Mahaney's theorem

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Mahaney's theorem is a theorem in

NP-complete, then P = NP. Also, if any sparse language is NP-complete with respect to Turing reductions, then the polynomial-time hierarchy
collapses to .[1]

Mahaney's argument does not actually require the sparse language to be in NP, so there is a sparse

NP-hard set if and only if P = NP. This is because the existence of an NP-hard sparse set implies the existence of an NP-complete sparse set.[2]

References