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Give an example of a model that has exactly two objects in its domain and on which all of the following four propositions are true.∃y∃x(x ≠ y ∧ Syxy) → ∀z(P z → Qz) ∀x∀y∀z(Sxyz ↔ (x = z ∧ y = a))∀x(Qx → x = b) a ≠ b ∧ (Qb ↔ ∃xSxbx)

Question

Give an example of a model that has exactly two objects in its domain and on which all of the following four propositions are true.∃y∃x(x ≠ y ∧ Syxy) → ∀z(P z → Qz) ∀x∀y∀z(Sxyz ↔ (x = z ∧ y = a))∀x(Qx → x = b) a ≠ b ∧ (Qb ↔ ∃xSxbx)

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Solution

The model that satisfies all the given propositions can be constructed as follows:

  1. Let's define the domain D = {a, b} where a ≠ b. This satisfies the condition that the model has exactly two objects in its domain.

  2. The predicate Sxyz is defined as Sxyz ↔ (x = z ∧ y = a). This means that Sxyz is true if and only if x is the same object as z and y is the same object as a.

  3. The predicate Qx is defined as Qx → x = b. This means that if Qx is true, then x must be the same object as b.

  4. The proposition ∃y∃x(x ≠ y ∧ Syxy) → ∀z(P z → Qz) is true because there are no objects z in the domain such that Pz is true and Qz is false. This is because the only object for which Qx can be true is b, and there are no objects in the domain for which Px is true and x ≠ b.

  5. The proposition a ≠ b ∧ (Qb ↔ ∃xSxbx) is true because a ≠ b and Qb is true if and only if there exists an object x in the domain such that Sxbx is true. This is satisfied because Sbbb is true according to the definition of Sxyz.

Therefore, the model with domain D = {a, b} and predicates Sxyz and Qx as defined above satisfies all the given propositions.

This problem has been solved

Similar Questions

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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2. Use a tree to test whether the following two propositions are equivalent. (Present the tree, and say whether they’re equivalent or not.) If they’re not equivalent, read off from your tree a model on which the propositions have different truth values (and indicate the path from which you are reading it off). ∀x(Ax → Gx) ∃x(Ax → Gx)

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