[Solved]: If NP is not a proper subset of coNP, why does NP not equal coNP?

Problem Detail: I am studying some lecture notes on the complexity of algorithms.
The notes give a proof that NP is not a proper subset of coNP.
However, they still assert that NP is a subset of coNP (which I agree with).
So, in this case, why does it not follow that NP is equal to coNP?

Asked By : CKKOY

Answered By : jmite

${2,3}$ is not a proper subset of ${3,4}$, yet the two clearly are not equal. Comparing sets is not like comparing numbers: two sets might not be comparable. Additionally, NP is not a subset of coNP, or at least, it is not known that this is the case. You are either misreading the textbook, or your textbook is wrong, since proving that $NP subseteq coNP$ would be a massive result. $Psubseteq coNP$, perhaps that is what you actually read?
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