 # Permutations, Lehmer Code, and Lexicographic Index

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The number of ways to arrange seventeen objects in a row is , or for short. The arrangements are called permutations. Combinatorial theorists use five different notational systems for permutations.

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Assume that the objects are labeled by the numbers 1 to 17. The first notation has positions on top and the numbers of the rearranged objects on the bottom. It can be read as a mapping of a finite set of numbers, where the numbers on top get mapped to those below.

The second notation is just the bottom row of the first notation. (5 14 9 2 1 11 16 6 7 4 8 17 10 13 15 3 12) is a permutation.

The third notation applies that mapping repeatedly to any starting number until there is a repetition. Then a new number is chosen to start the next cycle, and so on. (1 5)(2 14 13 10 4)(3 9 7 16)(6 11 8)(12 17)(15) is a cycle form. A permutation can be shown as a directed graph.

The fourth notation is the lexicographical index. The arrangement (5 14 9 2 1 11 16 6 7 4 8 17 10 13 15 3 12) is the hundred trillionth permutation if the permutations of 17 objects are sorted. Thus, 100000000000000 of 17! is a permutation.

The fifth notation is the Lehmer code, for example (4 12 7 1 0 6 9 2 2 1 1 5 1 2 2 0 0). The generating algorithm is basically "this number is greater than of the subsequent numbers." This is also known as the factorial number system representation of a number. Note that .

Gauss noted that every self-intersecting closed loop with crossings corresponds to a permutation by labeling every other intersection with 1 to . The reverse is not true, since permutations like (3 4 5 1 2) lead to nonplanar graphs.

Big numbers are needed to get to 27!, but in a slider, only discrete integer values are possible. Bigger numbers are split across sliders.

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Contributed by: Ed Pegg Jr (September 2015)
Open content licensed under CC BY-NC-SA

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Reference

 Wikipedia. "Lehmer Code." (Sep 3, 2015) en.wikipedia.org/wiki/Lehmer_code.

## Permanent Citation

Ed Pegg Jr

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