# Equivalence and Implication

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1 Equivalence and Alice E. Fischer CSCI 1166 Discrete Mathematics for Computing February 7 8, 2018 Alice E. Fischer Laws of Logic... 1/33

2 1 Logical Equivalence Contradictions and Tautologies Necessary and Sufficient Conditions Inside out and Backwards 5 Alice E. Fischer Laws of Logic... 2/33

3 Logical Equivalence Contradictions and Tautologies Logical Equivalence Contradictions and Tautologies Alice E. Fischer Laws of Logic... 3/33

4 Logical Equivalence: Logical Equivalence Contradictions and Tautologies Two propositions are logically equivalent ( ) if they have the same outcomes under all input conditions. We can determine whether two propositions are equivalent ( ) by comparing the last column in the truth tables for the two statements. P Q P Q Q P T T T T T F T T F T T T F F F F For example, The column for P Q is the same as the column for Q P, so the two propositions are equivalent. Alice E. Fischer Laws of Logic... 4/33

5 Example: The Supervisor s Critique Logical Equivalence Contradictions and Tautologies Do you remember John and his elevator design from last time? John s supervisor looked at his logic and told him It s too complex, O is just the opposite of M. Simplify! So John thought a bit and added a column with a simpler formula to his truth table. (O M) O M O M O M O M ( O M) O M T T F F F F F F T F F T T F T T F T T F F T T T F F T T F F F F Indeed, the boss was right. They are equivalent! Alice E. Fischer Laws of Logic... 5/33

6 Contradictions Logical Equivalence Contradictions and Tautologies A contradiction is a proposition that is always false, no matter what the input truth values are. For example, this is a contradiction: An odd number, X, equals 2 * some even number, Y. And this is a contradiction: P P Let P be All dogs are black. All dogs are black and not (all dogs are black). A contradiction can be symbolized by c. Alice E. Fischer Laws of Logic... 6/33

7 Tautologies Outline Logical Equivalence Contradictions and Tautologies A tautology is a proposition that is always true, no matter what the input truth values are. For example: Let Q be the proposition Today is Tuesday. Then this is a tautology: Q Q Today is Tuesday or today is not Tuesday. A tautology can be symbolized by t. Alice E. Fischer Laws of Logic... 7/33

8 A set of rules for logical calculation can be proven from the definitions using truth tables. They will be presented in the following slides. These laws are analogs of the laws for algebra and for sets that you already know. They are essential in making logical proofs and several are also important in programming. The following slides define the laws for symbolic logic. The corresponding set-theory law(s) will be given at the bottom of each slide. Alice E. Fischer Laws of Logic... 8/33

9 1. Commutative Laws Both and and or are commutative, that is, the left and right sides can be reversed without changing the meaning of the proposition. P Q Q P P Q Q P This is important in programming. Set theory: and are commutative. Given sets A and B, A B B A A B B A Alice E. Fischer Laws of Logic... 9/33

10 2. Associative Laws Both and and or are associative operators, that is, you may parenthesize the expression any way you wish without changing its meaning. (P Q) R P (Q R) (P Q) R P (Q R) This is important in programming. Set theory: and are associative. Given sets P, Q, and R, (P Q) R P (Q R) (P Q) R P (Q R) Alice E. Fischer Laws of Logic... 10/33

11 3. Distributive Laws Both and and or can be distributed over the other operator. P (Q R) (P Q) (P R) P (Q R) (P Q) (P R) This is important in programming. Set theory: and can be distributed over each other. Given sets P, Q, and R, P (Q R) (P Q) (P R) P (Q R) (P Q) (P R) Alice E. Fischer Laws of Logic... 11/33

12 4. Identity Laws The identity laws are based on the definitions of and and or. P t P P c P Set Theory: The set identity laws are based on definitions of the Universal set and the empty set. Given set A, A U A A A Alice E. Fischer Laws of Logic... 12/33

13 5. Negation Laws The negation laws are based on the definitions of and, or, and not. P P t P P c Set Theory: Complement Laws The set complement laws are based on definitions of the Universal set, empty set, and complement. Given set A, A A c U A A c Alice E. Fischer Laws of Logic... 13/33

14 6. Double Negative Law The double-negative law is based on the definitions of not. ( P) P Set Theory: Double Complement Law This is based on the definitions of complement. Given set A, (A c ) c A Alice E. Fischer Laws of Logic... 14/33

15 7. Idempotent Laws and, or are idempotent, meaning that the trivial computations have no effect and can be repeated without changing the meaning of the expression. P P P P P P Set Theory: intersection and union are idempotent. Given set A, A A A A A A Alice E. Fischer Laws of Logic... 15/33

16 8. Universal Bound Laws A contradiction and anything is a contradiction. A tautology or anything is a tautology. P c c P t t The shortcut evaluation of logical operators in C is based on these. Set Theory: The empty set is as small as you can get and the universal set is as large as you can get. Given set A, A A U U Alice E. Fischer Laws of Logic... 16/33

17 9. DeMorgan s Laws To distribute not over and, change the and to or. To distribute not over or, change the or to and. (P Q) P Q (P Q) P Q These laws are very important in programming and in databases. Set Theory: To distribute complement over, change the to. To distribute complement over, change the to. Given sets A, B, (A B) c A c B c (A B) c A c B c Alice E. Fischer Laws of Logic... 17/33

18 10. Absorption Laws A truth table can be used to verify these absorptions. P (P Q) P P (P Q) P These laws are important in proofs. Set Theory: Given sets A, B, A (A B) A That is, A a subset of A is still A. A (A B) A That is, A a superset of A is still A. Alice E. Fischer Laws of Logic... 18/33

19 11. Negations of t and c The negation of a tautology is a contradiction and vice versa. t c c t Set Theory: The complement of the Universal set is the empty set, and vice versa. U c c U Alice E. Fischer Laws of Logic... 19/33

20 Practice This section contains three logical expressions to simplified by applying the rules, and one law to prove using truth tables. Alice E. Fischer Laws of Logic... 20/33

21 1. Proving DeMorgan s Law Construct a truth table to prove that (P Q) P Q P Q P Q (P Q) P Q ( P Q) T T T F F F F T F F T F T T F T F T T F T F F F T T T T Compare the contents of the fourth and seventh columns. They are the same, so the two expressions are equivalent. Alice E. Fischer Laws of Logic... 21/33

22 2. Two small messes to simplify. (P Q) (P Q) P ( Q Q) P (t) P Initial expression. Use the distributive law, backwards. Use negation law. Use identity law. P ( Q P) (P Q) (P P) (P Q) P P (P Q) P Distributive law. Idempotent law Commutative law Absorption law. Alice E. Fischer Laws of Logic... 22/33

23 3. A big mess. (P ( ( P Q))) (P Q) Initial expression (P ( P) Q)) (P Q) DeMorgan s law. (P ( P Q)) (P Q) Double negative law. ((P P) Q) (P Q) Associative law. ( P Q) (P Q) Idempotent law. P ( Q Q) Distributive law, backwards. P (t) Negation law. P Identity law. Alice E. Fischer Laws of Logic... 23/33

24 Necessary and Sufficient Conditions Inside out and Backwards and its Truth Table Necessary and Sufficient Conditions Inside out and Backwards Alice E. Fischer Laws of Logic... 24/33

25 and its Truth Table Necessary and Sufficient Conditions Inside out and Backwards The symbol is called if... then in the textbook. It is more traditional to call it implies. The proposition P Q is read P implies Q or if P, then Q We say that P is the premise and Q is the conclusion. The truth table for P Q is: P Q P Q P P Q T T T F T T F F F F F T T T T F F T T T It is also true that P Q P Q Alice E. Fischer Laws of Logic... 25/33

26 Can be Confusing. Necessary and Sufficient Conditions Inside out and Backwards Sometimes, but not always, P Q means that P causes Q: I didn t increment my loop variable so my program had an infinite loop. Sometimes P Q is used to define a term: If a creature is bipedal, it has two feet. The conclusion can be true independently of the premise: If the moon is made of green cheese then UNH is a university. This is often called a vacuous truth. The implication is true when both the premise and the conclusion are false. if 0 = 1 then 1 = 2. A truth table is the most reliable way to figure out the correct interpretation of an implication. Alice E. Fischer Laws of Logic... 26/33

27 If and Only If Necessary and Sufficient Conditions Inside out and Backwards P Q is a conditional statement that can be read if P then Q. Now suppose that P Q and also Q P. Then we write P Q, which can be read P if and only if Q. This statement is called a biconditional. P Q P Q Q P P Q T T T T T T F F T F F T T F F F F T T T Alice E. Fischer Laws of Logic... 27/33

28 Necessary and Sufficient Conditions Necessary and Sufficient Conditions Inside out and Backwards A sufficient condition is a premise or set of premises that are enough, without any more premises, to prove the conclusion. If C is a sufficient condition for S, then C S. A necessary condition is a premise that must be true in order for the conclusion to be true. If C is a necessary condition for S, then C S. If a condition C is both necessary and sufficient for S, then C S. Alice E. Fischer Laws of Logic... 28/33

29 Inside Out and Backwards Necessary and Sufficient Conditions Inside out and Backwards There are four ways to reverse the proposition P Q: The contrapositive is The converse is Q P. The inverse is P Q. Q P. The negation is (P Q), which is equivalent to P Q. A proposition and its contrapositive are equivalent. A proposition is not equivalent to its converse, inverse, or negation. For example, If it is a bee, it can fly is not to If it can fly, it is a bee. Alice E. Fischer Laws of Logic... 29/33

30 The opposites of P Q: Necessary and Sufficient Conditions Inside out and Backwards Conditional P Q Inverse P Q Converse Q P Contrapositive Q P A conditional is not equivalent to its converse. A conditional is equivalent to its contrapositive. The inverse is equivalent to the converse P Q P Q P Q Q P Q P P Q T T F F T T T T T F F T F F T T F T T F T T F F F F T T T T T T Alice E. Fischer Laws of Logic... 30/33

31 Alice E. Fischer Laws of Logic... 31/33

32 of Terminology A conjunction is two propositions joined by. A disjunction is two propositions joined by. A negation is applied to a proposition. Two propositions are equivalent ( ) if the results in the last column of their truth tables are identical. An implication is two propositions joined by. A biconditional is two propositions joined by. Alice E. Fischer Laws of Logic... 32/33

33 More Terminology A proposition that is always true is called a tautology. A proposition that is always false is called a contradiction. Given a proposition P Q, The contrapositive is Q P (equivalent to proposition) The converse is Q P (equivalent to inverse) The inverse is P Q (equivalent to converse) The negation is (P Q) (equivalent to P Q) Alice E. Fischer Laws of Logic... 33/33

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