First-Order Theories

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1 First-Order Theories Ruzica Piskac Max Planck Institute for Software Systems, Germany Seminar on Decision Procedures 2012 Ruzica Piskac First-Order Theories 1 / 39

2 Acknowledgments Theories These slides were originally developed by Jochen Hoenicke. We kindly thank him for his permission to use them. Ruzica Piskac First-Order Theories 2 / 39

3 Theories In first-order logic function symbols have no predefined meaning: The formula = 3 is satisfiable. We want to fix the meaning for some function symbols. Examples: Equality theory Theory of natural numbers Theory of rational numbers Theory of arrays or lists Ruzica Piskac First-Order Theories 3 / 39

4 First-Order Theories Theories Definition (First-order theory) A First-order theory T consists of A Signature Σ - set of constant, function, and predicate symbols A set of axioms A T - set of closed (no free variables) Σ-formulae A Σ-formula is a formula constructed of constants, functions, and predicate symbols from Σ, and variables, logical connectives, and quantifiers The symbols of Σ are just symbols without prior meaning The axioms of T provide their meaning Ruzica Piskac First-Order Theories 4 / 39

5 Theory of Equality Theory of Equality T E Signature Σ = : {=,a,b,c,,f,g,h,,p,q,r, } =, a binary predicate, interpreted by axioms. all constant, function, and predicate symbols. Axioms of T E : 1 x. x = x (reflexivity) 2 x,y. x = y y = x (symmetry) 3 x,y,z. x = y y = z x = z (transitivity) 4 for each positive integer n and n-ary function symbol f, x 1,...,x n,y 1,...,y n. i x i = y i f(x 1,...,x n ) = f(y 1,...,y n ) (congruence) 5 for each positive integer n and n-ary predicate symbol p, x 1,...,x n,y 1,...,y n. i x i = y i (p(x 1,...,x n ) p(y 1,...,y n )) (equivalence) Ruzica Piskac First-Order Theories 5 / 39

6 Axiom Schemata Theories Theory of Equality Congruence and Equivalence are axiom schemata. 4 for each positive integer n and n-ary function symbol f, x 1,...,x n,y 1,...,y n. i x i = y i f(x 1,...,x n ) = f(y 1,...,y n ) (congruence) 5 for each positive integer n and n-ary predicate symbol p, x 1,...,x n,y 1,...,y n. i x i = y i (p(x 1,...,x n ) p(y 1,...,y n )) (equivalence) For every function symbol there is an instance of the congruence axiom schemata. Example: Congruence axiom for binary function f 2 : x 1,x 2,y 1,y 2. x 1 = y 1 x 2 = y 2 f 2 (x 1,x 2 ) = f 2 (y 1,y 2 ) A TE contains an infinite number of these axioms. Ruzica Piskac First-Order Theories 6 / 39

7 T-Validity and T-Satisfiability T-Validity and T-Satisfiability Definition (T-interpretation) An interpretation I is a T-interpretation, if it satisfies all the axioms of T. Definition (T-valid) A Σ-formula F is valid in theory T (T-valid, also T = F), if every T-interpretation satisfies F. Definition (T-satisfiable) A Σ-formula F is satisfiable in T (T-satisfiable), if there is a T-interpretation that satisfies F Definition (T-equivalent) Two Σ-formulae F 1 and F 2 are equivalent in T (T-equivalent), if F 1 F 2 is T-valid, Ruzica Piskac First-Order Theories 7 / 39

8 Example: T E -validity Theories T-Validity and T-Satisfiability Semantic argument method can be used for T E Prove F : a = b b = c g(f(a),b) = g(f(c),a) T E -valid. Suppose not; then there exists a T E -interpretation I such that I = F. Then, 1. I = F assumption 2. I = a = b b = c 1, 3. I = g(f(a),b) = g(f(c),a) 1, 4. I = x,y,z. x = y y = z x = z transitivity 5. I = a = b b = c a = c 4, 3 {x a,y b,z c} 6a I = a = b b = c 5, 7a I = 2 and 6a contradictory 6b. I = a = c 4, 5, (5, ) 7b. I = a = c f(a) = f(c) (congruence), 2 8ba. I = a = c I = 8bb. I = f(a) = f(c) 7b, 9bb. I = a = b 2, 10bb. I = a = b b = a (symmetry), 2 11bba. I = a = b I = 11bbb. I = b = a 10bb, 12bbb. I = f(a) = f(c) b = a g(f(a),b) = g(f(c),a) (congruence), I = g(f(a),b) = g(f(c),a) 8bb, 11bbb, 12bbb 3 and 13 are contradictory. Thus, F is T E -valid. Ruzica Piskac First-Order Theories 8 / 39

9 T-Validity and T-Satisfiability Decidability of T E Is it possible to decide T E -validity? T E -validity is undecidable. If we restrict ourself to quantifier-free formulae we get decidability: For a quantifier-free formula T E -validity is decidable. Ruzica Piskac First-Order Theories 9 / 39

10 Fragments of Theories Theories T-Validity and T-Satisfiability A fragment of theory T is a syntactically-restricted subset of formulae of the theory. Example: quantifier-free fragment of theory T is the set of quantifier-free formulae in T. A theory T is decidable if T = F (T-validity) is decidable for every Σ-formula F, i.e., there is an algorithm that always terminate with yes, if F is T-valid, and no, if F is T-invalid. A fragment of T is decidable if T = F is decidable for every Σ-formula F in the fragment. Ruzica Piskac First-Order Theories 10 / 39

11 Natural Numbers and Integers Natural Numbers and Integers Natural numbers N = {0,1,2, } Integers Z = {, 2, 1,0,1,2, } Three variations: Peano arithmetic T PA : natural numbers with addition and multiplication Presburger arithmetic T N : natural numbers with addition Theory of integers T Z : integers with +,,> Ruzica Piskac First-Order Theories 11 / 39

12 Natural Numbers and Integers Peano Arithmetic T PA (first-order arithmetic) Signature: Σ PA : {0, 1, +,, =} Axioms of T PA : axioms of T E, 1 x. (x + 1 = 0) (zero) 2 x,y. x + 1 = y + 1 x = y (successor) 3 F[0] ( x. F[x] F[x + 1]) x. F[x] (induction) 4 x. x + 0 = x (plus zero) 5 x,y. x + (y + 1) = (x + y) + 1 (plus successor) 6 x. x 0 = 0 (times zero) 7 x,y. x (y + 1) = x y + x (times successor) Line 3 is an axiom schema. Ruzica Piskac First-Order Theories 12 / 39

13 Expressiveness of Peano Arithmetic Natural Numbers and Integers 3x + 5 = 2y can be written using Σ PA as x + x + x = y + y We can define > and : 3x + 5 > 2y write as z. z 0 3x + 5 = 2y + z 3x + 5 2y write as z. 3x + 5 = 2y + z Examples for valid formulae: Pythagorean Theorem is T PA -valid x,y,z. x 0 y 0 z 0 xx + yy = zz Fermat s Last Theorem is T PA -valid (Andrew Wiles, 1994) n.n > 2 x,y,z.x 0 y 0 z 0 x n + y n = z n Ruzica Piskac First-Order Theories 13 / 39

14 Natural Numbers and Integers Expressiveness of Peano Arithmetic (2) In Fermat s theorem we used x n, which is not a valid term in Σ PA. However, there is the Σ PA -formula EXP[x,n,r] with 1 EXP[x,0,r] r = 1 2 EXP[x,i + 1,r] r 1. EXP[x,i,r 1 ] r = r 1 x EXP[x,n,r] : d,m. ( z. d = (m + 1)z + 1) ( i,r 1. i < n r 1 < m ( z. d = ((i + 1)m + 1)z + r 1 ) r 1 x < m ( z. d = ((i + 2)m + 1)z + r 1 x)) r < m ( z. d = ((n + 1)m + 1)z + r) Fermat s theorem can be stated as: n.n > 2 x,y,z,rx,ry.x 0 y 0 z 0 EXP[x,n,rx] EXP[y,n,ry] EXP[z,n,rx + ry] Ruzica Piskac First-Order Theories 14 / 39

15 Decidability of Peano Arithmetic Natural Numbers and Integers Gödel showed that for every recursive function f : N n N there is a Σ PA -formula F[x 1,...,x n,r] with F[x 1,...,x n,r] r = f(x 1,...,x n ) T PA is undecidable. (Gödel, Turing, Post, Church) The quantifier-free fragment of T PA is undecidable. (Matiyasevich, 1970) Remark: Gödel s first incompleteness theorem Peano arithmetic T PA does not capture true arithmetic: There exist closed Σ PA -formulae representing valid propositions of number theory that are not T PA -valid. The reason: T PA actually admits nonstandard interpretations For decidability: no multiplication Ruzica Piskac First-Order Theories 15 / 39

16 Natural Numbers and Integers Presburger Arithmetic T N Signature: Σ N : {0, 1, +, =} no multiplication! Axioms of T N : axioms of T E, 1 x. (x + 1 = 0) (zero) 2 x,y. x + 1 = y + 1 x = y (successor) 3 F[0] ( x. F[x] F[x + 1]) x. F[x] (induction) 4 x. x + 0 = x (plus zero) 5 x,y. x + (y + 1) = (x + y) + 1 (plus successor) 3 is an axiom schema. T N -satisfiability and T N -validity are decidable. (Presburger 1929) Ruzica Piskac First-Order Theories 16 / 39

17 Natural Numbers and Integers Theory of Integers T Z Signature: Σ Z : {..., 2, 1,0, 1, 2,..., 3, 2, 2, 3,..., +,, =, >} where..., 2, 1,0, 1, 2,... are constants..., 3, 2,2, 3,... are unary functions (intended meaning: 2 x is x + x) +,,=,> have the usual meanings. Relation between T Z and T N T Z and T N have the same expressiveness: For every Σ Z -formula there is an equisatisfiable Σ N -formula. For every Σ N -formula there is an equisatisfiable Σ Z -formula. Σ Z -formula F and Σ N -formula G are equisatisfiable iff: F is T Z -satisfiable iff G is T N -satisfiable Ruzica Piskac First-Order Theories 17 / 39

18 Example: Σ Z -formula to Σ N -formula Consider the Σ Z -formula F 0 : w,x. y,z. x + 2y z 7 > 3w + 4 Natural Numbers and Integers Introduce two variables, v p and v n (range over the nonnegative integers) for each variable v (range over the integers) of F 0 F 1 : w p,w n,x p,x n. y p,y n,z p,z n. (x p x n ) + 2(y p y n ) (z p z n ) 7 > 3(w p w n ) + 4 Eliminate by moving to the other side of > F 2 : w p,w n,x p,x n. y p,y n,z p,z n. x p + 2y p + z n + 3w p > x n + 2y n + z p w n + 4 Eliminate > and numbers: F 3 : w p,w n,x p,x n. y p,y n,z p,z n. u. (u = 0) x p + y p + y p + z n + w p + w p + w p = x n + y n + y n + z p + w n + w n + w n + u which is a Σ N -formula equisatisfiable to F 0. Ruzica Piskac First-Order Theories 18 / 39

19 Natural Numbers and Integers Example: Σ N -formula to Σ Z -formula. Example: The Σ N -formula x. y. x = y + 1 is equisatisfiable to the Σ Z -formula: x. x > 1 y. y > 1 x = y + 1. To decide T Z -validity for a Σ Z -formula F: transform F to an equisatisfiable Σ N -formula G, decide T N -validity of G. Ruzica Piskac First-Order Theories 19 / 39

20 Rationals and Reals Theories Σ = {0, 1, +,,, =, } Rationals and Reals Theory of Reals T R (with multiplication) x x = 2 x = ± 2 Theory of Rationals T Q (no multiplication) Note: Strict inequality 2x }{{} x+x x,y. z. x + y > z can be expressed as = 7 x = 2 7 x,y. z. (x + y = z) x + y z Ruzica Piskac First-Order Theories 20 / 39

21 Rationals and Reals Theory of Reals T R Signature: Σ R : {0, 1, +,,, =, } with multiplication. Axioms of T R : axioms of T E, 1 x,y,z. (x + y) + z = x + (y + z) (+ associativity) 2 x,y. x + y = y + x (+ commutativity) 3 x. x + 0 = x (+ identity) 4 x. x + ( x) = 0 (+ inverse) 5 x,y,z. (x y) z = x (y z) ( associativity) 6 x,y. x y = y x ( commutativity) 7 x. x 1 = x ( identity) 8 x. x 0 y. x y = 1 ( inverse) 9 x,y,z. x (y + z) = x y + x z (distributivity) (separate identies) 11 x,y. x y y x x = y (antisymmetry) 12 x,y,z. x y y z x z (transitivity) 13 x,y. x y y x (totality) 14 x,y,z. x y x + z y + z (+ ordered) 15 x,y. x 0 y 0 x y 0 ( ordered) 16 x. y. x = y y x = y y (square root) 17 for each odd integer n, x 0,...,x n 1. y. y n + x n 1 y n 1 + x 1 y + x 0 = 0 (at least one root) Ruzica Piskac First-Order Theories 21 / 39

22 Example Theories Rationals and Reals F: a,b,c. b 2 4ac 0 x. ax 2 + bx + c = 0 is T R -valid. As usual: x 2 abbreviates x x, we omit, e.g. in 4ac, 4 abbreviate and a b abbreviates a + ( b). 1. I = F assumption 2. I = y. bb 4ac = y 2 bb 4ac = y 2 square root, 3. I = d 2 = bb 4ac d 2 = (bb 4ac) 2, 4. I = d 0 0 d total 5. I = d 2 0 4, case distinction, ordered 6. I = 2a e = 1 inverse,, 7a. I = bb 4ac 0 1, 8a. I = x.axx + bx + c = 0 1, 9a. I = a(( b + d)e) 2 + b( b + d)e + c = 0 8a, 10a. I = ab 2 e 2 2abde 2 + ad 2 e 2 b 2 e + bde + c = 0 distributivity 11a. I = dd = bb 4ac 3, 5, 7a 12a. I = ab 2 e 2 bde + a(b 2 4ac)e 2 b 2 e + bde + c = 0 6, 11a, congruence 13a. I = 0 = 0 3, distributivity, inverse 14a. I = 13a, reflexivity Ruzica Piskac First-Order Theories 22 / 39

23 Example Theories Rationals and Reals F: a,b,c. bb 4ac 0 x. axx + bx + c = 0 is T R -valid. As usual: x 2 abbreviates x x, we omit, e.g., in 4ac, 4 abbreviate and a b abbreviates a + ( b). 1. I = F assumption 2. I = y. bb 4ac = y 2 bb 4ac = y 2 square root, 3. I = d 2 = bb 4ac d 2 = (bb 4ac) 2, 4. I = d 0 0 d total 5. I = d 2 0 4, case distinction, ordered 6. I = 2a e = 1 inverse,, 7b. I = bb 4ac 0 1, 8b. I = x.axx + bx + c = 0 1, 9b. I = aff + bf + c = 0 8b, 10b. I = (2af + b) 2 = bb 4ac field axioms, T E 11b. I = (2af + b) 2 0 analogous to 5 12b. I = bb 4ac 0 10b, 11b, equivalence 13b. I = 12b, 7b Ruzica Piskac First-Order Theories 23 / 39

24 Rationals and Reals Decidability of T R T R is decidable (Tarski, 1930) High time complexity: O(2 2kn ) Ruzica Piskac First-Order Theories 24 / 39

25 Rationals and Reals Theory of Rationals T Q Signature: Σ Q : {0, 1, +,, =, } no multiplication! Axioms of T Q : axioms of T E, 1 x,y,z. (x + y) + z = x + (y + z) (+ associativity) 2 x,y. x + y = y + x (+ commutativity) 3 x. x + 0 = x (+ identity) 4 x. x + ( x) = 0 (+ inverse) (one) 6 x,y. x y y x x = y (antisymmetry) 7 x,y,z. x y y z x z (transitivity) 8 x,y. x y y x (totality) 9 x,y,z. x y x + z y + z (+ ordered) 10 For every positive integer n: x. y. x = y + + y } {{ } n (divisible) Ruzica Piskac First-Order Theories 25 / 39

26 Rationals and Reals Expressiveness and Decidability of T Q Rational coefficients are simple to express in T Q Example: Rewrite as the Σ Q -formula 1 2 x y 4 x + x + x + y + y + y + y } {{ } 24 T Q is decidable Efficient algorithm for quantifier free fragment Ruzica Piskac First-Order Theories 26 / 39

27 Recursive Data Structures (RDS) Recursive Data Structures Data Structures are tuples of variables. Like struct in C, record in Pascal. In Recursive Data Structures, one of the tuple elements can be the data structure again. Linked lists or trees. Ruzica Piskac First-Order Theories 27 / 39

28 Recursive Data Structures RDS theory of LISP-like lists, T cons Σ cons : {cons, car, cdr, atom, =} where cons(a,b) list constructed by adding a in front of list b car(x) left projector of x: car(cons(a,b)) = a cdr(x) right projector of x: cdr(cons(a, b)) = b atom(x) true iff x is a single-element list Axioms: The axioms of A TE plus x,y. car(cons(x,y)) = x x,y. cdr(cons(x,y)) = y x. atom(x) cons(car(x), cdr(x)) = x x,y. atom(cons(x,y)) (left projection) (right projection) (construction) (atom) Ruzica Piskac First-Order Theories 28 / 39

29 Recursive Data Structures Axioms of Theory of Lists T cons 1 The axioms of reflexivity, symmetry, and transitivity of = 2 Congruence axioms x 1,x 2,y 1,y 2. x 1 = x 2 y 1 = y 2 cons(x 1,y 1 ) = cons(x 2,y 2 ) x,y. x = y car(x) = car(y) x,y. x = y cdr(x) = cdr(y) 3 Equivalence axiom x,y. x = y (atom(x) atom(y)) 4 x,y. car(cons(x,y)) = x (left projection) 5 x,y. cdr(cons(x,y)) = y (right projection) 6 x. atom(x) cons(car(x), cdr(x)) = x (construction) 7 x,y. atom(cons(x,y)) (atom) Ruzica Piskac First-Order Theories 29 / 39

30 Recursive Data Structures Decidability of T cons T cons is undecidable Quantifier-free fragment of T cons is efficiently decidable Ruzica Piskac First-Order Theories 30 / 39

31 Example: T cons -Validity Theories Recursive Data Structures We argue that the following Σ cons -formula F is T cons -valid: F : car(a) = car(b) cdr(a) = cdr(b) atom(a) atom(b) a = b 1. I = F assumption 2. I = car(a) = car(b) 1,, 3. I = cdr(a) = cdr(b) 1,, 4. I = atom(a) 1,, 5. I = atom(b) 1,, 6. I = a = b 1, 7. I = cons(car(a), cdr(a)) = cons(car(b), cdr(b)) 2, 3, (congruence) 8. I = cons(car(a), cdr(a)) = a 4, (construction) 9. I = cons(car(b), cdr(b)) = b 5, (construction) 10. I = a = b 7, 8, 9, (transitivity) Lines 6 and 10 are contradictory. Therefore, F is T cons -valid. Ruzica Piskac First-Order Theories 31 / 39

32 Arrays Theory of Arrays T A Signature: Σ A : { [ ],, =}, where a[i] binary function read array a at index i ( read(a,i) ) a i v ternary function write value v to index i of array a ( write(a,i,e) ) Axioms 1 the axioms of (reflexivity), (symmetry), and (transitivity) of T E 2 a,i,j. i = j a[i] = a[j] (array congruence) 3 a,v,i,j. i = j a i v [j] = v (read-over-write 1) 4 a,v,i,j. i j a i v [j] = a[j] (read-over-write 2) Ruzica Piskac First-Order Theories 32 / 39

33 Arrays Equality in T A Note: = is only defined for array elements not T A -valid, but is T A -valid. Also a[i] = e a i e = a a[i] = e j. a i e [j] = a[j], a = b a[i] = b[i] is not T A -valid: We only axiomatized a restricted congruence. T A is undecidable Quantifier-free fragment of T A is decidable Ruzica Piskac First-Order Theories 33 / 39

34 Theory of Arrays T = A Theories Arrays (with extensionality) Signature and axioms of T = A are the same as T A, with one additional axiom a,b. ( i. a[i] = b[i]) a = b (extensionality) Example: is T = A -valid. F : a[i] = e a i e = a T A = is undecidable Quantifier-free fragment of T A = is decidable Ruzica Piskac First-Order Theories 34 / 39

35 Combination of Theories How do we show that Theories Combination of Theories 1 x x 2 f(x) f(1) f(x) f(2) is (T E T Z )-unsatisfiable? Or how do we prove properties about an array of integers, or a list of reals...? Given theories T 1 and T 2 such that The combined theory T 1 T 2 has signature Σ 1 Σ 2 axioms A 1 A 2 Σ 1 Σ 2 = {=} Ruzica Piskac First-Order Theories 35 / 39

36 Nelson & Oppen Theories Combination of Theories qff = quantifier-free fragment Nelson & Oppen showed that if satisfiability of qff of T 1 is decidable, satisfiability of qff of T 2 is decidable, and certain technical requirements are met then satisfiability of qff of T 1 T 2 is decidable. Ruzica Piskac First-Order Theories 36 / 39

37 Combination of Theories Lists with equality T = cons T = cons : T E T cons Signature: Σ E Σ cons (this includes uninterpreted constants, functions, and predicates) Axioms: union of the axioms of T E and T cons T = cons is undecidable Quantifier-free fragment of T = cons is efficiently decidable Ruzica Piskac First-Order Theories 37 / 39

38 Example: T = cons-validity Theories Combination of Theories We argue that the following Σ = cons-formula F is T = cons-valid: F : car(a) = car(b) cdr(a) = cdr(b) atom(a) atom(b) f(a) = f(b) 1. I = F assumption 2. I = car(a) = car(b) 1,, 3. I = cdr(a) = cdr(b) 1,, 4. I = atom(a) 1,, 5. I = atom(b) 1,, 6. I = f(a) = f(b) 1, 7. I = cons(car(a), cdr(a)) = cons(car(b), cdr(b)) 2, 3, (congruence) 8. I = cons(car(a), cdr(a)) = a 4, (construction) 9. I = cons(car(b), cdr(b)) = b 5, (construction) 10. I = a = b 7, 8, 9, (transitivity) 11. I = f(a) = f(b) 10, (congruence) Lines 6 and 11 are contradictory. Therefore, F is T = cons-valid. Ruzica Piskac First-Order Theories 38 / 39

39 First-Order Theories Theories Decidability Theory Decidable QFF Dec. T E Equality T PA Peano Arithmetic T N Presburger Arithmetic T Z Linear Integer Arithmetic T R Real Arithmetic T Q Linear Rationals T cons Lists T cons = Lists with Equality T A Arrays T A = Arrays with Extensionality Ruzica Piskac First-Order Theories 39 / 39

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