Discrete Mathematics with Applications 4th Edition

Published by Cengage Learning
ISBN 10: 0-49539-132-8
ISBN 13: 978-0-49539-132-6

Chapter 8 - Relations - Exercise Set 8.2 - Page 458: 23

Answer

-S is Reflexive -S is not symmetric -S is Transitive

Work Step by Step

As X be a nonempty set and P(X) the power set of X. Defining the “subset” relation S on P(X) as follows: For all A, B ∈P(X), A S B ⇔ A ⊆ B. By telling that it is transitive,symmetric,reflexive or not. . S is reflexive : -S is reflexive⇔for all sub sets A of X, A S A. By definition of S, this means that for all subsets A of X, A ⊆ A. But this is true because every set is a subset of itself. --S is not symmetric: -S is symmetric⇔for all subsets A and B of X, if A S B then B S A. By definition of S, this means that for all subsets A and B of X, if A ⊆ B then B ⊆ A. But this is false because X (\ne=) ∅ and so there is an element a in X. As a counterexample, take A =∅, and B ={ a}. --S is transitive: - S is transitive⇔for all subsets A, B, and C of X, if A S B and B S C, then A S C. By definition of S, this means that for all subsets A, B, and C of X, if A ⊆ B and B ⊆ C then A ⊆ C. But this is true by the transitive property of subsets .
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