The following notion of a distillation algorithm comes from "On Problems Without Polynomial Kernels".

Let a language $L$ be given. A distillation algorithm for $L$ takes a given list of input strings $\{ x_i \}_{i \in [t]}$ and computes an output string $y$ such that:

(1) $y \in L$ if and only if there exists $i \in [t]$ such that $x_i \in L$

(2) $\vert y \vert \leq p(Max_{i\in[t]} \vert x_i \vert)$ for some polynomial $p$

(3) The algorithm computes $y$ in at most $q(\sum_{i\in[t]}\vert x_i \vert)$ time for some polynomial $q$

It has been shown that if there exists a distillation algorithm for an $NP$-complete problem, then $coNP \subseteq NP/poly$. Moreover, $PH = \Sigma_3$.

See details and discussion in:

  • "Infeasibility of Instance Compression and Succinct PCPs for NP"
  • "On Problems Without Polynomial Kernels"
  • "Lower bounds on kernelization"


  • Could there exist a distillation algorithm for a $PSPACE$-complete problem?
  • If such an algorithm existed, what complexity consequences would we get?
  • $\begingroup$ Any further references are welcomed. Thank you! :) $\endgroup$ Nov 19, 2016 at 1:00
  • 1
    $\begingroup$ By this paper and polynomial-time many-one reductions, "if there exists a distillation algorithm for an $NP$-complete problem, then" ​ NP $\subseteq$ coAM ​ and "there are non-uniform, statistical zero-knowledge proofs for all languages in NP." ​ ​ ​ ​ $\endgroup$
    – user6973
    Nov 19, 2016 at 3:13
  • $\begingroup$ @RickyDemer This is great!! Thank you for sharing. :) $\endgroup$ Nov 19, 2016 at 3:16
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    $\begingroup$ I now notice that the paper I linked to in fact only needs compression, which makes their results more general. ​ In particular, by Theorems 7.1 and 7.3, if there exists even a compression "algorithm for a $PSPACE$-complete problem" then PSPACE has non-uniform statistical zero-knowledge proofs. ​ ​ ​ ​ $\endgroup$
    – user6973
    Nov 19, 2016 at 6:14
  • 3
    $\begingroup$ I don't understand the last part of the question. AFAICS the existence of a distillation algorithm for a PSPACE-complete problem doesn't imply the existence of a dist algo for an NP-complete problem, or am I missing something? $\endgroup$ Nov 19, 2016 at 10:19

1 Answer 1


Theorem 15.3 of the recent "Parameterized Algorithms" textbook by Cygan et al. states the following:

"Let $L, R ⊆ \Sigma^*$ be two languages. If there exists an OR-distillation of L into R, then $L\in coNP / poly$"

So, I think that if there exists an OR-distillation from a PSPACE-complete language $L$ to itself, then $PSPACE \subseteq coNP/poly$, i.e. not only does the polynomial-hierarchy collapse, but also PSPACE collapses with it.

  • 2
    $\begingroup$ Theorem 7.1 of the paper I linked to in the comments is strictly qualitatively better. ​ Does Theorem 15.3 of that book handle larger-for-some-parameters error bounds than item 2 of 7.1? ​ ​ ​ ​ $\endgroup$
    – user6973
    Nov 19, 2016 at 17:33
  • $\begingroup$ You stated "also PSPACE collapses with it". In particular, I believe we get $\Sigma_3 = PSPACE$. I don't see a way to improve this, but I thought I would ask anyways. Can we get a better collapse that this? Say $\Sigma_2 = PSPACE$? $\endgroup$ Nov 29, 2016 at 8:07

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