2 research outputs found
Adjacencies in Permutations
A permutation on an alphabet , is a sequence where every element in
occurs precisely once. Given a permutation = (, , ,....., ) over the alphabet =0, 1, . . . , n1 the elements in two consecutive positions in
e.g. and are said to form an \emph{adjacency} if =+1. The concept of adjacencies is widely used in
computation. The set of permutations over forms a symmetric group,
that we call P. The identity permutation, I P where
I =(0,1,2,...,n1) has exactly n1 adjacencies. Likewise, the
reverse order permutation R=(n1, n2, n3, n4,
...,0) has no adjacencies. We denote the set of permutations in P with
exactly k adjacencies with P(k). We study variations of adjacency. % A
transposition exchanges adjacent sublists; when one of the sublists is
restricted to be a prefix (suffix) then one obtains a prefix (suffix)
transposition. We call the operations: transpositions, prefix transpositions
and suffix transpositions as block-moves. A particular type of adjacency and a
particular block-move are closely related. In this article we compute the
cardinalities of P(k) i.e. P (k) for
each type of adjacency in time. Given a particular adjacency and the
corresponding block-move, we show that and the
expected number of moves to sort a permutation in P are closely related.
Consequently, we propose a model to estimate the expected number of moves to
sort a permutation in P with a block-move. We show the results for
prefix transposition. Due to symmetry, these results are also applicable to
suffix transposition.Comment: 20 pages. 5 table
Sorting permutations with a transposition tree
The set of all permutations with symbols is a symmetric group denoted by
. A transposition tree, , is a spanning tree over its vertices
{} where the vertices are the positions of a
permutation and is in . is the operation and the edge set
denotes the corresponding generator set. The goal is to sort a given
permutation with . The number of generators of that suffices to
sort any constitutes an upper bound. It is an upper bound, on the
diameter of the corresponding Cayley graph i.e. . A
precise upper bound equals . Such bounds are known only for a few
trees. Jerrum showed that computing is intractable in general if
the number of generators is two or more whereas has generators. For
several operations computing a tight upper bound is of theoretical interest.
Such bounds have applications in evolutionary biology to compute the
evolutionary relatedness of species and parallel/distributed computing for
latency estimation. The earliest algorithm computed an upper bound
in a time by examining all in . Subsequently,
polynomial time algorithms were designed to compute upper bounds or their
estimates. We design an upper bound whose cumulative value for all
trees of a given size is shown to be the tightest for . We show
that is tightest known upper bound for full binary trees.
Keywords: Transposition trees, Cayley graphs, permutations, sorting, upper
bound, diameter, greedy algorithms.Comment: 13 pages. 4 figures, 5 table