I need a list of $\Sigma_2^p$ complete languages. There are two such problems listed in the Complexity Zoo, namely:

  • Minimum equivalent DNF. Given a DNF formula F and integer k, is there a DNF formula equivalent to F with k or fewer occurences of literals?
  • Shortest implicant. Given a formula F and integer k, is there a conjunction of k or fewer literals that implies F?

Another basic $\Sigma_2^p$ complete problem:

  • $\Sigma_i \text{SAT}$. Given a quantified boolean formula $\varphi$ of the form $\varphi = \exists \vec{u} \forall \vec{v}\, \phi(\vec{u}, \vec{v})$, is $\varphi$ valid?

However, I am hopefully looking for a problem which makes use of graphs (e.g. a clique related problem).

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    $\begingroup$ Take a look at this compendium: ovid.cs.depaul.edu/documents/phcom.pdf $\endgroup$ Jun 30, 2014 at 15:51
  • $\begingroup$ This looks extremely useful. Thanks a lot! $\endgroup$
    – gdiazc
    Jun 30, 2014 at 15:53
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    $\begingroup$ @HuckBennett: good survey! Why don't you turn it into an answer? $\endgroup$ Jun 30, 2014 at 16:02

3 Answers 3


Marcus Schaefer and Chris Umans have a nice Garey-and-Johnson-esque survey of complete problems in the polynomial hierarchy.


A rather recent result not included in the Schaefer and Umans paper is 2-CLIQUE COLOURING OF PERFECT GRAPHS.

See David Defossez, Complexity of clique-coloring odd-hole-free graphs. J. Graph Theory 62, 2 (October 2009), 139-156, and some recent improvements (2013): Hélio B. Macêdo Filho, Raphael C. S. Machado, Celina M. H. de Figueiredo, Hierarchical complexity of 2-clique-colouring weakly chordal graphs and perfect graphs having cliques of size at least 3 (the problem is still $\Sigma_2^p$-complete for weakly chordal graphs, and for perfect graphs having cliques of size at least 3).


Deciding the existence of an "evolutionarily stable strategy" in a normal-form game. See http://www.cs.duke.edu/~conitzer/ess.pdf .

The setup is a 2-player symmetric game. An evolutionarily stable strategy is a (randomized) strategy that is (a) a symmetric nash equilibrium, and (b) there are no good "symmetric deviations": in this equilibrium, if one player can deviate to some strategy and maintain equal utility, then the other player would do strictly worse to then also deviate to this strategy.


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