MATH3801 Exam 2017
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1. (a) Define the language of propositional logic, and define PROP, the set of propo- sitions. Show that -p1 is not an element of PROP.
(b) Let σ be the signature with propositional variables p1 , p2 , . . . and logical sym- bols -, → , and l. Define what it means for a function ν : PROP(σ) → {0, 1} to be a valuation.
(c) Let ϕ = (q1 → ((-q2 ) → q1 )), ψ 1 = ((p1 A p2 ) →l), and ψ2 = (p5 → p7 ). Draw the parsing trees for ϕ , ψ 1 , ψ2 , and ϕ[ψ1 /q1 , ψ2 /q2].
(d) Let σ be the signature with propositional variables p1 , p2 , . . . and logical sym- bols -, → , and l, and let σ \ be the signature with propositional variables q1 , . . . , qn , p1 , p2 , . . . and logical symbols -, → , and l. Give an inductive defi- nition of ϕ[ψi /qi]. State and prove the substitution theorem for σ and σ \ .
2. (a) Define what it means for a proposition to be satisfiable, for a set of propositions to be satisfiable, and for a proposition to be a tautology. State the compactness
theorem for propositional logic.
(b) Show the following:
(i) If {ϕ, ψ} 上 χ then ϕ 上 (ψ → χ).
(ii) If ϕ 上 (ψ1 v ψ2 ), ψ 1 上 χ, and ψ2 上 χ, then ϕ 上 χ .
(c) Let B = {s: N → {0, 1}} be the set of infinite binary strings. A sequence sn converges to s, written limn→o sn = s, if for all i there exists Ni such that for all n > Ni , sn (i) = s(i). A function f : B → {0, 1} is continuous if limn→o f (sn ) = f (limn→o sn ). Show that if f is continuous then there exists K such that: if s(i) = s\ (i) for i ≤ K then f (s) = f (s\ ).
3. (a) (i) Define what it means for a set Γ of propositions to be consistent.
(ii) Show that if Γ is consistent and Γ 上 (ϕ v ψ) then at least one of Γ u {ϕ} or Γ u {ψ} is consistent.
(iii) Show that if Γ is consistent then at least one of Γ u {ϕ} or Γ u {-ϕ} is consistent.
(b) Give natural deduction proofs of the following:
(i) ((ϕ → (ψ → χ)) → ((ϕ → ψ) → (ϕ → χ)))
(ii) ((-((-ϕ) v ψ)) → (-ψ))
(iii) ((ϕ → ψ) → ((-ϕ) v ψ))
4. (a) Define what it means for two L structures U and s to be elementary equivalent. Show that two finite groups are elementary equivalent if and only if they are isomorphic.
(b) Is there a set of sentences Γ in the language of groups such that if G is a group then G 上 Γ if and only if G is finite? Justify your answer.
(c) Find a sentence ϕ in the language of groups such that ϕ e Th(B2 ) and ϕ e/ Th(B3 ).
5. (a) Define the class of register machines.
(b) Define what it means for a function f : N0(k) → N0 to be computable.
(c) Find register machines which compute each of the following functions:
(i) f : N0(2) → N0 , f(n, m) = n + m.
(ii) f : N0(2) → N0 , f(n, m) = n - m, n > m, f(n, m) = 0 otherwise. (iii) f : N0(2) → N0 , f(n, m) = n - 2m, n > 2m, f(n, m) = 0 otherwise.
2023-04-12