For example, NP, co-NP, PLS, PPA all have known natural complete problems, while RP, ZPP, BPP and TFNP have no known complete problems (although such a problem may be discovered in the future).
舉例來說，NP，反NP，PLS，PPA 都有已知的完備問題，而RP，ZPP，BPP和TFNP則沒有已知的完備問題(雖然這不代表未來不會發現完備問題)。