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Consider the following two propositions.∀x∀y∀z((x 6 = y ∧ y 6 = z ∧ x 6 = z) → (¬P x ∨ ¬P y ∨ ¬P z))∃z(a = a → ∀yP y)(i) What is the maximum number of objects a model can have in its domain if both of the propositions are true on that model? Explain your answer.(ii) Give an example of a model that has the maximum number of objects in its domain and on which both of the propositions are true.

Question

Consider the following two propositions.∀x∀y∀z((x 6 = y ∧ y 6 = z ∧ x 6 = z) → (¬P x ∨ ¬P y ∨ ¬P z))∃z(a = a → ∀yP y)(i) What is the maximum number of objects a model can have in its domain if both of the propositions are true on that model? Explain your answer.(ii) Give an example of a model that has the maximum number of objects in its domain and on which both of the propositions are true.

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Solution

(i) The maximum number of objects a model can have in its domain if both of the propositions are true on that model is 2.

The first proposition states that for any three distinct objects, at least one of them does not satisfy P. This means that in any model where this proposition is true, there can be at most two objects that satisfy P.

The second proposition states that there exists an object (in this case, 'a') such that if it is identical to itself, then all objects satisfy P. Since 'a' is always identical to itself, this proposition is saying that all objects satisfy P.

Therefore, for both propositions to be true in the same model, there can be at most two objects in the domain of that model.

(ii) An example of a model that has the maximum number of objects in its domain and on which both of the propositions are true is a model with two objects, a and b, where P is true for a and false for b.

In this model, the first proposition is true because there are no three distinct objects, so the antecedent of the implication is always false, which makes the implication true. The second proposition is also true because 'a' is identical to itself and P is true for all objects in the domain (since it's only false for b, and b is not 'all objects').

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