It is common to define $P$-completeness with respect to log-space many-one reductions.

I am looking for a complexity class $C \subseteq \mathsf{L}$ such that there are $\mathsf{P}$-complete problems w.r.t. many-one $C$-reductions.

What is the smallest known many-one reduction class $C$ such that HornSAT is complete for $\mathsf{P}$ under $C$-reductions?

The question was originally posted on CS with no answer.

  • $\begingroup$ Maybe you mean all non-trivial problems: the empty language and the language whose complement is empty cannot be complete. $\endgroup$ Jul 29, 2015 at 13:58
  • $\begingroup$ @SashoNikolov Sure, I am not interested in trivial languages! $\endgroup$ Jul 29, 2015 at 14:28
  • $\begingroup$ I don't understand the question. If C=P then all problems in P except the trivial ones are complete for P under C-reductions, and this is the case independent of what C is. $\endgroup$
    – Kaveh
    Jul 29, 2015 at 15:30
  • $\begingroup$ @Kaveh What is the smallest such class $C$? For instance, is HornSAT complete for $P$ under $NC^1$ Reductions? $\endgroup$ Jul 29, 2015 at 15:47
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    $\begingroup$ I share Kaveh’s confusion about the first paragraph. But concerning the blue question, a reasonable encoding of Horn-SAT is P-complete under DLOGTIME reductions. $\endgroup$ Jul 29, 2015 at 17:09

1 Answer 1


It is easy to show that the Circuit Value Problem is complete for $\mathsf{P}$ under $\mathsf{AC^0}$ reductions (see András's comment below).

For an easier example consider $$A = \{\langle M,x,y\rangle \mid \text{$M$ accepts $x$ in $|y|$ steps} \}$$

If a class of reductions $C$ contains constant functions, pairing of strings, and functions where the size of their output can bound any polynomial; then $A$ is complete for $\mathsf{P}$ w.r.t. $C$.

  • 2
    $\begingroup$ For P-completeness of CVP under FO-reductions, see Exercise 3.28 in Immerman's Descriptive Complexity. $\endgroup$ Jul 30, 2015 at 15:05

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