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CS sometimes seems take for granted that $\mathcal O(\text{poly}(n))$ is "easy", while $\mathcal O\left(2^{poly(n)}\right)$ is "difficult". I am interested in research into "difficult" polynomial-time algorithms, where the best-case solution to the constructed problem runs in $\Theta(n^c)$, where $c$ can be chosen to be large; but the solution could be tested in $O(n)$ time.

Question:

Given an integer $c$, can we construct problems that would:

  • Take $\Theta\left(n^c\right)$ best-case-time to solve,
  • While taking $\tilde{\mathcal O}(n)$ time, and $\tilde{\mathcal O}(n)$ space, to test a solution?

($\tilde{\mathcal O}(n)$ is soft-big-oh, meaning $O(n \log^k n)$ for some $k$)

Something I note - I might be mistaken somewhere here - is that presumably, if there is a $\mathcal O(n)$ algorithm to test the solution, then perhaps there can be a $\mathcal O(n)$ reduction to $\rm k\text{-}SAT$. If so, and, if $\rm P=NP$, and there was a polynomial-time algorithm, say ${\rm S{\small OLVE}}\left(\Phi(\mathbf x)\right) \in O({|\mathbf x|}^{\alpha})$ time, then I think this might contradict our $\Theta(n^c)$ problem, if $\alpha < c$.

The motivation would be to research the possibility of having a "one-way-function", that is linear(ithmic)-time computable, and best-case "difficult"-polynomial-time invert-able, where "difficult" means a high degree polynomial, instead of the usual exponential-time definition of "difficult"; perhaps this might be able to be used for cryptography even if $\rm P=NP$ (like "post-P-equals-NP-cryptography", similar to how there is a field of "post-quantum-cryptography").

Realz Slaw
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2 Answers2

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If you believe in the exponential time hypothesis, then you can construct such an example by padding SAT. The ETH states that solving SAT on $n$ variables takes time $2^{\Omega(n)}$; let's say the time is $T(n)$. We can assume that SAT instances consist of at most $O(n^3)$ clauses, and so have length at most $\tilde{O}(n^3)$. Pad such an instance by adding $N = T(n)^{1/c}$ (where $c > 1$ need not be an integer) spaces. According to the ETH, the resulting languages requires time $\Omega(n^c)$ to solve in the worst case (the "best case" time complexity of a problem is almost always $\tilde{O}(n)$, depending on your model of computation and how devious the problem is), but witnesses can be verified in time $\tilde{O}(n)$ and $O(\log n)$ space; most of these resources are spent on checking that the input is well-formed.

The same idea would work even with much weaker hypothesis, such as P$\neq$NP; I'll leave you the details.

Yuval Filmus
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if am understanding your question right, there are probably many examples of this based on "fixed parameters" of NP complete problems. eg finding a $k$-clique in a graph takes $O(n^{\sqrt k})$ time and can be verified in $O(n)$ time (a $k$ edge clique has $\sqrt k$ vertices).

vzn
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