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In this illustration, P and NPC are two disjoint set.

We know that NPC is non-empty. If P $\cap$ NPC $=\varnothing$, then there are elements in NP which are not in P. Doesn't this imply that P $\neq$ NP?

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    $\begingroup$ You're right that the illustration you link to assumes that P ≠ NP. Note that on the P versus NP Wikipedia page, it has the caption "Diagram of complexity classes provided that P ≠ NP." $\endgroup$
    – Henry Cohn
    Commented Apr 21, 2014 at 12:10

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If there is even a single NPC problem in P, then P = NP. NPC problems are defined to be the hardest problems in NP in a suitable sense: every other NP problem reduces to them in polynomial time.

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