Is that correct? I'm not sure. NP problems are verifiable in deterministic polynomial time. That includes the hardest of NP problems (NP-complete problems, e.g. travelling salesman and graph coloring). Are there problems that are "harder" than that? I never learned about anything outside NP—I know there are other complexity classes out there (like probabilistic classes), but I don't know how they compare to NP. My guess is that anything outside NP is essentially a non-deterministic problem altogether, meaning the "answer" isn't formally a simple "yes" or "no."
