1 Section 7.3 Formal Proofs in Predicate Calculus All proof rules for propositional calculus extend to predicate calculus. Example. … k.  x p(x) P k+1.

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1 Section 7.3 Formal Proofs in Predicate Calculus All proof rules for propositional calculus extend to predicate calculus. Example. … k.  x p(x) P k+1.  x p(x)   x p(x)P k+2.  x p(x) 1, 2, MP … But we need additional proof rules to reason with most quantified wffs. For example, suppose we want to prove that the following wff is valid.  x  y p(x, y)   y  x p(x, y). We might start with Proof: 1.  x  y p(x, y) P But what do we do for the next line of the proof? We’re stuck if we want to use proof rules. We need more proof rules. Free to Replace For a wff W(x) and a term t we say t is free to replace x in W(x) if W(t) has the same bound occurrences of variables as W(x). Example. Let W(x) =  y p(x, y). Then W(y) =  y p(y, y), so y is not free to replace x in W(x). W(ƒ(x)) =  y p(ƒ(x), y), so ƒ(x) is free to replace x in W(x). W(c) =  y p(c, y), so c is free to replace x in W(x). W(x) =  y p(x, y), so x is free to replace x in W(x).

2 Universal Instantiation (UI) if t is free to replace x in W(x). Existential Generalization (EG) Existential Instantiation (EI) Universal Generalization (UG) If  x W(x) occurs on some line of a proof, then W(c) may be placed on any subsequent line of the proof subject to the following restrictions: Choose c to be a new constant in the proof and such that c does not occur in the statement to be proven. If W(x) occurs on some line of a proof, then  x W(x) may be placed on any subsequent line of the proof subject to the following restrictions: Among the wffs used to obtain W(x), x is not free in any premise and x is not free in any wff obtained by EI. if t is free to replace x in W(x). Special cases that satisfy the restriction:

3 Restrictions on quantifier inference rules are necessary Example.  x  y p(x, y)   y  x p(x, y) is invalid. Here is an attempted proof. 1.  x  y p(x, y) P 2.  y p(x, y)1, UI 3.p(x, c)2, EI 4.  x p(x, c)3, UG (NO, x on line 3 is free in wff obtained by EI) 5.  y  x p(x, y)4, EG NOT QED1–5, CP Example.  x p(x)   x p(x) is invalid. Here is an attempted proof. 1.  x p(x) P 2.p(x) 1, EI (NO, x is not a new constant in the proof) 3.  x p(x) 2, UG (NO, x on line 2 is free in wff obtained by EI) NOT QED1–3, CP Example.  x p(x)   x q(x)   x (p(x)  q(x)) is invalid. Here is an attempted proof. 1.  x p(x) P 2.  x q(x) P 3.p(c)1, EI 4.q(c)2, EI (NO, c is not a new constant in the proof) 5.p(c)  q(c)3, 4, Conj 6.  x (p(x)  q(x))5, EG NOT QED1–6, CP

4 Example. p(x)   x p(x) is invalid. Here is an attempted proof. 1.p(x)P 2.  x p(x)1, UG (NO, x is free in a premise) NOT QED1, 2, CP Example.  x  y p(x, y)   y p(y, y) is invalid. Here is an attempted proof. 1.  x  y p(x, y) P 2.  y p(y, y) 1, UI (NO, y is not free to replace x in  y p(x, y)) NOT QED1, 2, CP Example.  x p(x, ƒ(x))   x p(x, x) is invalid. Here is an attempted proof. 1.  x p(x, ƒ(x)) P 2. p(x, ƒ(x)) 1, UI 3.  x p(x, x)2, EG (NO, p(x, ƒ(x)) ≠ p(x, x)(x/t) for any term t) NOT QED1–3, CP Example.  x p(x, ƒ(x))   y  x p(x, y) is invalid. Here is an attempted proof. 1.  x p(x, ƒ(x)) P 2.  y  x p(x, y) 1, EG (NO, ƒ(x) is not free to replace y in  x p(x, y)) NOT QED1, 2, CP Example.  x p(x)  p(c) is invalid. Here is an attempted proof. 1.  x p(x) P 2. p(c) 1, EI (NO, c occurs in statement to be proved) NOT QED1, 2, CP

5 Now Some Valid Wffs Example.  x  y p(x, y)   y p(y, y) is valid. Here is an attempted proof. 1.  x  y p(x, y)P 2.  y p(y, y)1, UI (NO, y is not free to replace x in  y p(x, y)) NOT QED1, 2, CP But here is a correct proof. 1.  x  y p(x, y)P 2.  y p(x, y) 1, UI 3.p(x, x)2, UI 4.  x p(x, x) 3, UG 5.p(y, y) 4, UI 6.  y p(y, y) 5, UG QED1–6, CP. Quiz. Find a proof of the statement that uses IP. Example/Quiz.  x (A(x)  B(x))  (  x A(x)   x B(x)) is valid. Find a proof. 1.  x (A(x)  B(x)) P 2.  x A(x) P [for  x A(x)   x B(x)] 3. A(x) 2, UI 4. A(x)  B(x)1, UI 5. B(x)3, 4, MP 6.  x B(x) 5, UG 7.  x A(x)   x B(x) 2–6, CP QED1, 7, CP.

6 Example/Quiz. Prove that the following wff is valid using IP.  x ¬ p(x, x)   x  y  z (p(x, y)  p(y, z)  p(x, z))   x  y ¬ (p(x, y)  p(y, x)). 1.  x ¬ p(x, x) P 2.  x  y  z (p(x, y)  p(y, z)  p(x, z)) P 3.  x  y (p(x, y)  p(y, x)) P [for  x  y ¬ (p(x, y)  p(y, x)], T 4.p(a, b)  p(b, a) 3, EI, EI 5.p(a, b)  p(b, a)  p(a, a)2, UI, UI, UI 6.p(a, a) 4, 5, MP 7.¬ p(a, a) 1, UI 8.False6, 7, Contr 9.  x  y ¬ (p(x, y)  p(y, x) 3–8, IP QED1, 2, 9, CP. Example/Quiz. Use IP to prove that the  x  y (p(x)  p(y)) is valid. 1.  x  y (p(x)  ¬ p(y)) P [for IP] 2.  y (p(c)  ¬ p(y)) 1, EI 3.p(c)  ¬ p(c) 2, UI 4. p(c)3, Simp 5.¬ p(c) 4, Simp 6. False4, 5, Contr QED1–6, IP. Quiz. Find a CP proof of the statement. 1.  x ¬ p(x, x) P 2.  x  y  z (p(x, y)  p(y, z)  p(x, z)) P 3.¬ p(x, x)1, UI 4.p(x, y)  p(y, x)  p(x, x) 2, UI, UI, UI 5.¬ (p(x, y)  p(y, x)) 3, 4, MT 6.  x  y ¬ (p(x, y)  p(y, x))5, UG, UG QED1– 6, CP.

7 Group Quiz. Divide the class into six subgroups and assign each group one of the following six wffs to prove using CP (no IP and no T’s). Assume that x does not occur free in C. 1.  x (A(x)  C)  (  x A(x)  C). 2.(  x A(x)  C)   x (A(x)  C). 3.(C   x A(x))   x (C  A(x)). 4.(C   x A(x))   x (C  A(x)). 5.  x (C  A(x))  (C   x A(x)). 6.  x (A(x)  C)  (  x A(x)  C). Solutions. 1.  x (A(x)  C)  (  x A(x)  C). 1.  x (A(x)  C)P 2.  x A(x) P[for  x A(x)  C ] 3.A(d)2, EI 4.A(d)  C1, UI 5.C3, 4, MP 6.  x A(x)  C2–5, CP QED1, 6, CP. 2. (  x A(x)  C)   x (A(x)  C). 1.  x A(x)  C P 2.A(x) P [for A(x)  C] 3.  x A(x) 2, EG 4.C1, 3, MP 5.A(x)  C 2–4, CP 6.  x (A(x)  C)5, UG QED1, 5–6, CP.

8 3. (C   x A(x))   x (C  A(x)). 1.C   x A(x)P 2.C P [for C  A(x)] 3.  x A(x) 1, 2, MP 4.A(x)3, UI 5.C  A(x) 2–4, CP 6.  x (C  A(x)) 5, UG QED1, 5–6, CP. 4. (C   x A(x))   x (C  A(x)). 1.C   x A(x) P 2.C P [for C  A(?)] 3.  x A(x) 1, 2, MP 4.A(d) 3, EI 5.C  A(d) 2–4, CP 6.  x (C  A(x))5, EG QED1, 5–6, CP. 5.  x (C  A(x))  (C   x A(x)). 1.  x (C  A(x)) P 2.C P [for C   x A(x)] 3.C  A(d) 1, EI 4.A(d)2, 3, MP 5.  x A(x) 4, EG 6.C   x A(x) 2–5, CP QED1, 6, CP. 6.  x (A(x)  C)  (  x A(x)  C). 1.  x (A(x)  C) P 2.  x A(x) P [for  x A(x)  C ] 3.A(d)  C 1, EI 4.A(d) 2, UI 5.C 3, 4, MP 6.  x A(x)  C 2–5, CP QED1, 6, CP.