10
Fagin’s Theorem
•Recall Second Order Logic, SO:
–May have formulas of the form 9 R.f or 8 R. f, where R is a relation symbol
•Every SO formula can be written in prenex form like this:
where each Qi is either 9 or 8, and f is in FO
[WHY ???]
where each Qi is either 9 or 8, and f is in FO
[WHY ???]
Q1 R1 … Qm Rm. f