Discrete Mathematics and Its Applications, Seventh Edition

Published by McGraw-Hill Education
ISBN 10: 0073383090
ISBN 13: 978-0-07338-309-5

Chapter 1 - Section 1.5 - Nested Quantifiers - Exercises - Page 65: 9

Answer

a. $\forall xL(x, Jerry)$ b. $\forall x \exists y L(x, y)$ c. $\exists y \forall xL(x, y)$ d. $\neg \exists x \forall y L(x,y)$ e. $\exists y \neg L(Lydia, y)$ f.$\exists y \forall x \neg L(x, y)$ g.$\exists x(\forall y L(x, y) \land \forall z((\forall wL(w, z)) \rightarrow z = x))$ h.$\exists y \exists z(L(Lynn, y) \land L(Lynn, z) \forall y \ne z \land \forall w(L( Lynn, w) \rightarrow (w=y \lor w = z))$ (i) $\forall xL(x, x)$ (j) $\exists x \forall y(L(x,y) \leftrightarrow x = y)$

Work Step by Step

a. "Everybody” means ”All people in the world”. $\forall xL(x, Jerry)$ b. ’Everybody” means "All people in the world”. ’’Somebody” means ’’There exists a person in the world” $\forall x \exists y L(x, y)$ c. ’’Somebody” means "There exists a person in the world”. ’’Everybody” means " All people in the world”. $\exists y \forall xL(x, y)$ d. ’’Nobody" means ’’There does not exists a person in the world”. ’’Everybody" means “All people in the world”. $\neg \exists x \forall y L(x,y)$ e. We could rewrite the given sentence as "Lydia does not love somebody". "Somebody" means “There exists a person in the world" $\exists y \neg L(Lydia, y)$ f. We could rewrite the given sentence as ’’There is somebody whom everybody does not love". (Note: Lydia refers to x and somebody refers to y. while the statement of y occurs before the statement of x) $\exists y \forall x \neg L(x, y)$ g. We could rewrite the given sentence as "There is somebody x whom everyone loves and all people that are loved by everyone, then this person has to be x". $\exists x(\forall y L(x, y) \land \forall z((\forall wL(w, z)) \rightarrow z = x))$ h. We could rewrite the given sentence as "There are two people y and z that Lynn loves and these two people are different and for all people that Lynn loves, these people then have to be either y or z". $\exists y \exists z(L(Lynn, y) \land L(Lynn, z) \forall y \ne z \land \forall w(L( Lynn, w) \rightarrow (w=y \lor w = z))$ (i) We could rewrite the given sentence as "Every person x loves x (himself/herself)" $\forall xL(x, x)$ (j) We could rewrite the given sentence as "There is a person x who loves y if and only if y is x (himself/herself)". $\exists x \forall y(L(x,y) \leftrightarrow x = y)$
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