- https://math.stackexchange.com/questions/361422/why-isnt-np-conp "Why isn't NP = coNP?"
- https://stackoverflow.com/questions/17046440/whats-the-difference-between-np-and-co-np
- https://cs.stackexchange.com/questions/9795/is-the-open-question-np-co-np-the-same-as-p-np
- https://mathoverflow.net/questions/31821/problems-known-to-be-in-both-np-and-conp-but-not-known-to-be-in-p