Assignment 12
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1. Let L = {f,g,R,S, c} where f is a unary function symbol,g is a binary function symbol, R is a binary relation symbol, S is a ternary relation symbol, and c is a constant symbol. Show that each of the following L-formulas are axioms of our proof system.
(a) (Rx1 x2 → (Scx1 x2 → Rx1 x2 ))
(b) (Ax2 ∃x3 fx2 ≈ x3 → ∃x3 fgcx1 ≈ x3 )
(c) Ax7Ax3 (Ax1 (∃x7 Rcx7 → Sx1 x7 c) → (Ax1 ∃x7 Rcx7 → Ax1 Sx1 x7 c))
(d) Ax3Ax1Ax4 (x2 ≈ x5 → (gx2 x1 ≈ x5 → gx5 x1 ≈ x5 ))
2. Let x, y, and z denote first order variables.
(a) Provide the justifications for each line of the following derivation, which shows that {x ≈ y} ⊢ y ≈ x.
(1) x ≈ y
(2) x ≈ x
(3) (x ≈ y → (x ≈ x → y ≈ x))
(4) (x ≈ x → y ≈ x)
(5) y ≈ x
(b) Show that ⊢ (x ≈ y → y ≈ x), without appealing to the Com- pleteness Theorem.
(c) Show that {x ≈ y, y ≈ z} ⊢ x ≈ z, without appealing to the Completeness Theorem.
3. Complete the “Proof of (8)” on page 10 of the Week 12 Slides. That is, show that if x and y are variables, and if Ψ is an atomic L-formula and Ψ(ˆ) is obtained from Ψ by replacing some occurrences of x in Ψ by y, that (x ≈ y → (Ψ → Ψ(ˆ))) is logically valid.
2023-07-31