# Computational Logic. Recall of First-Order Logic. Damiano Zanardini

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1 Computational Logic Recall of First-Order Logic Damiano Zanardini UPM European Master in Computational Logic (EMCL) School of Computer Science Technical University of Madrid Academic Year 2009/2010 D. Zanardini Computational Logic Ac. Year 2009/ / 1

2 Syntax of a first-order languagep An alphabet A consists of variable symbols: x, y, z, w, x,... function symbols: f ( ), g(, ),... (arity = no. of args: f /1, g/2) predicate symbols: p( ), q(, ),..., = /2 connectives:,,,, quantifiers:, constants (a, b, c,... ) = 0-arity functions propositions = 0-arity predicates Metalanguage F and G denote arbitrary formulæ D. Zanardini Computational Logic Ac. Year 2009/ / 1

3 Syntax of a first-order languagep variable symbols: x, y, z constant (0-ary functions) symbols: a, b, c, tom, 0, 1 function symbols: f /1, g/2 predicate symbols: p/0, q/2, cat/1, +/3 Terms a variable is a term if t 1,..., t n are terms and f is an n-ary (n 0, thus including constants) function symbol, then f (t 1,..., t n ) is a term Examples a f (tom) f (1 a() a(1) f (2) g(g, g(1)) q(0, f (1)) a + y g(1, f (a)) g(0c) cat(cat(0)) +(a, f (z), c) f (f (f (x))) D. Zanardini Computational Logic Ac. Year 2009/ / 1

4 Syntax of a first-order languagep Atoms variable symbols: x, y, z constant (0-ary functions) symbols: a, b, c, tom, 0, 1 function symbols: f /1, g/2 predicate symbols: p/0, q/2, cat/1, +/3 if t 1,..., t n are terms and p is an n-ary (n 0) predicate symbol, then p(t 1,..., t n ) is an atom Examples cat(g(x, y)) p q q( xp(x), xp(f (x))) q(p, p) f (q(a, a)) q(a, f (1)) +(a, f (z), c) q(0,, z) cat(a, 1) cat(cat(f (1))) D. Zanardini Computational Logic Ac. Year 2009/ / 1

5 Syntax of a first-order languagep variable symbols: x, y, z constant (0-ary functions) symbols: a, b, c, tom, 0, 1 function symbols: f /1, g/2 predicate symbols: p/0, q/2, cat/1, +/3 Formulæ an atom is a formula if F and G are formulæ, and x is a variable, then F, (F G), (F G), (F G), (F G), xf and xf are also formulæ Examples (p q(a, f (x))) p zf (1) a(q(a, 0) g(0, a)) p ( cat(a) ( xp ycat(y))) q(a, b) tom D. Zanardini Computational Logic Ac. Year 2009/ / 1

6 Syntax of a first-order languagep Literals A literal is an atom or the negation of an atom p, p, q(a, f (1)), q(a, f (1)), cat(g(x, y)), cat(g(x, y)) Precedence parentheses give an order between operators, but can make a formula quite unreadable we can use an order of precedence between operators in order to remove some parentheses without introducing ambiguity ((p q) (p r)) p q p r p ( q p) r ((p q) p) r (p q) (p r) p ( q p r) D. Zanardini Computational Logic Ac. Year 2009/ / 1

7 Syntax of a first-order languagep Literals A literal is an atom or the negation of an atom p, p, q(a, f (1)), q(a, f (1)), cat(g(x, y)), cat(g(x, y)) Precedence parentheses give an order between operators, but can make a formula quite unreadable we can use an order of precedence between operators in order to remove some parentheses without introducing ambiguity ((p q) (p r)) p q p r p ( q p) r ((p q) p) r (p q) (p r) p ( q p r) D. Zanardini Computational Logic Ac. Year 2009/ / 1

8 Syntax of a first-order languagep Literals A literal is an atom or the negation of an atom p, p, q(a, f (1)), q(a, f (1)), cat(g(x, y)), cat(g(x, y)) Precedence parentheses give an order between operators, but can make a formula quite unreadable we can use an order of precedence between operators in order to remove some parentheses without introducing ambiguity {,, } higher precedence than {, } {, } higher precedence than {, } the scope of in yp(y) q(y) is only p(y), not the whole formula D. Zanardini Computational Logic Ac. Year 2009/ / 1

9 Syntax of a first-order languagep Free and bounded variable occurrences an occurrence of the variable x is bounded in F if it is in the scope of a quantifier x or x x(p(1, x, y) q(x)) q(x) otherwise, it is said to be free x(p(1, x, y) q(x)) q(x) a formula if closed if it contains no free occurrences occurrences, not variable symbols, can be free or bounded D. Zanardini Computational Logic Ac. Year 2009/ / 1

10 Syntax of a first-order languagep Substitution F (x) denotes a formula where x occurs free somewhere F (x/t) denotes a formula where every free occurrence of x has been replaced by a term t provided x does not occur free in the scope of any y or y for y occurring in t F s(x) ( y(p(x) q(y))) F (x/f (y, y)) cannot be done xp(x) is the same as yp(y) (general result: xf (x) yf (x/y)) if you are not sure about the name of variables, it is never a mistake to rename all the bounded occurrences of a variable D. Zanardini Computational Logic Ac. Year 2009/ / 1

11 Syntax of a first-order languagep Alternative notation &,, xf x. F, x F xf x. F, x F p, q, r P, Q, R More than first-order second-order logic: it allows quantification over functions and predicates (e.g., mathematical induction) p(p(0) k(p(k) p(s(k))) np(n)) higher-order logic allows quantification over functions and predicates of any order (as in functional programming) D. Zanardini Computational Logic Ac. Year 2009/ / 1

12 Semantics of a first-order languagep Interpretations An interpretation I is a pair (D, I ), where D is a set (the domain of the universe) and I maps symbols to individuals or functions constants: I (a) = d D variables: I (x) = d D functions: I (f /n) = F : D n D I (f (t 1,..., t n)) = F(I (t 1),..., I (t n)) = F(d 1,..., d n) D predicates: I (p/n) = P : D n {t, f} I (p(t 1,..., t n)) = P(I (t 1),..., I (t n)) = P(d 1,..., d n) {t, f} an interpretation assigns an element of D to any term, and a truth value to any predicate applied to terms P is an n-ary relation R: P(d 1,..., d n ) = t iff d 1,..., d n R D. Zanardini Computational Logic Ac. Year 2009/ / 1

13 Semantics of a first-order languagep Evaluation of a formula Assigning a truth value to a formula, according to: the chosen interpretation of constants, functions and predicates the rules for evaluation (see also truth tables) I ( F ) = t iff I (F ) = f I (F G) = t iff I (F ) = I (G) = t I (F G) = f iff I (F ) = I (G) = f I (F G) = f iff I (F ) = t and I (G) = f I (F G) = t iff I (F ) = I (G) I ( xf (x)) = t iff I (F (x/c)) = t for every constant c I ( xf (x)) = t iff I (F (x/c)) = t for at least one constant c it is required that every element d of D is denoted by at least one constant I (instead of I) will often denote an interpretation when D is clear D. Zanardini Computational Logic Ac. Year 2009/ / 1

14 Semantics of a first-order languagep Example (propositional): (p q) (q r) r first interpretation: I (p) = f, I (q) = f, I (r) = f (f f) (f f) f t t f t f f D. Zanardini Computational Logic Ac. Year 2009/ / 1

15 Semantics of a first-order languagep Example (propositional): (p q) (q r) r second interpretation: I (p) = f, I (q) = f, I (r) = t (f f) (f t) t t t t t t t D. Zanardini Computational Logic Ac. Year 2009/ / 1

16 Semantics of a first-order languagep Example (propositional): (p q) (q r) r this example only needs truth tables, for all possible interpretations p q r p q q r (p q) (q r) (p q) (q r) r t t t t t t t t t f t f f t t f t f t f t t f f f t f t f t t t t t t f t f t f f t f f t t t t t f f f t t t f D. Zanardini Computational Logic Ac. Year 2009/ / 1

17 Semantics of a first-order languagep Example (first-order): x(m(a, x) p(x)) yq(s(y)) first interpretation: D = {0, 1, 2, 3,..} I (a) = 0 I (s(x)) = S(I (x)) = the successor of I (x) p(x) means that x is even q(x) means that x is odd m(x, y) means that x < y I evaluates the formula to t (try it!) D. Zanardini Computational Logic Ac. Year 2009/ / 1

18 Semantics of a first-order languagep Example (first-order): x(m(a, x) p(x)) yq(s(y)) second interpretation: D =,,, I (a) = x s(x) x p(x) t t t and this evaluates to f x q(x) t f t m y x t t t f f t t f f D. Zanardini Computational Logic Ac. Year 2009/ / 1

19 Semantics of a first-order languagep Satisfiable formulæ An interpretation I = (D, I ) satisfies a formula F on D iff I (F ) = t (also written I(F ) = t). In this case, I is a model of F F is satisfiable (written SAT (F )) iff it has at least one model F is unsatisfiable (written UNSAT (F )) iff it has no models that is, all interpretations are countermodels F is valid (written VAL(F )) iff every interpretation is a model this is denoted by = F, and amounts to say UNSAT ( F ) With a set of formulæ {F 1,.., F n }: (D, I ) satisfies {F 1,.., F n } iff I (F i ) = t on D for every i {F 1,.., F n } is satisfiable iff there is such an interpretation Example: x(m(a, x) p(x)) yq(s(y)) this formula is satisfiable but not valid D. Zanardini Computational Logic Ac. Year 2009/ / 1

20 Logical consequencep Logical consequence Given a set of formulæ Γ = {F 1,.., F n } and a formula G over the same language, G is a logical consequence of Γ (written Γ = G) iff every interpretation satisfying Γ also satisfies G, or, equivalently, there is no interpretation which satisfies Γ but not G Important (in some sense, it is a matter of convenience) {F 1,.., F n } = G iff = (F 1.. F n ) G To decide Γ = G can be very hard We have to take all models of Γ and verify that they all satisfy G, or find a counterexample D. Zanardini Computational Logic Ac. Year 2009/ / 1

21 Logical consequencep Example: {p (q r), p q} = r equivalent to = ((p (q r)) (p q)) r p q r p (q r) p q (p (q r)) (p q) G t t t t t t t t t f f t f t t f t t f f t t f f t f f t f t t t f f t f t f t f f t f f t t f f t f f f t f f t D. Zanardini Computational Logic Ac. Year 2009/ / 1

22 Logical consequencep Example: {p (q r), p q} = r equivalent to = ((p (q r)) (p q)) r p q r p (q r) p q (p (q r)) (p q) G t t t t t t f t t f f t f t t f t t f f t t f f t f f t f t t t f f t f t f t f f t f f t t f f t f f f t f f t D. Zanardini Computational Logic Ac. Year 2009/ / 1

23 Logical consequencep Example: { xp(x), xq(x)} = x(p(x) q(x)) D = {1, 2, 3, 4,..} p(x): x is even q(x): x is odd It is easy to see that this interpretation makes both premises true (indeed, there exist even numbers and there exist odd numbers), but does not satisfy the conclusion (no numbers are both even and odd) I ( xp(x)) = t I ( xq(x)) = t I ( x(p(x) q(x))) = f Therefore, this deduction is incorrect Example: { xp(x), xq(x)} = x(p(x) q(x)) this is, of course, a correct deduction (or valid, see a few slides above) D. Zanardini Computational Logic Ac. Year 2009/ / 1

24 Syntax vs. semanticsp Formal systems A proof formal system consists of: a formal language (alphabet and rules for building formulæ) a set of logical axioms (i.e., valid formulæ, which do not require proof) a set of inference rules for proving new formulæ a definition of proof Theories A theory T is a formal system extended with a set Γ of non-logical axioms (i.e., formulæ taken for granted) T [Γ] if Γ =, then T is the basic theory of the formal system D. Zanardini Computational Logic Ac. Year 2009/ / 1

25 Syntax vs. semanticsp Proofs A proof of a formula G in a theory T [Γ] (written T [Γ] G) is a finite sequence of formulæ such that each formula of the sequence is either a logical or non-logical axiom of the theory; or the result of applying an inference rule to previous formulæ in the sequence G is the last formula in the sequence Theorems (of a theory) a theorem of the theory T [Γ] is a formula for which there is at least one proof in T [Γ] D. Zanardini Computational Logic Ac. Year 2009/ / 1

26 Syntax vs. semanticsp Proof example: T [p (q r), p q] r ❶ p (q r) first premise ❷ p ( q r) interdefinition of, and on ❶ ❸ ( p q) r associativity on ❷ ❹ (p q) r De Morgan on ❸ ❺ p q r interdefinition of, and on ❹ ❻ p q second premise ❼ r modus ponens on ❺, ❻ Another approach to prove validity Instead of looking at all the possible models of a formula, we exploited our knowledge of logical rules we also say that ((p (q r)) (p q)) r is a tautology D. Zanardini Computational Logic Ac. Year 2009/ / 1

27 Syntax vs. semanticsp Theorem (Validity) Every theorem of T is logically valid: if T G then = G Theorem (Completeness) In a first-order theory T, all valid formulæ are theorems of T : if = G then T G the rest of the course will be basically about finding such formulæ Theorem (Deduction) T [F 1,.., F n ] G iff T (F 1.. F n ) G D. Zanardini Computational Logic Ac. Year 2009/ / 1

28 Syntax vs. semanticsp Theorem (Validity) Every theorem of T is logically valid: if T G then = G Theorem (Completeness) In a first-order theory T, all valid formulæ are theorems of T : if = G then T G the rest of the course will be basically about finding such formulæ Theorem (Deduction) T [F 1,.., F n ] G iff T (F 1.. F n ) G T [F 1,.., F n ] G iff VAL((F 1.. F n ) G) D. Zanardini Computational Logic Ac. Year 2009/ / 1

29 Syntax vs. semanticsp Theorem (Validity) Every theorem of T is logically valid: if T G then = G Theorem (Completeness) In a first-order theory T, all valid formulæ are theorems of T : if = G then T G the rest of the course will be basically about finding such formulæ Theorem (Deduction) T [F 1,.., F n ] G iff T (F 1.. F n ) G T [F 1,.., F n ] G iff VAL((F 1.. F n ) G) T [F 1,.., F n ] G iff UNSAT (F 1.. F n G) D. Zanardini Computational Logic Ac. Year 2009/ / 1

30 Syntax vs. semanticsp Completeness vs. Incompleteness in theories where Gödel s first incompleteness theorem holds, the completeness theorem (also Gödel s) holds as well incompleteness: an effectively generated (whose set of axioms is a recursively enumerable set) theory which is powerful enough to express elementary arithmetic cannot be both consistent (there is no statement so that both the statement and its negation are provable from the axioms) and complete (for any statement in the axioms language, either that statement or its negation is provable from the axioms) there is a statement which is true in the theory, but cannot be proven nor disproven (the Gödel sentence) but such statement is not a logical consequence of the theory (i.e., there are non-standard models, not isomorphic to the standard one, of the theory where it does not hold) D. Zanardini Computational Logic Ac. Year 2009/ / 1

31 What you ABSOLUTELY have to getp Pay attention to these sentences (do they make sense?) the interpretation I is satisfiable there exists a valid model for F a formula F makes an interpretation I false in order to prove that a formula is satisfiable, we need a true interpretation a literal is a negated or non-negated term we are not asking if these sentences are true or false, only if they make sense D. Zanardini Computational Logic Ac. Year 2009/ / 1

32 What you ABSOLUTELY have to getp Names and individuals Names are only there to denote things there is no problem in taking female or even odd(4) to be true if we are always consistent about it never forget that you are dealing with symbols! D. Zanardini Computational Logic Ac. Year 2009/ / 1

33 What you ABSOLUTELY have to getp Types in first-order logic! Equality a function of arity n will never have arity m n in the same formal system a predicate of arity n will never have arity m n in the same formal system a function is not a predicate a predicate is not a function the predicate = /2 is special in the sense that its meaning is often given for granted in a formal system (i.e., being equal means being the same element of the domain) anyway, we could choose to redefine it! D. Zanardini Computational Logic Ac. Year 2009/ / 1

34 What you ABSOLUTELY have to getp Interpretations and imagination When we want to prove that a formula is not valid, we need one interpretation which makes it false you can choose anything you want as D and I it s ok if we take that function s/1 to map 45 to! D. Zanardini Computational Logic Ac. Year 2009/ / 1

35 What you ABSOLUTELY have to getp Implication that the left-hand side of an implication is false is enough to say that the implication is true that the left-hand side of an implication is false is enough to say that the implication is true that the left-hand side of an implication is false is enough to say that the implication is true that the left-hand side of an implication is false is enough to say that the implication is true that the left-hand side of an implication is false is enough to say that the implication is true that the left-hand side of an implication is false is enough to say that the implication is true that the left-hand side of an implication is false is enough to say that the implication is true D. Zanardini Computational Logic Ac. Year 2009/ / 1

36 What you ABSOLUTELY have to getp Implication, cont. all red apples are good x((apple(x) red(x)) good(x)) there are apples which are red and good all apples are red and good there are no apples which are red but not good the set of good apples is a superset of the set of red apples whenever an apple is not good, it cannot be red whenever an apple is not good, it must be red there exists a yellow apple which is bad x(apple(x) yellow(x) bad(x)) whenever an apple is yellow, it is bad there is at least an object which is bad and yellow, and is an apple every time an apple is good, it is not yellow D. Zanardini Computational Logic Ac. Year 2009/ / 1

37 What you ABSOLUTELY have to getp Implication, cont. all red apples are good x((apple(x) red(x)) good(x)) there are apples which are red and good NO all apples are red and good NO there are no apples which are red but not good YES the set of good apples is a superset of the set of red apples YES whenever an apple is not good, it cannot be red YES whenever an apple is not good, it must be red NO there exists a yellow apple which is bad x(apple(x) yellow(x) bad(x)) whenever an apple is yellow, it is bad NO there is at least an object which is bad and yellow, and is an apple YES every time an apple is good, it is not yellow NO D. Zanardini Computational Logic Ac. Year 2009/ / 1

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