2.2: Logical Equivalence: The Laws of Logic

Transcription

1 Example (2.7) For primitive statement p and q, construct a truth table for each of the following compound statements. a) p q b) p q Here we see that the corresponding truth tables for two statement p q and p q are exactly the same. Definition (2.2) Two statement s 1, s 2 are said to be logically equivalent, and we write s 1 s 2, when the statement s 1 is true (respectively, false) if and only if the statement s 2 is true (respectively, false). From the table, we know that p q p q.

2 Example (2.7) For primitive statement p and q, construct a truth table for each of the following compound statements. a) p q b) p q Here we see that the corresponding truth tables for two statement p q and p q are exactly the same. Definition (2.2) Two statement s 1, s 2 are said to be logically equivalent, and we write s 1 s 2, when the statement s 1 is true (respectively, false) if and only if the statement s 2 is true (respectively, false). From the table, we know that p q p q.

3 Example (2.7) For primitive statement p and q, construct a truth table for each of the following compound statements. a) p q b) p q Here we see that the corresponding truth tables for two statement p q and p q are exactly the same. Definition (2.2) Two statement s 1, s 2 are said to be logically equivalent, and we write s 1 s 2, when the statement s 1 is true (respectively, false) if and only if the statement s 2 is true (respectively, false). From the table, we know that p q p q.

4 Example (2.7) For primitive statement p and q, construct a truth table for each of the following compound statements. a) p q b) p q Here we see that the corresponding truth tables for two statement p q and p q are exactly the same. Definition (2.2) Two statement s 1, s 2 are said to be logically equivalent, and we write s 1 s 2, when the statement s 1 is true (respectively, false) if and only if the statement s 2 is true (respectively, false). From the table, we know that p q p q.

5 Example (2.8) Construct a truth table for each of the following statements a) (p q), b) p q, c) (p q), d) p q, where p, q are primitive statements. Here, a crucial difference emerges: The negation of the conjunction of two primitive statement p, q results in the disjunction of their negations p, q, whereas the negation of the disjunction of these same statements p, q is logically equivalent to the conjunction of their negations p, q.

6 Example (2.8) Construct a truth table for each of the following statements a) (p q), b) p q, c) (p q), d) p q, where p, q are primitive statements. Here, a crucial difference emerges: The negation of the conjunction of two primitive statement p, q results in the disjunction of their negations p, q, whereas the negation of the disjunction of these same statements p, q is logically equivalent to the conjunction of their negations p, q.

7 The Laws of Logic 1/2 For any primitive statements p, q, r, any tautology T 0, and any contradiction F 0, 1) p p Laws of Double Negation 2) (p q) p q DeMorgan s Laws (p q) p q 3) p q q p Commutative Laws p q q p 4) p (q r) (p q) r Associative Laws p (q r) (p q) r 5) p (q r) (p q) (p r) Distributive Laws p (q r) (p q) (p r)

8 The Laws of Logic 2/2 6) p p p Idempotent Laws p p p 7) p F 0 p Identity Laws p T 0 p 8) p p T 0 Inverse Laws p p F 0 9) p T 0 T 0 Domination Laws p F 0 F 0 10) p (p q) p Absorption Laws p (p q) p Remark: T 0 =tautology F 0 =contradiction

9 Definition (2.3 Dual) Let s be a statement. If s contains no logical connectives other than and, then the dual of s, denoted s d, is the statement obtained from s by replacing each occurrence of and by and, respectively, and each occurrence of T 0 and F 0 by F 0 and T 0, respectively. Theorem (2.1 The Principle of Duality) Let s and t be statements that contain no logical connectives other than and. If s t, then s d t d.

10 Definition (2.3 Dual) Let s be a statement. If s contains no logical connectives other than and, then the dual of s, denoted s d, is the statement obtained from s by replacing each occurrence of and by and, respectively, and each occurrence of T 0 and F 0 by F 0 and T 0, respectively. Theorem (2.1 The Principle of Duality) Let s and t be statements that contain no logical connectives other than and. If s t, then s d t d.

11 Two substitution rules: 1) Suppose that the compound statement P is a tatulogy. If p is a primitive statement that appears in P and we replace each occurrence of p by the same statement q, then the resulting compound statement P 1 is also a tatulogy. 2) Let P be a compound statement where p is an arbitrary statement that appears in P, and let q be a statement such that q p. Suppose that in P we replace one or more occurrences of p by q. Then this replacement yields the compound statement P 1. Under these circumstances P 1 P.

12 Example (2.12) Negate and simplify the compound statement (p q) r. Example (2.15) Verify the following compound statements are logically equivalent: a) (p q) ( q p) b) (q p) ( p q) The statement q p is called the contrapositive of the implication p q. The statement q p is called the converse of p q; p q is called the inverse of p q.

13 Simplification of compound statements: Example (2.16, 2.17, 2.18) Let p, q, r, t denote primitive statements. Simplify each of the following compound statements: a) (p q) ( p q) b) [ [(p q) r] q] c) (p q r) (p t q) (p t r)