'''Permutations'''
** http://www.cut-the-knot.org/do_you_know/permutation.shtml
* ''Being able to count elements in the set V means the set can be written as {v1, v2, ..., vn}. However, a set may be counted in many different ways.''
* Various ways to define a permutation
** http://www.cut-the-knot.org/do_you_know/Perm.shtml 
*''There are many ways one can define a permutation, none of which is simple.''


'''Arrays'''
* An analysis of permutations in arrays
** http://hal.archives-ouvertes.fr/docs/00/45/65/58/PDF/main.pdf
* ''In order to analyze this kind of properties, we define an abstract interpretation working on multisets of values, and able to discover invariant equations about such multisets.'' 
* Question and Answers
** http://www.velocityreviews.com/forums/t131940-permutations-of-instances-in-array.html
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'''Trees'''
* Permutations as Trees
** http://www.luschny.de/math/factorial/combi/PermutationTrees.html
* ''A permutation tree is a labeled rooted tree that has vertex set {0,1,2,..,n} and root 0, and in which each child is larger than its parent and the children are in ascending order from the left to the right. The power of a permutation tree is the number of descendants of the root. The height of a permutation tree is the number of descendants of the root on the longest chain starting at the root and ending at a leaf. The width of a permutation tree is the number of leafs.''
* ''The correspondence with the permutation is given by traversing the periphery of the tree starting at the right hand side of the root and recording a node whenever the node's right edge is passed.'' 

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