57 research outputs found
On the Structure and the Number of Prime Implicants of 2-CNFs
Let be the maximum number of prime implicants that any -CNF on n
variables can have. We show that
Breaking the PPSZ Barrier for Unique 3-SAT
The PPSZ algorithm by Paturi, Pudl\'ak, Saks, and Zane (FOCS 1998) is the
fastest known algorithm for (Promise) Unique k-SAT. We give an improved
algorithm with exponentially faster bounds for Unique 3-SAT.
For uniquely satisfiable 3-CNF formulas, we do the following case
distinction: We call a clause critical if exactly one literal is satisfied by
the unique satisfying assignment. If a formula has many critical clauses, we
observe that PPSZ by itself is already faster. If there are only few clauses
allover, we use an algorithm by Wahlstr\"om (ESA 2005) that is faster than PPSZ
in this case. Otherwise we have a formula with few critical and many
non-critical clauses. Non-critical clauses have at least two literals
satisfied; we show how to exploit this to improve PPSZ.Comment: 13 pages; major revision with simplified algorithm but slightly worse
constant
A Randomized Algorithm for 3-SAT
In this work we propose and analyze a simple randomized algorithm to find a
satisfiable assignment for a Boolean formula in conjunctive normal form (CNF)
having at most 3 literals in every clause. Given a k-CNF formula phi on n
variables, and alpha in{0,1}^n that satisfies phi, a clause of phi is critical
if exactly one literal of that clause is satisfied under assignment alpha.
Paturi et. al. (Chicago Journal of Theoretical Computer Science 1999) proposed
a simple randomized algorithm (PPZ) for k-SAT for which success probability
increases with the number of critical clauses (with respect to a fixed
satisfiable solution of the input formula). Here, we first describe another
simple randomized algorithm DEL which performs better if the number of critical
clauses are less (with respect to a fixed satisfiable solution of the input
formula). Subsequently, we combine these two simple algorithms such that the
success probability of the combined algorithm is maximum of the success
probabilities of PPZ and DEL on every input instance. We show that when the
average number of clauses per variable that appear as unique true literal in
one or more critical clauses in phi is between 1 and 1.9317, combined algorithm
performs better than the PPZ algorithm
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