Transcript Lecture 2 - Satisfiablity

MCS 312: NP Completeness
and Approximation algorthms
Instructor
Neelima Gupta
[email protected]
Table of Contents
Circuit Satisfiablity is NP Complete
(CNF) SAT is NP Complete
3(CNF) SAT is NP Complete
C SAT is NP Complete
The Cook’s Theorem
Thanks to Stuti Chawla (28) MCS '11
The CIRCUIT-SAT is in NP
Do it yourself
Circuit SAT or C SAT is a set of all combinatorial
circuits such that C is satisfiable
<c> : <c> is satisfiable
All NP Hard problems should be reducible
to CSAT for it to be NP Hard
Thanks to Stuti Chawla (28) MCS '11
Proof :
Let ‘P’ be an arbitrary problem in NP.
P is a decision problem (say).
Let ‘L’ be the language corresponding to P.
Since P Є NP, there exists an algorithm A that
verifies P (or L) in polynomial time.
This implies that for every x Є L, there exists a certificate y,
polynomial in the length of x,
such that A(x,y) = 1 (by def. of NP)
Thanks to Stuti Chawla (28) MCS '11
Aim : To reduce the given NP problem, ‘P’ to CSAT
The algorithm ‘A’ changes the state of the system
from one configuration to another
Suppose |x| = n
Then |y| = O(nk), const k
(by def.)
A runs in time
T(n) = O(|x| + |y|)
= O(nk)
Thanks to Stuti Chawla (28) MCS '11
Reduction of an NP problem to Circuit-SAT
Claim:
• Cx is satisfiable if there exists a certificate y
such that A(x, y) = 1.
• If Cx is satisfiable then there exists a certificate
y such that A(x, y) = 1.
Proof: The claim follows from the construction
that Cx (y) = A(x, y).
Reduction of an NP problem to Circuit-SAT
Claim: Reduction runs in polynomial time.
Proof: We’ll prove that the size of the circuit Cx is polynomial in
|x|.
1. Size of the program for A is independent of the size of x. (Size
of any program is independent of its input size, its execution
time may depend but not the size of the code.)
2. |y| is polynomial in |x|
3. Working Storage Area in each configuration = O(T(n)) where
T(n) is the running time of A. (That’s true for any algorithm)
4. Size of M is polynomial in the length of a configuration.
5. Number of configurations is T(n).
Hence proved.
The CIRCUIT-SAT is in NP
We shall construct a 2-input , polynomial
time algorithm that can verify CIRCUIT SAT.
The CNF-SAT Problem
Terminology
• Boolean Formula is a parenthesized expression formed from Boolean
variables using Boolean operations, such as AND, OR, NOT, IMPLIES, IF
AND ONLY IF.
• A clause is formed as the OR of Boolean variables or their negation called
literals.
• A Boolean Formula is in Conjunctive Normal Form(CNF) if it is formed as a
collection of subexpressions, called clauses, which are combined using
AND.
• For example, the following Boolean formula is in CNF
(x1’ + x2 + x4 + x7’) (x3 + x5’) (x2’ + x4 + x6’ + x8) (x1 + x3 + x5 + x8’)
The CNF-SAT Problem
Statement : CNF SAT takes a Boolean
formula in CNF as input and asks if there is
an assignment of Boolean values to its
variables so that the formula evaluates to 1.
The CNF-SAT Problem
To Prove: CNF-SAT is NP Complete
Step 1: Show that CNF-SAT belongs to NP
It is possible to design a polynomial time algorithm
which takes the following 2 inputs and checks
whether the assignment specified by S satisfies
every clause in I:
• An instance I of the problem
• A proposed solution S
CNF-SAT is NP-Hard
Step 2: Show that CNF-SAT is NP hard
We shall do this through the Local-Replacement approach
by trying to reduce the CIRCUIT-SAT problem to CNF_SAT
in polynomial time.
Given: A Boolean circuit C
Assume that each AND / OR gate has 2 inputs, and
each NOT gate has 1 input
CNF-SAT is NP Hard: Reduction
from Circuit-SAT
CNF SAT is NP-Hard
• ϕ = y4 ∧(y1  x1 ∧x2)
∧(y1  ~x3)
∧(y3  x2 ∧y2)
∧(y4
 y3 ∨ y1)
Note that for every assignment of xi’s there is an assignment of yj’s.
These values of xi’s and yj’s will always saitisfy the last four clauses
above by construction
Suppose the the circuit is satisfiable. Then there exists an assignment
of xi’s which makes y4 true.. Hence the above formula is satisfiable.
If the circuit is not satisfiable. Then every assignment of xi’s will force
y4 to be set to false. Hence the above formula is not satisfiable.
Converting Implications to CNF
•
Create a formula Bg corresponding to each gate g in C as follows:
1. If g is an AND gate with inputs a and b (which could be either xi‘s or yi’s)
and output c, then Bg = (c ↔ (a.b)).
2. If g is an OR gate with inputs a and b, and output c, then Bg = (c ↔
(a+b))
3. If g is a NOT gate with input a and output b, then Bg = (b ↔ a’)
•
Convert each Bg to CNF as follows
1. Construct a truth table for Bg.
2. Derive a formula for Bg in CNF form.
( If you don’t know how to do it directly then convert it into DNF and then convert it
into CNF by De Margan’s Law)
CNF-SAT : An Example
Conversion of BG1=
(y1 ↔ (x1.x2)) to CNF
y1 x1 x2
1
1
1
1
0
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
Bg1 = (y1 ↔
(x1.x2))
1
0
0
1
0
1
0
1
CNF-SAT : An Example
• DNF formula for BG1’ =
x1.x2.y1’ + x1.x2’.y1 + x1’.x2.y1 + x1’.x2’.y1
• CNF formula for BG1 =
(x1’+x2’+y1)(x1’+x2+y1’)(x1+x2’+y1’)(x1+x2+y1’)
Similarly CNF formula for BG3 =
(x2’+y2’+y3)(x2’+y2+y3’)(x2+y2’+y3’ )(x2+y2+y3’)
CNF-SAT : An Example
Conversion of BG2=
(y2 ↔ x3’) to CNF
x3 y2 Bg2 = (y2 ↔
x3’)
0 0
0
0 1
1
1 0
1
1 1
0
CNF-SAT : An Example
• DNF formula for BG2’ =
x3’.y2’ + x3.y2
• CNF formula for BG2 =
(x3+y2)(x3’+y2’)
BG3 is similar to BG1
CNF-SAT : An Example
y1 y3 y4 Bg4 = (y4 ↔
Conversion of BG4=
(y1 V y3))
(y4 ↔ (y1 V y3)) to CNF 0 0 0
1
0
0
0
1
1
1
1
0
1
1
0
0
1
1
1
0
1
0
1
0
1
0
0
1
0
1
0
1
CNF-SAT : An Example
• DNF formula for BG4’ =
y1’.y3’.y4 + y1’.y3.y4’ + y1.y3’.y4’ + y1.y3.y4’
• CNF formula for BG4 =
(y1+y3+y4’)(y1+y3’+y4)(y1’+y3+y4)(y1’+y3’+y4)
CNF-SAT : An Example
CNF for the entire circuit is given by
Ф=y4(x1’+x2’+y1)(x1’+x2+y1’)(x1+x2’+y1’)(x1+x2+y1’)(x3+y2)(x3’+y2’)(x2’+y2’+
y3)(x2’+y2+y3’)(x2+y2’+y3’)(x2+y2+y3’)(y1+y3+y4’)(y1+y3’+y4)(y1’+y3+y4)(y1
’+y3’+y4)
Note that for every assignment of xi’s there is an assignment of yj’s.
These values of xi’s and yj’s will always satisfy the above clauses
(but y4) by construction.
Suppose the circuit is satisfiable. Then there exists an assignment of
xi’s which makes y4 true.. Hence the above formula is satisfiable.
If the circuit is not satisfiable. Then every assignment of xi’s will force
y4 to be set to false. Hence the above formula is not satisfiable.
THE 3SAT PROBLEM
Statement: This problem takes a Boolean
formula S in CNF, having exactly 3 literals,
and asks if S is satisfiable.
Observation : This is a restricted version of
the CNF-SAT problem.
THE 3SAT PROBLEM
To prove: 3SAT is NP-Complete
Step 1: 3SAT is obviously in NP (by restriction: 3 SAT is a special
case of CNF SAT)
Step 2: Show that 3SAT is NP hard
Observe that Restriction form of NP hardness proof does
not apply to this situation. WHY ?
We shall use the local replacement form of proof for this
and try to reduce the CNF-SAT problem to 3SAT in
polynomial time.
THE 3SAT PROBLEM
Given : A Boolean formula C in CNF.
Perform the following local replacement for each clause Ci
• If Ci = (a), that is, it has a single term, replace Ci
Si = (a + b + c) . (a + b’ + c) . (a + b + c’) . (a + b’
where b and c are new variables not used anywhere
•
•
•
If Ci = (a+b), that is, it has 2 terms, replace Ci with
Si = (a + b + c) . (a + b + c’)
where c is a new variable not used anywhere else.
If Ci = (a+b+c), that is, it has 3 terms, set Si = Ci
If Ci = (a1 + a2 + ……. + ak), that is, it has k>3 terms, replace Ci with
Si = (a1 + a2 + b1) . (b1’ + a3 + b2) . (b2’ + a4 + b3)……..( bk-3’+ ak-1 + ak)
where b1, b2,…..... bk-1 are new variables not used anywhere else.
in C:
with
+ c’)
else.
Reducing SAT to 3SAT
• If Ci = 1 then let a_r be true..
• if r=1 or 2, then (a1 \/ a2 \/ b1) is satisfied so set all
b_i’s to false to satisfy all other clauses;
• if r=k-1 or k, then (~b_{r-3} \/ a_{r-1} \/ a-r) is
satisfied so set all b_i’s to true to satisfy all other
clauses;
• if 2 < r < k-1, then (~b_{r-2} \/ a_r \/ b_{r-1}) is
satisfied so set b1 = b2 = ... = b_{r-2} = true to satisfy
the first r-2 clauses and set b_{i-1} = b_i = ... = b_{r-3}
= false to satisfy
the remaining clauses.
Hence, Si is satisfied.
Reducing SAT to 3SAT
• If Si is satisfiable then:
• if none of the new variables b1, ..., b{r-3} is set to false,
then (~b{r-3} \/ a{r-1} \/ ar) can only be satisfied by setting
either a{r-1} = true or ar = true;
• If b1 is set to false, then (a1 \/ a2 \/ b1) can only be
satisfied by setting either a1 = true or a2 = true;
• else let b_r be the first new variable set to false i.e. b1 = b2 =
... = b{r-1} = true: then (~b{r-1} \/ a{r+1} \/ br) can
only
be satisfied by setting a{r+1} = true;
• Thus in all the cases, one of the original literals must be set to
true. Hence Ci is satisfied.
Reducing SAT to 3SAT
Reduction is polynomial time
• Each clause increases in size by at most a constant
factor and the computations involved are simple
substitutions.
T h u s
3 S AT
i s
N P - C o m p l e t e .
POLYNOMIAL TIME VARIANTS OF SAT
T h e fo l l o w i n g 2 va r i a nt s o f t h e SAT p ro b l e m c a n
be solved in polynomial time, and therefore belong
to the complexity class P.
•
If all clauses contain at most one positive literal, then the
Boolean formula is called a Horn Formula, and a satisfying
truth assignment can be found by greedy
algorithm.
•
If the clauses have only 2 literals, then SAT can be solved in
linear time by finding the strongly connected components
o f
a
p a r t i c u l a r
g r a p h
constructed from the instance.
We’ll not do them here. Do them yourself.
The 2SAT Problem
• Instance: Collection C = {c1, …, cm} of clauses
on a set U of n Boolean variables such that
|ci| = 2 for 1 ≤ i ≤ m.
• Question: Is there a truth assignment for the
variables in U that satisfies all clauses in C?
The 2SAT Problem
2SAT can be solved by formulating it as a graph
algorithm:
• Let Ф be an instance of 2SAT. Construct a
directed graph G(Ф) such that vertices of G(Ф)
are the variables of Ф and their negations.
• There is an arc (x, y) in G(Ф) if and only if
there is a clause (x’ + y) or (y + x’) in Ф.
The 2SAT Problem
Note: a + b is equivalent to each of a’ ⇒ b and b’ ⇒ a.
Thus a 2SAT formula may be viewed as a set of
implications. Accordingly, if we have the following formula:
(a’ + b) (b’ + c) (c’+ d)
then we have a string of implications a ⇒ b ⇒ c ⇒ d, which
leads to a ⇒ d,
• If for some variable a, there is a string of implications
a ⇒ ... ⇒ a’, and another string of implications
a’ ⇒ ... ⇒ a, then the formula is not satisfiable,
otherwise the formula is satisfiable.
The 2SAT Problem
Consider the following formula
(x1+x2) (x2’+x3) (x1’+x2’) (x3+x4) (x3’+x5)
(x4’ + x5’) (x3’ + x4).
The implication graph is as follows:
The 2SAT Problem
x1
x2’
x4’
x3’
x5
x5’
x3
x4
x2
x1’
The 2SAT Problem
• The 2SAT problem thus reduces to the graph problem
of finding strongly connected components (SCC) in
the implication graph.
• A 2SAT formula is unsatisfiable if and only if some
variable and its complement reside in the same SCC.
• As SCC is known to have a linear-time solution and
the implication graph is constructible in
linear time, it is clear that 2SAT may be decided
under the same time bound.
Horn Formula
References: 1. Chapter 5 Greedy Algorithms (S. Dasgupta, C.H. Papadimitriou, and U.V. Vazirani)
2. http://www.cs.berkeley.edu/~daw/teaching/cs170-s03/Notes/lecture15.pdf
(UC Berkeley|CS 170: Ecient Algorithms and Intractable Problems
:Lecturer: David Wagner)
In Horn formulas, knowledge about variables is represented by two kinds of clauses :
1. Implications, whose left-hand side is an AND of any number of positive literals and whose
right-hand side is a single positive literal. These express statements of the form
x1 ^ x2 ^… ^ xk implies xk+1
which can alternatively be written as follows:
x’1 v x’2 v… v x’k v xk+1
•
Here, if the conditions on the left hold, then the one on the right must
also be true.
• A degenerate type of implication is the singleton implies x, meaning
simply that x is true.
2. Pure negative clauses, consisting of an OR of any number of negative literals, as in (u’ v v’ v
y’)
Horn Formula
• The implications tell us to set some of the variables to true.
• The negative clauses encourage us to make them false.
Strategy for solving a Horn formula :
 start with all variables set to false.
 proceed to set some of them to true, one by one, only if an
implication would otherwise be violated.
 once done with this phase, when and all implications are
satisfied, turn to the negative clauses and make sure they are
all satisfied.