Ex.1. Give a Search Problem whose deciding Problem is in co-NP.

Assuming 3SAT is in NP then asking wether a given Boolean formula has a Solution is a search problem in NP right?

Then would asking wether a given Boolean formula has no Solution be in co-NP?

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    $\begingroup$ What do you think? Use the definitions to try and prove that your answer is correct. $\endgroup$ – Yuval Filmus Jun 26 '18 at 15:40

Specifically speaking, 3-SAT is an example of NP-Complete problem. But your problem is only considered with NP problems.

We have the decision problem:

Is a SAT problem in NP?

Obviously, it is. Because you could always verify a "certificate" in time linear to your input size (values of your literals, that is a sequences of 0s and 1s).

For your question:

Is a un-SAT problem in co-NP? (complement to SAT problem)

We could find the following definition:

A decision problem X is a member of co-NP if and only if its complement X is in the complexity class NP [1].

Therefore, un-SAT is co-NP by definition.

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