# COMS4236: Introduction to Computational Complexity. Summer 2014

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1 COMS4236: Introduction to Computational Complexity Summer 2014 Mihalis Yannakakis Lecture 17

2 Outline conp NP conp Factoring Total NP Search Problems

3 Class conp Definition of NP is nonsymmetric with respect to Yes, No conp { L L * L NP } A decision problem P is in conp if the complement of its Yes language LP is in NP its No language (set of No instances) is in NP Certificate version A language L is in conp if there is a polynomial-time decidable binary predicate R(.,.) and constant c such that L = { x y. y x c predicate R(x,y) is true }

4 conp-complete problems Complements of NP-complete problems UNSAT: Given Boolean formula, is it unsatisfiable? TAUTOLOGY (VALIDITY): Given Boolean formula, is it a tautology (valid), i.e. satisfied by all truth assignments? NONHAMILTONICITY: Given a (undirected or directed) graph, is it nonhamiltonian? NON 3-COLORABILITY: Given an undirected graph, is it the case that it has no 3-coloring? NODE COVER LOWER BOUND: Given graph G and number k, does every node cover of G have k nodes? INDEPENDENT SET UPPER BOUND: Given a graph G and number k, does every independent set of G have k nodes?

5 Properties P NP, P conp, thus, P NP conp NP is closed under union, intersection conp is also closed under union, intersection NP (and conp) closed under complement iff NP=coNP - conjectured not

6 Relations between P, NP, conp Conjectured relation: all distinct NP NP conp P conp Other possibilities: NP conp P = NP conp NP = conp P P=NP=coNP

7 P = NP? Fundamental Questions Is it always as easy to generate a proof as it is to check a proof that is given to us? NP = conp? Is it always possible to provide simple convincing evidence that something does not exist, as it is to show that something exists by exhibiting it? P = NP conp? If there is a simple convincing proof both for the presence and the absence of a property, does this mean we can test the property efficiently?

8 Relations between P, NP, conp and their complete problems Conjectured relation: all distinct NP NPC conpc NP conp P conp If an NP-complete problem is in P then P=NP If an NP-complete problem is in conp then NP=coNP If NP conp then no NP-complete problem can be in conp - equivalently, no problem in NP conp can be NP-complete

9 NP conp Short, easy to check certificates both for the Yes and the No instances Examples: Graph Bipartiteness: bipartite nodes can be partitioned into two sets V1, V2 so that all edges connect a node in V1 with a node in V2 nonbipartite there is an odd length cycle Graph Planarity planar can draw on the plane so that no edges intersect nonplanar contains a homeomorph of K5 or K33 (Kuratowski s theorem) K5 K33 These particular properties happen to be in fact in P

10 NP conp and Optimization Optimization problems whose decision version is in NP: assume solutions have polynomial size (#bits) and cost (or values) can be computed in polynomial time If there is a polynomial time (or even NP) optimality testing algorithm, which, given an instance and a solution, tests that the solution is optimal, then the decision version is in conp, and hence probably not NP-complete. Proof: Given an instance x and a bound k, guess a solution s, verify that it is optimal, and verify that its cost (or value) is worse than k (i.e. cost(s) >k for a minimization problem, or value(s)<k for maximization). Examples: Linear Programming Maximum flow problem Maximum matching. These particular problems turn out to be in fact in P

11 Primality Input: Positive integer N (given in binary: input size n=logn) Question: Is N prime? The straightforward algorithm (try out all numbers < N to see if any divides N) is pseudopolynomial, not polynomial Primes conp, i.e. Composites NP: guess a factor q and verify that q divides N Primes NP. Not as obvious [Pratt 1975] Primes P. Much harder [Agrawal, Kayal, Saxena 2002]

12 Factoring Input: Positive number N i Output: The prime factorization of N, 1 i 2 i N p1p2 p k k Most common cryptographic scheme (RSA) based on presumed difficulty of the factoring problem Decision version Input: Number N, number b Question: Does N have a (nontrivial) factor b? (Note: N has a factor b iff it has a prime factor b) - Can factor N with polynomially many calls to an algorithm for the decision problem: Use binary search to find the smallest (nontrivial) factor of N (it will be a prime), divide N by the factor, and repeat.

13 Factoring Decision NP conp Decision NP: Guess a factor p > 1 and b, and check that p divides N Decision conp: Guess the prime factorization of N: i 1 i 2 i N p, verify the factoring and primality of the pi 1p2 p k k (can use the primality algorithm in P or the NP algorithm: guess certificates C(pi) for all the pi and verify them) Check that no pi is b If NP conp=p then Factoring Decision P Factoring P In other words, Factoring P NP conp P Factoring can be done in polynomial time in a Quantum Computer model [Shor 1994] more powerful than ordinary computers?

14 Hardness of Factoring? Probably cannot use NP-hardness to argue that Factoring is an intractable problem: If Factoring is NP-hard then NP=coNP This holds even for a more general notion of polynomial reduction (and NP-hardness) called Cook reduction. Problem A Cook reduces to problem B if there is an algorithm for problem A that uses a subroutine for B and which runs in polynomial time apart from the subroutine calls Thus if the subroutine for B is polynomial-time then the algorithm A also polynomial-time. If SAT Cook reduces to Factoring then can get NP algorithm for UNSAT (a conp-complete problem): replace the subroutine calls with an NP algorithm that guesses and verifies the factorization.

15 NP conp and Completeness Factoring P NP conp P Does the converse hold? Can we argue that Factoring is complete for NP conp? No complete problems known for NP conp Basic Obstacle: NP conp is a semantic class. No effective syntactic characterization in terms of a class of machines, as we had with deterministic and nondeterministic time- and space-complexity classes, where we could just limit the amount of time or space used by a TM

16 NP Search problems Binary relation RP relates every input x to 0, 1, or more correct outputs y of problem P as in certificate-based definition of NP. Assume RP is polynomially balanced: y poly( x ) and polynomial-time decidable. NP Search problem P: Given input x, compute a y such that RP (x,y) - anyone is fine or determine that there is none. Examples: Given formula, output a satisfying assignment (if the formula is satisfiable) Given integer N, output its prime factorization (Book calls these `function problems, but they are possibly partial and multivalued functions, i.e. relations)

17 Total Function and Search problems P Search problem P is total if every input x relates to one or more correct outputs y; i.e., for each x there is at least one y such that RP (x,y) Search problem is a (total) function problem, if every x relates to exactly one y Example: Factoring is a total function problem. Total NP search problems cannot be NP-hard (under Cook reductions) unless NP=coNP. For total function problems, P P NP conp P (HW exercise)

18 Total Function and Search problems P There is a number of open problems of this type that are not known to be in P various games (equilibria, optimal strategies) locally optimal solutions to optimization problems stable solutions in neural nets verification of certain properties of systems...

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