17 Mar 2005
CS 3243 - Logical Inference
5
The DPLL algorithm
Determine
if an input propositional logic sentence (in CNF) is satisfiable.
Improvements
over truth table enumeration:
1.Early
termination
A
clause is true if any literal is true.
A
sentence is false if any clause is false.
2.Pure symbol
heuristic
Pure
symbol: always appears with the same "sign" in all clauses.
e.g.,
In the three clauses (A Ú ØB), (ØB Ú ØC), (C Ú A), A
and B are pure, C is impure.
Make
a pure symbol literal true.
3.Unit clause
heuristic
Unit
clause: only one literal in the clause
The
only literal in a unit clause must be true.
What are correspondences between DPLL and in general CSPs?
Least
constraining value
Most
constrained value