Question: Prove that N P ^ N P cap coNP = N P . Be sure you understand what this means. A language is in

Prove that N P^N P \cap coNP=N P. Be sure you understand what this means. A language is in N P^N P \cap c o N P if it can be written in the form {x | w: M^A(x, w)=1}, where M is a poly-time oracle TM, and A is a language in NP \cap coNP. In other words, A has both an "NP-style" algorithm and a "coNP-style" one. More formally, there are polytime TMs M_1 and M_2 such that A={a | w_1: M_1(a, w_1)=1}={a | w_2: M_2(a, w_2)=1}. Hint: compare the situation to N P^N P or N P^coNP, both of which are equal to \Sigma _2. Think about where the additional quantifier comes from in these cases, and why you might not need one in our problem.

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