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What do pigeonhole formulas demonstrate?

  • A.Resolution is unsound for certain classes of formula
  • B.Satisfiability is decidable only for small formulas
  • C.Some formulas require exponentially long resolution proofs
  • D.Some formulas cannot be written in conjunctive normal form

Answer: C

The statement that n plus one objects cannot fit into n boxes is easy for a person and provably hard for resolution, requiring exponentially many steps. Solvers based on resolution therefore fail on them at modest sizes, and stronger proof systems are needed. Normal form, soundness and decidability are unaffected. A family of formulas that separates human reasoning from a particular proof system is instructive about the limits of that system rather than about difficulty in general.

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