Nested Quantifiers Nested Quantifiers Needed to express statements

Nested Quantifiers

Nested Quantifiers Needed to express statements with multiple variables Example 1: “x+y = y+x for all real numbers” x y(x+y = y+x) where the domains of x and y are real numbers Example 2: “Every real number has an inverse” x y(x + y = 0) where the domains of x and y are real numbers Each quantifier is within the scope of the preceding quantifier: x y(x+y=0) can be viewed as x Q(x) where Q(x) is y P(x, y) where P(x, y) is (x+y=0)

Order of Quantifiers Example 1: Let P(x, y) be the statement “x+y=y+x” Assume that U are real numbers Then x y. P(x, y)≡ y x. P(x, y)≡T because for any combination of x and y the statement holds Example 2: Let Q(x, y) be the statement “x+y=0” Assume that U are real numbers. Then x y. P(x, y)≡T, because for every x there is always an inverse y but y x. P(x, y)≡F because for a given y not every x will add up to zero

Quantifications of Two Variables General rules: Statement When True? When False P(x, y) is true for every pair x, y. There is a pair x, y for which P(x, y) is false. For every x there is a y for There is an x such that which P(x, y) is true. P(x, y) is false for every y. There is an x for which P(x, y) is true for every y. For every x there is a y for which P(x, y) is false. There is a pair x, y for which P(x, y) is true. P(x, y) is false for every pair x, y

Translating Math Statements Example : “The sum of two positive integers is always positive” Solution: 1. Rewrite the statement to make the implied quantifiersand domainsexplicit: “For every two positive integers, their sum is positive. ” 2. Introduce variablesx and y: “For all positive integers x and y, x+y is positive. ” 3. Introduce quantifiers: x y ((x > 0)∧ (y > 0)→ (x + y > 0)) where the domain of both variables is integers

Translation from English Decide on the domain; choose the obvious predicates; express in predicate logic. Example 1: “Brothers are siblings. ” domain: all people x y (B(x, y) → S(x, y)) Example 2: “Siblinghood is symmetric. ” x y (S(x, y) → S(y, x)) Example 3: “Everybody loves somebody. ” x y L(x, y)

Translation from English Example 4: “There is someone who is loved by everyone. ” y x L(x, y) Example 5: “There is someone who loves someone. ” x y L(x, y) Example 6: “Everyone loves himself” x L(x, x)

Negating Nested Quantifiers Recall De Morgan’s laws for quantifiers ¬ x P(x) ≡ x ¬P(x) Starting from left to right, successively apply these laws to nested quantifiers Example: ¬ w a f (P(w, f ) ∧ Q(f, a)) ≡ ≡ w ¬ a f (P(w, f ) ∧ Q(f, a)) by De Morgan’s for ≡ w a ¬ f (P(w, f ) ∧ Q(f, a)) by De Morgan’s for ≡ w a f ¬(P(w, f ) ∧ Q(f, a)) by De Morgan’s for ≡ w a f (¬P(w, f ) ∨ ¬Q(f, a)) by De Morgan’s for ∧