(5-2)=5*4*3=60. On the third chair (5-2) people can sit on the chair. On the second chair (5-1) people can sit on the chair. If there are 3 chairs and 5 people, how many permutations are there? Well, for the first chair, 5 people can sit on it. MathWorld-A Wolfram Web Resource.Ok, let's start by an example. On Wolfram|Alpha Permutation Cite this as: Skiena,ĭiscrete Mathematics: Combinatorics and Graph Theory with Mathematica. "Permutations: Johnson's' Algorithm."įor Mathematicians. "Permutation Generation Methods." Comput. Knuth,Īrt of Computer Programming, Vol. 3: Sorting and Searching, 2nd ed. "Generation of Permutations byĪdjacent Transpositions." Math. "Permutations by Interchanges." Computer J. "Arrangement Numbers." In Theīook of Numbers. The permutation which switches elements 1 and 2 and fixes 3 would be written as (2)(143) all describe the same permutation.Īnother notation that explicitly identifies the positions occupied by elements before and after application of a permutation on elements uses a matrix, where the first row is and the second row is the new arrangement. There is a great deal of freedom in picking the representation of a cyclicĭecomposition since (1) the cycles are disjoint and can therefore be specified inĪny order, and (2) any rotation of a given cycle specifies the same cycle (Skienaġ990, p. 20). This is denoted, corresponding to the disjoint permutation cycles (2)Īnd (143). The unordered subsets containing elements are known as the k-subsetsĪ representation of a permutation as a product of permutation cycles is unique (up to the ordering of the cycles). (Uspensky 1937, p. 18), where is a factorial.
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