NP-complete and NP-hard problems

NP-complete and NP-hard problems • Transitivity of polynomial-time many-one reductions • Definition of complexity class NP • Nondeterministic computation • Problems that can be verified • The P = NP Question • Concept of NP-hard and NP-complete problems

Π1 ≤p Π2 & Π2 ≤p Π3 Π1 ≤p Π3 • Let R1 be the reduction used to prove Π1 ≤p Π2 • Let R2 be the reduction used to prove Π2 ≤p Π3 • Let x be an input to Π1 • Define R3(x) to be R2(R1(x))

Answer-preserving argument • Because R1 is a reduction between Π1 and Π2, we know that R1(x) is a yes input instance of Π2 iff x is a yes input instance of Π1 • Because R2 is a reduction between Π2 and Π3, we know that R2(R1(x)) is a yes input instance of Π3 iff R1(x) is a yes input instance of Π2 • Applying transitivity of iff, we get that R3(x) is a yes input of Π3 iff x is a yes input instance of Π1

Polynomial-time Argument • Let R1 take time nc1 • Let R2 take time nc2 • Let n be the size of x • Then the R1 call of R3 takes time at most nc1 • Furthermore, R1(x) has size at most max(n,nc1) • Therefore, the R2 call of R3 takes time at most max(nc2, (nc1)c2) = max (nc2, nc1 c2) • In either case, the total time taken by R3 is polynomial in n

Traditional definition of NP • Turing machine model of computation • Simple model where data is on an infinite capacity tape • Only operations are reading char stored in current tape cell, writing a char to current tape cell, moving tape head left or right one square • Deterministic versus nondeterministic computation • Deterministic: At any point in time, next move is determined • Nondeterministic: At any point in time, several next moves are possible • NP: Class of problems that can be solved by a nondeterminstic turing machine in polynomial time

Turing Machines A Turing machine has a finite-state-control (its program), a two way infinite tape (its memory) and a read-write head (its program counter) Finite State Control Head …. …. 1 0 1 0 0 0 1 1 0 1 1 Tape

Deterministic Computation Nondeterministic Computation Nondeterministic Running Time • We measure running time by looking at height of computation tree, NOT number of nodes explored • Both computation have same height 4 and thus same running time

Yes Result No Result ND computation returning yes • If any leaf node returns yes, we consider the input to be a yes input. • If all leaf nodes return no, then we consider the input to be a no input.

Showing a problem is in NP • Hamiltonian Path • Input: Undirected graph G = (V,E) • Y/N Question: Does G contain a HP? • Nondeterministic polynomial-time solution • Guess a hamiltonian path P (ordering of vertices) • V! possible orderings • For binary tree, V log V height to generate all guesses • Verify guessed ordering is correct • Return yes/no if ordering is actually a HP

2 Guess Phase Nondeterministic --------------- Verify Phase Deterministic 3 1 Yes input graph 123 132 231 321 213 312 2 Guess Phase Nondeterministic --------------- Verify Phase Deterministic 3 1 No input graph 123 132 231 321 213 312 Illustration

Alternate definition of NP • Preliminary Definitions • Let Π be a decision problem • Let I be an input instance of Π • Let Y(Π) be the set of yes input instances of Π • Let N(Π) be the set of no input instances of Π • Π belongs to NP iff • For any I ε Y(Π), there exists a “certificate” [solution] C(I) such that a deterministic algorithm can verify I ε Y(Π) in polynomial time with the help of C(I) • For any I ε N(Π), no “certificate” [solution] C(I) will convince the algorithm that I ε Y(Π).

2 Guess Phase Nondeterministic --------------- Verify Phase Deterministic 3 1 Yes input graph 123 132 231 321 213 312 2 Guess Phase Nondeterministic --------------- Verify Phase Deterministic 3 1 No input graph 123 132 231 321 213 312 Connection • Certificate [Solution] • A Hamiltonian Path • C(I1): 123 or 321 • C(I2): none • Verification Alg: • Verify certificate is a possible HP • Check for edge between all adjacent nodes in path

Example: Clique Problem • Clique Problem • Input: Undirected graph G = (V,E), integer k • Y/N Question: Does G contain a clique of size ≥k? • Certificate • A clique C of size at least k • Verification algorithm • Verify this is a potential clique of size k • Verify that all nodes in C are connected in E

Proving a problem is in NP • You need to describe what the certificate C(I) will be for any input instance I • You need to describe the verification algorithm • usually trivial • You need to argue that all yes input instances and only yes input instances have an appropriate certificate C(I) • also usually trivial (typically do not require)

Example: Vertex Cover • Vertex Cover • Input: Undirected graph G = (V,E), integer k • Y/N Question: Does G contain a vertex cover of size ≤k? • Vertex cover: A set of vertices C such that for every edge (u,v) in E, either u is in C or v is in C (or both are in C) • Certificate • A vertex cover C of size at most k • Verification algorithm • Verify C is a potential vertex cover of size at most k • Verify that all edges in E contain a node in C

Example: Satisfiability • Satisfiability • Input: Set of variables X and set of clauses C over X • Y/N Question: Is there a satisfying truth assignment T for the variables in X such that all clauses in C are true? • Certificate? • Verification algorithm?

Example: Unsatisfiability • Unsatisfiability • Input: Set of variables X and set of clauses C over X • Y/N Question: Is there no satisfying truth assignment T for the variables in X such that all clauses in C are true? • Certificate and Verification algorithm? • Negative certificate and Negative verification algorithm?

Example: Exact Vertex Cover • Exact Vertex Cover • Input: Undirected graph G = (V,E), integer k • Y/N Question: Does the smallest vertex cover in G have size exactlyk? • Vertex cover: A set of vertices C such that for every edge (u,v) in E, either u is in C or v is in C (or both are in C) • Certificate and Verification algorithm? • Negative certificate and Negative verification algorithm?

Definition of NP-hard • A problem П is NP-hard if • for all П’ εNP П’ ≤p П holds. • Intuitively, an NP-hard problem П is at least as hard (defined by membership in P) as any problem in NP

NP P NP-complete Definition of NP-complete • A problem П is NP-complete if • П is NP-hard and • П is in NP • Intuitively, an NP-complete problem П is the hardest problem in NP • That is, if П is in P, then P=NP • If P ≠ NP, then П is not in P P=NP=NP-complete OR

Importance of NP-completenessImportance of “Is P=NP” Question • Practitioners view • There exist a large number of interesting and seemingly different problems which have been proven to be NP-complete • The P=NP question represents the question of whether or not all of these interesting and different problems belong to P • As the set of NP-complete problems grows, the question becomes more and more interesting

Graph Theory Network Design Sets and Partitions Storage and Retrieval Sequencing and Scheduling Mathematical Programming Algebra and Number Theory Games and Puzzles Logic Automata and Languages Program Optimization Miscellaneous List of Problem Types from Garey & Johnson, 1979

Importance of NP-completenessImportance of “Is P=NP” Question • Theoretician’s view • NP is exactly the set of problems that can be “verified” in polynomial time • Thus “Is P=NP?” can be rephrased as follows: • Is it true that any problem that can be “verified” in polynomial time can also be “solved” in polynomial time? • Hardness Implications • It seems unlikely that all problems that can be verified in polynomial time also can be solved in polynomial time • If so, then P≠NP • Thus, proving a problem to be NP-complete is a hardness result as such a problem will not be in P if P≠NP.

Proving a problem П is NP-complete • Proving a problem П is NP-complete • Show П is in NP (usually easy step) • Prove for all П’ εNP П’ ≤p П holds. • Show that П’ ≤p П for some NP-hard problemП’ • This only works if we have a known NP-hard problem П’ to reduce from • Also depends on transitivity property proven earlier • We need to prove the existence of a first NP-hard problem • Cook-Levin Thm • Developing new reductions is a skill or art form • Over time, it gets easier

Select the right source problem 3-SAT: The old reliable. When none of the other problems seem to work, this is the one to come back to. Integer Partition: A good choice for number problems. 3-Partition: A good choice for proving “strong” NP-completeness for number problems. Vertex Cover: A good choice for selection problems. Hamiltonian Path: A good choice for ordering problems.

Some history • Cook: “The complexity of theorem-proving procedures” STOC 1971, pp. 151-158 • Polynomial-time reductions • NP complexity class • SAT is NP-complete • Levin: “Universal sorting problems”, Problemi Peredachi Informatsii 9:3 (1973), pp. 265-266 • Independent discovery of many of the same ideas • Karp: “Reducibility among combinatorial problems”, in Complexity of Computer Computations, 1972, pp. 85-103 • Showed 21 problems from a wide variety of areas are NP-complete

NP-complete and NP-hard problems