Introduction To Algorithms, Third Edition (international Edition)
Introduction To Algorithms, Third Edition (international Edition)
3rd Edition
ISBN: 9780262533058
Author: Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, Clifford Stein
Publisher: TRILITERAL
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Chapter D, Problem 2P

a.

Program Plan Intro

To prove that |R(A)|=2r and conclude that A defines a permutation on Sn only if A has full rank.

Given information:

  • r is the rank of matrix A.
  • Preimage of y is defined by P(A,y)={x:Ax=y} , for given n×n matrix A and given a value yR(A) .

Explanation:

It can be assumed without generality loss, that the first r columns of A are linearly independent.

Then for x1,x3Sn , it can be concluded that Ax1Ax2 based on following two conditions.

  • In the first r entries x1 and x2 are not identical. And x1 , x2
  • have 0’s in the remaining entries.

Now Ax1Ax2 is true since the first r entries of each are a linear combination of the first r rows of A , and it is impossible to have two different linear combinations that are equal of them because they are independent.

Now it is must that |R(A)|2r as there are at least 2r non-equivalent vectors xSn . Alternatively, x is a vector that doesn’t consists of 0’s in the coordinates that larger than r . Therefore Ax=xiai where ai is the ith column of A . Now as each of the last nr columns of A is in fact a linear combination of the first r columns of A , this can be rewritten as a linear combination of the first r columns of A .

Since all of these has been already, |R(A)|=2r . The range cannot include all 2n elements of Sn if A doesn’t have full rank so, permutation can be defined possible by A

b.

Program Plan Intro

To prove that |P(A,y)|=2nr if r is the rank of n×n matrix A and yR(A) .

c.

Program Plan Intro

To prove that |B(S',m)|=2r and that for each block in B(S',m) exactly 2mr numbers in S map to that block.

d.

Program Plan Intro

To show that number of linear permutations of Sn is much less than number of permutations of S using counting argument.

e.

Program Plan Intro

To provide an example of a value n and permutation of Sn that cannot be achieved by any linear permutation.

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