Sequent calculus

Question

Hi, 

I have doubt regarding sequent calculus. How can i find the counter example?  I have written down the solution with counter examples, could you please help to solve this problem?

check the validity of !!((c->a)->b)

Thanks

Answer 1

You have replaced (c->a)->b by !(!c&a)&a, but it should be rather !(!c|a)|a since an implication phi->psi can be defined as !phi | psi.

Comments

got it..after that how i can find the counterexample?

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Well, have a look at the slides. Every leaf node where the two sets of formulas are disjoint gives you an assignment of the variables.