MATH0037 2019
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1. (a) Let ϕ = ((a → b) → (c A d)), ψ 1 = (a V d) and ψ2 = (-e). Draw the parsing trees for ϕ [ψ1 /a, ψ2 /b, ψ2 /d] and ψ1 [ϕ/a].
(b) Consider the set of propositions in the language with logical connectives → and
1. Find, for each of the following
● (-ϕ),
● (ϕ V ψ),
● (ϕ A ψ),
a formula equivalent to it which uses only the logical symbols → and 1.
(c) Let PROP be the set of propositions in the language with logical connectives V and -. Define what it means for a function ν : PROP → {0, 1} to be a valuation.
(d) Show that there is no proposition ϕ using only the logical symbol → and the propositional variable a such that (ϕ → (-a)) is a tautology.
2. (a) State and prove Konig’s lemma. State the compactness theorem for proposi- tional logic.
(b) Suppose that {Xn | n ∈ N} is a family of finite subsets of N. Suppose that for
each finite subset I of N there is a subset DI ζ N such that |DI n Xn | = 1 for all n ∈ I . Show that there exists a subset D ∈ N such that |D n Xn | = 1 for all n ∈ N. Hint: Consider the set of sequences σ : N → N with the property that σ(n) ∈ Xn .
3. (a) Define what it means for a set of propositions Γ to be inconsistent. Show that
if Γ is consistent and Γ 上 (ϕ → ψ) then Γ u {ϕ, (-ψ)} is not consistent.
(b) Let PROP be the set of formulas in the language with logical connectives → and
1. Define what it means for Γ to be a maximal consistent set of propositions, and show that the function ν : PROP → {0, 1} defined by「ϕⅡ] = 1 iff ϕ ∈ Γ is a valuation when Γ is maximal consistent.
(c) Show that
{((-ϕ) → ψ), (ϕ → χ), (ψ → χ)} 上 χ
4. Recall that two L–structures A and B are elementarily equivalent, written A = B, if for all sentences ϕ , A 乍 ϕ if and only if B 乍 ϕ .
(a) Find a sentence ϕ in the language of groups such that S3 乍 ϕ but S4 ϕ . (b) State the compactness theorem for first order logic.
(c) A graph G is connected if, for any pair of distinct vertices a and b, there is a sequence x1 , . . . , xn of vertices such that a = x1 , b = xn , and xi is connected by an edge to xi+1 for all i = 1, . . . , n 一 1. Let E be the graph with vertex set 勿 and an edge between a and b if and only if |a 一 b| = 1. Show that there is a disconnected graph F such that F = E .
5. (a) Let X ∈ N. Define what it means for X to be recursively enumerable, and what it means for X to be recursive.
(b) Show that the following sets are recursive:
(i) {n2 }
(ii) {n | gcd(n, 2019) 1}, where gcd(n, m) is the greatest common divisor of
m and n.
(c) Prove that there is a subset of N which is not recursively enumerable.
2023-04-12