What is known about the languages:

$$S_f = \{ [M] \ | \ \exists{x} \ \text{s.t.} \\ M \ \text{accepts} \ x \ \text{in} \ f(|[M]|) \ \text{steps}, \\ \ |x| \leq f(|[M]|) \}$$

I used to think that in order to check whether a string $[M]$ is in $S_f$ you cannot avoid simulating $M$ on all possible $x$ whose length is less than $f(|[M]|)$.
But for a polynomial $f$, verifying a candidate solution $x$ takes polynomial time so that $S_f \in NP$, while trying all of them is exponential, which would prove $P \neq NP$ ...

So my reasoning is spectacularly wrong, but I can't find out why:

  1. Maybe $S_f$ is in fact not in $NP$ when $f$ is polynomial?
  2. Maybe it is possible to check whether a string $[M]$ is in $S_f$ without trying most of the possible $x$?
  3. Something else?
  • $\begingroup$ Can you identify a more specific question? "What is known about...?" sounds overly broad. Also we expect that you ask one question per post, not two or three. I'm not sure what "appartenance" means -- that might have been a translation failure. $\endgroup$
    – D.W.
    Sep 12, 2022 at 20:58
  • $\begingroup$ Thanks, indeed it was a huge translation failure. There is a single question: "Why is my reasoning wrong" $\endgroup$
    – agemO
    Sep 13, 2022 at 5:21

1 Answer 1


Your reasoning is wrong because you assert something without proof. You just assert that the only way to do it is to try all $x$, but there is no justification given for this assertion, and we don't know whether that assertion is true. Just because you can only think of one way to solve a problem, doesn't mean it is the only way. There could be another way that you haven't thought of yet. As you indicate, given the relationship to the P vs NP question, your assertion amounts to assuming something that we don't have a proof of. It'd be like saying "Well, P is different from NP, because the only way to solve a NP-complete problem is to try all possible solutions". We have no proof of such a claim (and indeed, such a claim is unlikely to be true).

You might also be interested in the strong exponential time hypothesis. That too is unproven, but it represents a more precise formalization of a conjecture that might well be true, and is closely connected to the topic you are asking about.

  • $\begingroup$ So 1. is true but we don't know about 2. ? $\endgroup$
    – agemO
    Sep 13, 2022 at 5:47

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