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Computability and Logic

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PROBLEMS 125<br />

10.14 Show that the following pairs are equivalent:<br />

(a) ∀xF(x)&∀yG(y) <strong>and</strong> ∀u(F(u)&G(u)).<br />

(b) ∀xF(x) ∨∀yG(y) <strong>and</strong> ∀u∀v(F(u) ∨ G(v)).<br />

(c) ∃xF(x)&∃yG(y) <strong>and</strong> ∃u∃v(F(u)&G(v)).<br />

(d) ∃xF(x) ∨∃yG(y) <strong>and</strong> ∃u(F(u) ∨ G(u)).<br />

[In (a), it is to be understood that u may be a variable not occurring free in<br />

∀xF(x) or∀yG(y); in particular, if x <strong>and</strong> y are the same variable, u may be<br />

that same variable. In (b) it is to be understood that u <strong>and</strong> v may be any distinct<br />

variables not occurring free in ∀xF(x) ∨∀yG(y); in particular, if x does not<br />

occur in free in ∀yG(y) <strong>and</strong> y does not occur free in ∀xF(x), then u may be<br />

x <strong>and</strong> y may be v. Analogously for (d) <strong>and</strong> (c).]

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