GATE2009 q26 Revision as of 20:02, 14 July 2014 by Arjun Suresh (talk | contribs) (diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

Consider the following well-formed formulae:
I. $¬∀x(P(x))$   II. $¬∃x(P(x))$   III. $¬∃x(¬P(x))$   IV. $∃x(¬P(x))$
Which of the above are equivalent?

(A) I and III

(B) I and IV

(C) II and III

(D) II and IV

Solution by Happy Mittal

A formula $∀x(P(x))$ is equivalent to formula $¬∃x(¬P(x))$ i.e. add $¬$ inside and outside, and convert $∀$ to ∃$.
So, $¬∀x(P(x))$ is equivalent to $∃x(¬P(x))$.

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