 # Discrete Mathematics Chapter 1 Logic and proofs 1282020

• Slides: 48 Discrete Mathematics Chapter 1 Logic and proofs 12/8/2020 1 Logic = the study of correct reasoning p Use of logic p n In mathematics: p n In computer science: p 12/8/2020 to prove theorems to prove that programs do what they are supposed to do 2 Section 1. 1 Propositions A proposition is a statement or sentence that can be determined to be either true or false. p Examples: p n n 12/8/2020 “John is a programmer" is a proposition “I wish I were wise” is not a proposition 3 Connectives If p and q are propositions, new compound propositions can be formed by using connectives p Most common connectives: n n n 12/8/2020 Conjunction AND. Inclusive disjunction OR Exclusive disjunction OR Negation Implication Double implication Symbol ^ Symbol v Symbol ~ Symbol 4 Truth table of conjunction The truth values of compound propositions can be described by truth tables. p Truth table of conjunction p p 12/8/2020 p T T F q T F T p^q T F F F p ^ q is true only when both p and q are true. 5 Example Let p = “Tigers are wild animals” p Let q = “Chicago is the capital of Illinois” p p ^ q = "Tigers are wild animals and Chicago is the capital of Illinois" p p ^ q is false. Why? p 12/8/2020 6 Truth table of disjunction q q The truth table of (inclusive) disjunction is q T F T pvq T T T F F F p q is false only when both p and q are false q q 12/8/2020 p T T F Example: p = "John is a programmer", q = "Mary is a lawyer" p v q = "John is a programmer or Mary is a lawyer" 7 Exclusive disjunction p q “Either p or q” (but not both), in symbols p q q T F T pvq F T T F F F p q is true only when p is true and q is false, or p is false and q is true. q q 12/8/2020 p T T F Example: p = "John is programmer, q = "Mary is a lawyer" p v q = "Either John is a programmer or Mary is a lawyer" 8 Negation p p Negation of p: in symbols ~p ~p T F F T ~p is false when p is true, ~p is true when p is false n n 12/8/2020 p Example: p = "John is a programmer" ~p = "It is not true that John is a programmer" 9 More compound statements Let p, q, r be simple statements p We can form other compound statements, such as p n n n 12/8/2020 (p q)^r p (q^r) (~p) (~q) (p q)^(~r) and many others… 10 Example: truth table of (p q)^r 12/8/2020 p q r (p q) ^ r T T T F F T T T F F F F T F F F 11 1. 2 Conditional propositions and logical equivalence A conditional proposition is of the form “If p then q” p In symbols: p q p Example: p n n n 12/8/2020 p = " John is a programmer" q = " Mary is a lawyer " p q = “If John is a programmer then Mary is a lawyer" 12 Truth table of p q q 12/8/2020 p q T T F F F T T F F T p q is true when both p and q are true or when p is false 13 Hypothesis and conclusion In a conditional proposition p q, p is called the antecedent or hypothesis q is called the consequent or conclusion q If "p then q" is considered logically the same as "p only if q" p 12/8/2020 14 Necessary and sufficient A necessary condition is expressed by the conclusion. p A sufficient condition is expressed by the hypothesis. p n Example: n If John is a programmer then Mary is a lawyer" Necessary condition: “Mary is a lawyer” Sufficient condition: “John is a programmer” n 12/8/2020 15 Logical equivalence q q Two propositions are said to be logically equivalent if their truth tables are identical. p q ~p q p q T T F T F F T T Example: ~p q is logically equivalent to p q 12/8/2020 16 Converse p The converse of p q is q p p T T F q T F T p q T F T q p T T F F F T T These two propositions are not logically equivalent 12/8/2020 17 Contrapositive p The contrapositive of the proposition p q is ~q ~p. p T q T p q T ~q ~p T T F F T F T F F T T They are logically equivalent. 12/8/2020 18 Double implication p The double implication “p if and only if q” is defined in symbols as p q p q (p q) ^ (q p) T T T F F F F T T p q is logically equivalent to (p q)^(q p) 12/8/2020 19 Tautology p A proposition is a tautology if its truth table contains only true values for every case n 12/8/2020 Example: p p v q p pvq T T F F T 20 Contradiction p A proposition is a tautology if its truth table contains only false values for every case n 12/8/2020 Example: p ^ ~p p p ^ (~p) T F F F 21 De Morgan’s laws for logic p The following pairs of propositions are logically equivalent: ~ (p q) and (~p)^(~q) n ~ (p ^ q) and (~p) (~q) n 12/8/2020 22 1. 3 Quantifiers A propositional function P(x) is a statement involving a variable x p For example: p n p x is an element of a set D n p 12/8/2020 P(x): 2 x is an even integer For example, x is an element of the set of integers D is called the domain of P(x) 23 Domain of a propositional function In the propositional function P(x): “ 2 x is an even integer”, the domain D of P(x) must be defined, for instance D = {integers}. p D is the set where the x's come from. p 12/8/2020 24 For every and for some Most statements in mathematics and computer science use terms such as for every and for some. p For example: p n n 12/8/2020 For every triangle T, the sum of the angles of T is 180 degrees. For every integer n, n is less than p, for some prime number p. 25 Universal quantifier p One can write P(x) for every x in a domain D n p 12/8/2020 In symbols: x P(x) is called the universal quantifier 26 Truth of as propositional function p The statement x P(x) is n n True if P(x) is true for every x D False if P(x) is not true for some x D Example: Let P(n) be the propositional function n 2 + 2 n is an odd integer n D = {all integers} p P(n) is true only when n is an odd integer, false if n is an even integer. p 12/8/2020 27 Existential quantifier For some x D, P(x) is true if there exists an element x in the domain D for which P(x) is true. In symbols: x, P(x) p p 12/8/2020 The symbol is called the existential quantifier. 28 Counterexample p The universal statement x P(x) is false if x D such that P(x) is false. p The value x that makes P(x) false is called a counterexample to the statement x P(x). n n 12/8/2020 Example: P(x) = "every x is a prime number", for every integer x. But if x = 4 (an integer) this x is not a primer number. Then 4 is a counterexample to P(x) being true. 29 Generalized De Morgan’s laws for Logic p If P(x) is a propositional function, then each pair of propositions in a) and b) below have the same truth values: a) ~( x P(x)) and x: ~P(x) "It is not true that for every x, P(x) holds" is equivalent to "There exists an x for which P(x) is not true" b) ~( x P(x)) and x: ~P(x) "It is not true that there exists an x for which P(x) is true" is equivalent to "For all x, P(x) is not true" 12/8/2020 30 Summary of propositional logic p In order to prove the universally quantified statement x P(x) is true n n 12/8/2020 It is not enough to show P(x) true for some x D You must show P(x) is true for every x D p In order to prove the universally quantified statement x P(x) is false n n It is enough to exhibit some x D for which P(x) is false This x is called the counterexample to the statement x P(x) is true 31 1. 4 Proofs p 12/8/2020 A mathematical system consists of n Undefined terms n Definitions n Axioms 32 Undefined terms p Undefined terms are the basic building blocks of a mathematical system. These are words that are accepted as starting concepts of a mathematical system. n 12/8/2020 Example: in Euclidean geometry we have undefined terms such as p Point p Line 33 Definitions p A definition is a proposition constructed from undefined terms and previously accepted concepts in order to create a new concept. n n n 12/8/2020 Example. In Euclidean geometry the following are definitions: Two triangles are congruent if their vertices can be paired so that the corresponding sides are equal and so are the corresponding angles. Two angles are supplementary if the sum of their measures is 180 degrees. 34 Axioms An axiom is a proposition accepted as true without proof within the mathematical system. p There are many examples of axioms in mathematics: p n Example: In Euclidean geometry the following are axioms Given two distinct points, there is exactly one line that contains them. p Given a line and a point not on the line, there is exactly one line through the point which is parallel to the line. p 12/8/2020 35 Theorems p 12/8/2020 A theorem is a proposition of the form p q which must be shown to be true by a sequence of logical steps that assume that p is true, and use definitions, axioms and previously proven theorems. 36 Lemmas and corollaries p A lemma is a small theorem which is used to prove a bigger theorem. p A corollary is a theorem that can be proven to be a logical consequence of another theorem. n 12/8/2020 Example from Euclidean geometry: "If the three sides of a triangle have equal length, then its angles also have equal measure. " 37 Types of proof A proof is a logical argument that consists of a series of steps using propositions in such a way that the truth of theorem is established. p Direct proof: p q p n 12/8/2020 A direct method of attack that assumes the truth of proposition p, axioms and proven theorems so that the truth of proposition q is obtained. 38 Indirect proof q The method of proof by contradiction of a theorem p q consists of the following steps: 1. Assume p is true and q is false 2. Show that ~p is also true. 3. Then we have that p ^ (~p) is true. 4. But this is impossible, since the statement p ^ (~p) is always false. There is a contradiction! 5. So, q cannot be false and therefore it is true. q OR: show that the contrapositive (~q) (~p) is true. q 12/8/2020 Since (~q) (~p) is logically equivalent to p q, then theorem is proved. 39 Valid arguments Deductive reasoning: the process of reaching a conclusion q from a sequence of propositions p 1, p 2, …, pn. p The propositions p 1, p 2, …, pn are called premises or hypothesis. p The proposition q that is logically obtained through the process is called the conclusion. p 12/8/2020 40 Rules of inference (1) 1. Law of detachment or modus ponens n n n 12/8/2020 p q p Therefore, q 2. Modus tollens n n n p q ~q Therefore, ~p 41 Rules of inference (2) 3. Rule of Addition n n p Therefore, p q 5. Rule of conjunction n n 4. Rule of simplification n n 12/8/2020 n p q Therefore, p ^ q p^q Therefore, p 42 Rules of inference (3) 6. Rule of hypothetical syllogism np q nq r n Therefore, p r 7. Rule of disjunctive syllogism np q n ~p n Therefore, q 12/8/2020 43 Rules of inference for quantified statements 1. Universal instantiation 3. Existential instantiation n x D, P(x) n x D, P(x) n d D n Therefore P(d) for some d D n Therefore P(d) 2. Universal generalization 4. Existential generalization n P(d) for some d D n P(d) for any d D n Therefore x, P(x) n Therefore x, P(x) 12/8/2020 44 1. 5 Resolution proofs p p Due to J. A. Robinson (1965) A clause is a compound statement with terms separated by “or”, and each term is a single variable or the negation of a single variable n p p Example: p q (~r) is a clause (p ^ q) r (~s) is not a clause Hypothesis and conclusion are written as clauses Only one rule: n n n 12/8/2020 p q ~p r Therefore, q r 45 1. 6 Mathematical induction p Useful for proving statements of the form n A S(n) where N is the set of positive integers or natural numbers, A is an infinite subset of N S(n) is a propositional function 12/8/2020 46 Mathematical Induction: strong form q Suppose we want to show that for each positive integer n the statement S(n) is either true or false. n n 12/8/2020 1. Verify that S(1) is true. 2. Let n be an arbitrary positive integer. Let i be a positive integer such that i < n. 3. Show that S(i) true implies that S(i+1) is true, i. e. show S(i) S(i+1). 4. Then conclude that S(n) is true for all positive integers n. 47 Mathematical induction: terminology Basis step: p Inductive step: p p Conclusion: 12/8/2020 Verify that S(1) is true. Assume S(i) is true. Prove S(i) S(i+1). Therefore S(n) is true for all positive integers n. 48