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In 21 and 22 , rewrite each statement without using variables or the symbol \(\forall\) or \(\exists\). Indicate whether the statement is true or false. Use the laws for negating universal and existential statements to derive the following rules: a. \(\sim(\forall x \in D(\forall y \in E(P(x, y))))\) \(\equiv \exists x \in D(\exists y \in E(\sim P(x, y)))\) b. \(\sim(\exists x \in D(\exists y \in E(P(x, y))))\) \(\equiv \forall x \in D(\forall y \in E(\sim P(x, y)))\)

Short Answer

Expert verified
The rules derived for negating universal and existential statements are as follows: a. \(\sim(\forall x \in D(\forall y \in E(P(x, y)))) \equiv \exists x \in D(\exists y \in E(\sim P(x, y)))\) b. \(\sim(\exists x \in D(\exists y \in E(P(x, y)))) \equiv \forall x \in D(\forall y \in E(\sim P(x, y)))\)

Step by step solution

01

Identify and analyze the given statement

The given statement: \(\sim(\forall x \in D(\forall y \in E(P(x, y))))\) Here, the negation symbol \(\sim\) is applied to the universal statements involving x and y, in the domain sets D and E, respectively. We have to rewrite this with existential statements.
02

Apply the law of negating universal statements

When negating a universal statement \(\forall\), we replace it with an existential statement \(\exists\), and negate the given proposition P(x, y): \(\sim (\forall x \in D (\forall y \in E(P(x, y)))) \equiv \exists x \in D(\sim (\forall y \in E(P(x, y))))\)
03

Apply the law of negating universal statements again

Now we need to negate the remaining universal statement \(\forall y \in E(P(x, y))\). Use the same method as in step 2: \(\exists x \in D(\sim (\forall y \in E (P(x, y)))) \equiv \exists x \in D (\exists y \in E (\sim P(x, y)))\) So, we have derived the rule for part a: \(\sim (\forall x \in D (\forall y \in E(P(x, y)))) \equiv \exists x \in D(\exists y \in E(\sim P(x, y)))\) For the second part of the exercise, which is question b:
04

Identify and analyze the given statement

The given statement: \(\sim(\exists x \in D(\exists y \in E(P(x, y))))\) Here, the negation symbol \(\sim\) is applied to the existential statements involving x and y, in the domain sets D and E, respectively. We have to rewrite this with universal statements.
05

Apply the law of negating existential statements

When negating an existential statement \(\exists\), we replace it with a universal statement \(\forall\), and negate the given proposition P(x, y): \(\sim (\exists x \in D (\exists y \in E(P(x, y)))) \equiv \forall x \in D(\sim (\exists y \in E(P(x, y))))\)
06

Apply the law of negating existential statements again

Now we need to negate the remaining existential statement \(\exists y \in E(P(x, y))\). Use the same method as in step 5: \(\forall x \in D(\sim (\exists y \in E (P(x, y)))) \equiv \forall x \in D (\forall y \in E (\sim P(x, y)))\) So, we have derived the rule for part b: \(\sim (\exists x \in D (\exists y \in E(P(x, y)))) \equiv \forall x \in D(\forall y \in E(\sim P(x, y)))\)

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Universal Quantification
Universal quantification is a fundamental concept in discrete mathematics, where it declares that a particular property or relation holds for all elements of a certain set. In logical terms, it is denoted by the symbol \( \forall \), which translates to 'for all' or 'for every'. Imagine you have a set of numbers and you want to express the idea that all numbers in this set are positive. You'd use universal quantification to do so.

When negating a universally quantified statement, the logical assertion flips from saying 'for all' to 'there exists', which leads to existential quantification. The negation process involves not just changing the quantifier but also negating the proposition that follows. This negation is crucial in constructing logical arguments and proving theorems, and it is often used when finding counterexamples to alleged universal truths.
Existential Quantification
In contrast to universal quantification, existential quantification deals with the existence of at least one element in a domain that satisfies a given property. It is indicated by the \( \exists \) symbol and essentially states 'there exists'. For example, saying 'there exists a number in the set that is even' employs existential quantification.

The negation of an existential quantified statement transforms it into a universal quantified statement asserting that no element in the domain possesses the property in question. This switch from 'there exists' to 'for all' with a negated proposition is part of logical negation and is widely used in problem-solving and proof construction in discrete mathematics.
Logic Laws
Logic laws are the backbone of mathematical reasoning. They provide the set of rules that govern how statements can be logically combined and manipulated. Among these laws are the rules for negating quantified statements, which have been demonstrated in the exercise solution. These laws ensure that the negation of a universal quantifier becomes an existential quantifier, and vice versa, while also affecting the proposition being quantified by negating it.

It's important to note that these logical equivalences are not arbitrary but are derived from the very definitions of universal and existential quantifiers. Mastery of logic laws enables students to navigate the complexities of mathematical arguments and is critical for success in more advanced topics.
Discrete Mathematics
Discrete mathematics is an area of mathematics that deals with objects that can assume only distinct, separated values. It encompasses a wide range of topics, including logic, set theory, combinatorics, graph theory, and algorithms. The concepts of quantification are part and parcel of discrete mathematics as they are central to forming precise and accurate statements about mathematical entities.

The practice of negating quantified statements is a fundamental skill within discrete mathematics. It enables students to explore the truth values of statements, construct proofs by contradiction, and is particularly valuable when formulating algorithms that rely on specific conditions. Discrete mathematics, being foundational to computer science as well, underscores the importance of understanding quantification and logic laws.

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Most popular questions from this chapter

Indicate whether the arguments in \(21-26\) are valid or invalid. Support your answers by drawing diagrams. [Adapted from Lewis Carroll.] Nothing intelligible ever puzzles \(m e\). Logic puzzles me. . Logic is unintelligible.

In exercises \(28-33\), reorder the premises in each of the arguments to show that the conclusion follows as a valid consequence from the premises. It may be helpful to rewrite the statements in ifthen form and replace some statements by their contrapositives. Exercises 28-30 refer to the kinds of Tarski worlds discussed in Example 2.1.12 and 2.3.1. Exercises 31 and 32 are adapted from Symbolic Logic by Lewis Carroll.* 1\. If an object is not blue, then it is not a triangle. 2\. If an object is not above all the gray objects, then it is not a square. 3\. Every black object is a square. 4\. Every object that is above all the gray objects is above all the triangles. \(\therefore\) If an object is black, then it is above all the blue objects.

Let the domain of \(x\) be the set \(\mathbf{Z}\) of integers, and let \(\operatorname{Odd}(x)\) be " \(x\) is odd," Prime \((x)\) be " \(x\) is prime," and Square \((x)\) be " \(x\) is a perfect square." (An integer \(n\) is said to be a perfect square if, and only if, it equals the square of some integer. For example, 25 is a perfect square because \(25=5^{2}\).) a. \(\exists x\) such that Prime \((x) \wedge \sim \operatorname{Odd}(x)\). b. \(\forall x, \operatorname{Prime}(x) \rightarrow \sim \operatorname{Square}(x) .\) c. \(\exists x\) such that \(\operatorname{Odd}(x) \wedge \operatorname{Square}(x)\).

Use universal instantiation or universal modus ponens to fill in valid conclusions for the arguments in \(2-4 .\) If an integer \(n\) equals \(2 \cdot k\) and \(k\) is an integer, then \(n\) is even, 0 equals \(2 \cdot 0\) and 0 is an integer.

How could you determine the truth or falsity of the following statements for the students in your discrete mathematics class? Assume that students will respond truthfully to questions that are asked of them. a. There is a student in this class who has dated at least one person from every residence hall at this school. b. There is a residence hall at this school with the property that every student in this class has dated at least one person from that residence hall. c. Every residence hall at this school has the property that if a student from this class has dated at least one person from that hall, then that student has dated at least two people from that hall.

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