In computational complexity theory, the Immerman–Szelepcsényi theorem states that nondeterministic space complexity classes are closed under complementation. It was proven independently by Neil Immerman and Róbert Szelepcsényi in 1987, for which they shared the 1995 Gödel Prize. In its general form the theorem states that NSPACE("s"("n")) = co-NSPACE("s"("n")) for any function "s"("n") ≥ log "n". The result is equivalently stated as NL = co-NL; although this is the special case when "s"("n") = log "n", it implies the general theorem by a standard padding argument. The result solved the second LBA problem.