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October 28

Resolution Lower Bound Example

I am looking for an example of a 3-CNF formula, $\phi$, and a clause $C$ with at most 3 variables, that can be derived from $\phi$ using resolution, and any resolution derivation of $C$ from $\phi$ must derive also some 5-variable clause (in some derivation step).

For example, any derivation of $C=(b+c+d)$ from $\phi =(a+b+c)(\overline{a}+d+e)(b+c+\overline{e})$ must derive also some 4-variable clause (i.e, must derive either $(b+c+d+e)$ or $(\overline{a}+d+b+c)$). עברית (talk) 14:46, 28 October 2017 (UTC)

Try $C=(c+d+e)$ and $\phi =(b+h+i)(c+f+\overline{h})(d+e+\overline{f})(c+g+\overline{i})(d+e+\overline{g})(\overline{b}+c+j)(d+e+\overline{j}).$ Not my area of expertise and I only checked by hand so there could have been errors, but allowing only clauses with 4 or fewer variables seems to only lead to dead ends no matter which order you eliminate the variables. No real methodology here, just took an expression with a 5 term clause, where the expression resolves to a 3 term clause, then added more variables to get an expression with only 3 term clauses, then checked for alternate derivations with no 5 term clauses. Seems like with a bit more systematic approach you could generate expressions which require clauses of arbitrary length. --RDBury (talk) 06:55, 29 October 2017 (UTC)