NP-Completeness | Tractable and Intractable Problems | Polynomial time reduction | DAA
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NP-Completeness
Most of the problems
considered up to now can be solved by algorithms in worst-case polynomial time.
There are many problems and it is not necessary that all the problems have an
apparent solution. This concept, somehow, can be applied to solving the problem
using computers. The computer can solve: some problems in a limited time e.g.
sorting, some problems require an unmanageable amount of time e.g. Hamiltonian
cycles, and some problems cannot be solved e.g. I-lasting Problem. In this
section, we concentrate on the specific class of problems called NP-complete
problems (will be defined later).
Tractable and Intractable Problems
We call problems as tractable or easy if the problem can be solved using polynomial-time
algorithms. The problems that cannot be solved in polynomial time but require a
super polynomial-time algorithm are called intractable or hard problems. There
are many problems for which no algorithm with running time better than
exponential time is known some of them are, the traveling salesman problem,
Hamiltonian cycles, circuit satisfiability, etc.
Polynomial-time reduction
Given two problems A and
B, a polynomial-time reduction from A to B is a polynomial-time function f that
transforms the instances of A into instances of B such that the output of the algorithm for the problem A on input instance x must be the same as the output
of the algorithm for the problem B on input instance fix} as shown in the figure
below. If there is polynomial-time computable function f such that it is
possible to reduce A to B, then it is denoted as A <=p B. The function
f described above is called the reduction function and the algorithm for
computing f is called the reduction algorithm.
P and NP classes and NP-completeness
The set of problems that
can be solved using the polynomial-time algorithm is regarded as class P. The
problems that are verifiable in polynomial time constitute the class NP. The
class of NP-complete problems consists of those problems that are NP as well as
they are as hard as any problem in NP (more on this later). The main concern of
studying NP-completeness is to understand how hard the problem is. So if we can
find some problem as NP-complete then we try to solve the problem using methods
like approximation, rather than searching for the faster algorithm for solving
the problem exactly.
Complexity Class P
P is the class of
problems that can be solved in polynomial time on a deterministic effective
computing system (ECS). Loosely speaking, all computing machines that exist in
the real world are deterministic ECSs. So P is the class of things that can be
computed in polynomial time on real computers.
Complexity Class NP
NP is the class of
problems that can be solved in polynomial time on a non—deterministic effective
computing system (ECS) or we can say that “NP is the class of problems that can
be solved in super polynomial time on a deterministic effective computing
system {EC S)”. Loosely speaking, all computing machines that exist in the real
world are deterministic ECSs. So NP is the class of problems that can be
computed in super polynomial time on real computers. But problems of class NP
are verifiable in polynomial time. Using the above idea we say the problem is in
class NP (nondeterministic polynomial time} if there is an algorithm for the
problem that
verifies the problem in polynomial time. For e.g. Circuit satisfiability problem
(SAT) is the question “Given a Boolean combinational circuit, is it satisfiable?
i.e. does the circuit has an assignment sequence of truth values that produces the
output of the circuit as l?" Given the circuit satisfiability problem take
a circuit x and a certificate y with the set of values that produce output 1, we
can verify that whether the given certificate satisfies the circuit in polynomial
time. So we can say that the circuit satisfiability problem is NP.
The complexity of Class NP-Complete
NP-complete problems are
those problems that are the hardest problems in class NP. We define some problems
say A, is NP-complete if
a. A ∈ NP,
and
b. B <=p A, for every B ∈
NP.
The problem satisfying property b is called NP-hard.
NP-Complete problems arise in many domains like boolean logic; graphs, sets, and partitions; sequencing, scheduling, allocation; automata and language
theory; network design; compilers, program optimization; hardware design
optimization; number theory, algebra, etc.
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