In this note, we are going to state the PCP theorem and its relation to the hardness of approximating some NP-hard problem.
PCP Theorem: the Interactive Proof View
Intuitively, a PCP (Probabilistically Checkable Proof) system is an interactive proof system where the verifier is given random bits and he is allowed to look into the proof in many locations. If the string is indeed in the language, then there exists a proof so that the verifier always accepts. However, if is not in the language, no prover can convince this verier with probability more than . The proof has to be short i.e., of size at most . This class of language is designated as PCP[r(n), q(n)].
Theorem A (PCP theorem). Every NP language has a highly efficient PCP verifier. In particular,