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 Q
i
is either
9
or
8
, and
f
is in FO
[WHY ???]
Q
1
R
1
… Q
m
R
m
.
f