Let $n$ be the number of variables in the input formula and $m$ the number of clauses. Define $s_k = \inf\{\delta : k\text{-SAT can be solved in } 2^{\delta n} \text{ time}\}$. The strong exponential time hypothesis says that $\lim \limits_{k \to \infty} s_k=1$. I have two questions here:

  1. What if the input size is very large, say, $m=\Theta(3^n)$? Simply reading the input formula needs $\omega(2^n)$ time. In this case, $\lim \limits_{k \to \infty} s_k$ would be larger than $1$.
  2. A paper in SAT Conference 2021 says, "The strong exponential time hypothesis conjectures that the SAT problem cannot be solved in time $O^*(c^n)$ for some constant $c<2$." I haven't found the equivalent statement in Impagliazzo and Paturi's original paper. Why is this statement equivalent to $\lim \limits_{k \to \infty} s_k = 1$?

1 Answer 1

  1. That can't happen. The maximum number of distinct possible clauses is $(2n)^k = O(n^k)$, which is polynomial in $n$ (since here $k$ is fixed).

  2. Here I imagine that there notation $O^*(c^n)$ probably refers to something like $O((nL)^{O(1)} c^n)$. If you could solve SAT in $O(1.5^n)$ time, then you could solve $k$-SAT in $O(1.5^n)$ time for all $k$, thus $s_k \le \log_2 1.5 < 1$ for all $k$, we cannot have $s_k \to 1$.

  • $\begingroup$ Thanks a lot. Let $A$ denote the statement that $s_k \to 1$, and $B$ denote the equivalent statement. However, your answer to question 2 only shows that $\neg B \implies \neg A$. Could you please explain why $\neg A \implies \neg B$, i.e., if $\lim \limits_{k\to\infty} s_k < 1$, we can solve SAT in $O^*(c^n)$ time where $c<2$? $\endgroup$
    – Soha
    Jan 25 at 2:50
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    $\begingroup$ @Soha, My answer proves that $s_k \to 1$ implies SAT can't be solved in $O^*(c^n)$ for $c<2$ (by showing that if SAT can be solved in $O^*(c^n)$ for $c<2$, then $s_k \not\to 1$, the contrapositive). I don't know whether it is possible to prove the other direction. $\endgroup$
    – D.W.
    Jan 25 at 7:00
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    $\begingroup$ I think the following may be a correct way to prove the other direction. For an SAT instance, each clause may contain no more than $n$ literals, which means it cannot be harder than an $n$-SAT instance. So if $s_k \to s_{\infty} \ne 1$, we would be able to solve the instance with an $n$-SAT solver in $O^*(2^{s_{\infty}n})$ time. $\endgroup$
    – Soha
    Jan 31 at 11:54

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