# Need An Online Exam Taken Between The Hours Of 1400 And 1700 May 5Th.

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INTRODUCTION TO COMPUTATION THEORY AND LOGIC exam needs to be taken, online exam between the hours of 1400 and 1700, previous year exam example provided
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Show all work. Question 1. 1a (8 marks). Construct a CNF (not DNF) with the following truth-table. X Y Z 0 0 0 0 0 0 1 1 0 1 0 1 0 1 1 0 X Y Z 1 0 0 1 1 0 1 0 1 1 0 1 1 1 1 0 1b (12 marks). Construct a resolution proof that the following CNF is inconsistent UWX, UW X , U WX , U W X, UY, U Y , V WY, V WY , V W Y, V W Y , V X, V X Question 2 Consider a first order theoryK which has a multiplication operator ×, usually omitted so xy means x× y, and has various proper axioms to ensure an equality theory (with = representing equality). It also has an axiom (associativity) ∀x1∀x2∀x3(x1(x2x3)) = ((x1x2)x3) Let A′, A,B′, B be the formulae below A′(x1) : ∀x2(x1x2 = x2), A(x1) : ∃x1A ′(x1), B ′(x1) : ∀x2(x2x1 = x2), B(x1) : ∃x1B ′(x1) and let K+A, K+A+B be the theories formed by adding A (respectively A,B) as an axiom (axioms). 2a (8 marks). Show that the interpretation I with domain {a, b}, =I the identity relation on {a, b}, and ×I a b a a b b a b is a model of K + A. (Show this informally without bringing in the idea of ‘snapshots.’) 2b (4 marks). Determine whether or not it is a model of K + A+ B. 2c (8 marks). Give a careful proof within K as a first-order theory (with equality: you may use equality reasoning informally) that A′(x1) ∧ A ′(x3) ∧ B ′(x4) =⇒ x1 = x3. (Hint: the proof should be guided by the fact that x1x4 = x1 = x4.) Question 3. 3a (6 marks). Prove that, in any first-order theory K, under a certain condition A(t) =⇒ ∃xiA(xi) is a theorem, stating the required condition. 3b (4 marks). Give an example showing that (3a) is not true (in every interpretation) if that condition is not met. 3c (5 marks). (Note: a countable set D is one which can be listed as a countable sequence d1, d2, . . ., so for every nonempty subset X of D there is a smallest i such that di ∈ X.) Suppose that K is a first-order theory, P (x1, x2) an atomic formula, and M is a model of K, with countable domain D, such that M |= ∃x2P (x1, x2) Show that for every a ∈ D there exists a b ∈ D such that PM(a, b) is true. 3d (5 marks) (Continuing 3c). Show that one can define a function F : D → D such that for every a ∈ D, M |= P (a, F (a)). (F can be used, for example, in connection with Skolem functions.) Question 4 Referring to the proof in the notes that it is consistent to assume that infinite integers exist, the idea was to extend PA with a new constant symbol a plus infinitely many formulae which together imply that a is infinite. 4a (6 marks) Construct an interpretation of this extension, with domain N∪{∞}, extending the definitions of s(),+,×, so that every axiom of PA except possibly Axiom 4 (induction), is true in this interpretation. Also, you may ignore the equality axioms. 4a (14 marks) Show that the 7 axioms mentioned in (4a) are true in the interpretation. You may skip the induction axiom.