NL complement closure
NL=coNL: complements of nondeterministic logspace decision languages also lie in NL.
Key knowledge
NL=coNL: complements of nondeterministic logspace decision languages also lie in NL. The reachability proof certifies counts of reachable vertices, making nonreachability testable with nondeterministic logarithmic workspace.
Conditions and scope
- Complement is relative to all finite strings in the specified input alphabet.
- Flipping each accepting/rejecting branch of an NTM is insufficient: acceptance has an existential branch quantifier. This theorem does not imply NP=coNP.
Sources · 2
References
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Massachusetts Institute of Technology2 sources
Evidence and review
Lecture 20 slides 9–12, Immerman–Szelepcsényi theorem and counted-reachability construction, with completion on Lecture 21 slides 2–6.
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