Notes by Lex Toumbourou

4.101 Introduction to predicate logic

• Predicate](../../../../permanent/predicate-logic.md)
  • Propositional logic has some limitations:
    • Cannot precisely express meaning of complex math statements.
    • Only studies propositions, which are statements with known truth values.
  • Predicate logic overcomes the limitations and can be used to build more complex reasoning.
  • Example 1:
    • Given statements:
      • "All men are mortal."
      • "Socrates is a man."
    • Naturally, it follows that: "Socrates is mortal"
    • Propositional logic can't express this reasoning, but predicate logic can enable us to formalise it.
  • Example 2:
    • "x squared is equal to 4."
    • It's not a proposition, as its truth value is a function depending on x.
    • We need predicate logic.

4.103 What are Predictates?

• Consider statement $x^2 = 4$
  • It's not a proposition as its true value depends on $x$
  • Therefore, it can't be expressed using propositional logic.
  • Can be expressed using predicate logic.
• Predicate
  • Predicates behave as functions whose values $T$ or $F$ depend on their variables.
  • Predicates become propositions when their variables are given actual values.
• The statement above has 2 parts:
• The variable x: the subject of the statement.
• The predicate "squared is equal to 4": the property the subject of the statement can have.
• The statement can be formalised as $P(x)$ where P is the predicate "squared is equal to 4" and x is the variable.
• Evaluate for certain values of x:
  • P(2) is True
  • P(3) is False
• A predictate can depend on multiple values:
  • Let $P(x, y)$ denote "$x^2 > y$":
    • $P(-2, 3)$ = $(4 > 3) \text{ is } \mathbf{ True}$
    • $P(2, 4)$ = $4(2^2 >4) \text{ is } \mathbf{False}$
• Let Q(x, y, z) denote x + y < z
  • Q(2, 4, 5) = (6 < 5) is F
  • Q(2, 4, z) is not a proposition
• Logical operations from propositional logic carry over to predicate logic
  • If $P(x)$ denotes $x^2 < 16$, then:
    • $P(1) \lor P(-5) \equiv (1 < 16) \lor (25 < 16) \equiv T \lor F \equiv T$
    • $P(1) \land P(-5) \equiv T \land F \equiv F$
    • $P(3) \land P(y)$ is not a proposition. It becomes a proposition when y is assigned a value.

4.105 Quantification

• Quantification
  • Quantification expresses the extent to which a predicate is true over a range of elements
  • They express the meaning of the words all and some.
  • Two most important ones:
    • Universal quantifier
    • Existential quantifier
    • Example
      • "All men are mortal"
      • "Some computers are not connected to the network"
  • A third quantifier called "uniqueness quantifier".
• Universal Quantifier
  • The universal quantifier of predictate P(x) is proposition:
    • P(x) is true for all values of x in the universe of discourse.
  • We use the notation: $\forall x P(x)$, which is read "for all x"
  • If the universe of discourse is finish, say $\{n_1, n_2, \ldots, n_3\}$ then the universal quanifier is simply the conjunction of the propositions over all elements:
    • $\forall x P(x) \Leftrightarrow P(n_{1}) \land P(n_{2}) \land \ldots \land P(n_k)$
  • Example 1:
    • $P(x)$: "x must take a Discrete Mathematics course"
    • $Q(x)$: "x is a Computer Science student."
    • Where, the university of discourse for both $P(x)$ and $Q(x)$ is all university students.
  • Let's express the following statements:
    • Every CS student must take a discrete math course
      • $\forall \ \ x \ \ Q(x) \rightarrow P(x)$
    • Everybody must take a discrete maths course or be a CS student
      • $\forall \ \ x \ \ (P(x) \lor Q(x))$
    • Everybody must take a discrete maths course and be a CS student
      • $\forall \ \ x \ \ (P(x) \land Q(x))$
  • Example 2:
    • Formalise statement S:
      • S: "For every x and every y, x + y > 10"
    • Let $P(x, y)$ by the statement x + y > 10, where the universe of discourse for x, y is the set of all integers.
    • The statement S is: $\forall x \forall y P(x, y)$
    • Can also be written as: $\forall x, y \ \ P(x, y)$
• Existential Quantifier
  • The existential quantification of a predicate $P(x)$is the proposition:
    • "There exists a value x in the universe of discourse such that P(x) is true."
  • We use the notation: $\exists \ x \ P(x)$, which reads "there exists x".
  • If the universe of discourse is finite, say $\{n_1, n_2, \ldots, n_k\}$ then the existential quantifier is simply the disjunction of propositions over all the elements:
    • $\exists \ x \ P(x) \Leftrightarrow P(n_1) \lor P(n_2) \lor \ldots \lor P(n_k)$
  • Example 1
    • Let $P(x, y)$ denote the statement "x + y = 5".
    • The expression E: $\exists \ x \ \exists \ y \ P(x, y)$ means:
      • There exists a value x and a value y in the universe of discourse such that $x + y = 5$ is true.
    • For instance
      • If the universe of discourse is positive integers, E is True.
      • If the universe of discourse is negative integers, E is False.
  • Example 2
    • Let $a, b, c$ denote fixed real numbers.
    • And S be the statement: "There exists a real solution to $ax^2 + bx - c = 0$"
    • S can be expressed as $\exists \ x \ P(x)$ where:
      • $P(x)$ is $ax^2 + bx - c = 0$ and the universe of discourse for x is the set of real numbers.
    • Let's evaluate the truth value of S:
      • When $b^2 >= 4ac, S \text{ is true , as } P(-b \mp \sqrt(b^2 - 4ac)) / 2a = 0$
      • When $b^2 < 4ac, S\text{ is false }$ as there is no real number x that can satisfy the predicate.
• Uniqueness quantifier
  • Special case of "existential quantifier".
  • The uniqueness quantifier of prediction P of x is the proposition:
    • There exists a unique value of x in the universe such that P of x is true.
    • We use the notation: $\exists ! x \ P(x)$: read as there exists a unique x.
  • Example:
    • Let P(x) denote the statement: $x^2 = 4$
    • The expression $E$: $\exists ! x \ P(x)$ means:
      • There exists a unique value x in the universe of discourse such that $x^2 = 4$ is true.
    • For instance
      • If the universe of discourse is positive integers, E is True (as x = 2 is the unique solution)
      • If the universe of discourse is integers, E is False (as x = 2 and x = -2 are both solutions)

4.107 Nested quantifiers

• Nested quantifiers
  • To express statements with multiple variables we use nested quantifiers
    • $\forall x \forall y P(x, y)$ - P(x, y) is true for every pair x, y
    • $\exists x \ \exists y \ P(x, y)$ - There is a pair x, y for which P(x, y) is true.
    • $\forall x \ \exists y \ P(x, y)$ - For every x, there is a y for whih P(x, y) i true.
    • $\exists x \ \forall y \ P(x, y)$ - there is an x for which P(x, y) is true for every y.
• Binding variables
  • A variable is said to be bound if it is within the scope of a quantifier.
  • A variable is free if it is not bound by a quantifier or particular values.
  • Example
    • Let P be a propositional function
    • And S the statement: $\exists \ x \ P(x, y)$
    • We can say that:
      • x is bound
      • y is free
• Logical operations
  • Logical operations can be applied to quantified statements
  • Example
    • If P(x) denotes "x > 3" and Q(x) denotes "x squared is even" then
      • $\exists \ x \ (P(x) \lor Q(x)) \equiv T (ex. x = 4)$
      • $\forall \ x \ (P(x) \rightarrow Q(x) \equiv F (ex. x = 5))$
• Order of operations
  • When nested quantifiers are of the same type, the order does not matter.
  • With quantifiers of different types, the order does matter.
  • Example
    • $\forall x \ \forall y P(x, y) \equiv \forall y \ \forall x \ P(x, y)$
    • $\exists x \ \exists y \ P(x, y) \equiv \exists y \ \exists x \ P(x, y)$
    • $\forall x \ \exists y \ P(x, y)$ is different from $\exists y \ \forall x \ P(x, y)$
• Precendence of quantifiers
  • The quantifiers $\forall$ and $\exists$ have a higher precendence than all logical operators
  • Example
    • P(x) and Q(x) denote two propositional functions.
    • $\forall x \ P(x) \lor Q(x)$ is the disjunction of $\forall x \ P(x) \text{ and } Q(x)$ rather than $\forall x \ (P(x) \text{ and } Q(x))$