## Sunday, 23 June 2013

### NP completeness

Consider a reduction of problem A to problem B. What is the most precise claim you
can make about problem B for each of the following situations?

a) A is NP-complete and the reduction is in polynomial time.
NP-hard. (At least as hard as NP-complete problem.)

b) A is in polynomial time and the reduction is also in polynomial time.
B could be anything.

c) A is NP-complete and the reduction is in Pspace.
B could be anything.

d) A is in nondeterministic polynomial time and the reduction is in polynomial time.
B is at least as hard as A, but nothing more can be said.

e) A requires exponential time and the reduction is in polynomial time.
B must requires polynomial time for deciding.

f) A is Pspace complete and the reduction is in Pspace.
B could be anything.
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Suppose you could reduce an NP complete problem to a polynomial time problem in
polynomial time. What would be the consequence?
What if the reduction required exponential time?

If we could reduce an NP-complete problem to a problem in P, then NP will
be equal to P. If the reduction required exponential time, then there is not
special consequence. (In fact any NP-complete problem can be reduced to
a polynomial time solvable problem using exponential time reduction.)