Black-box (oracle)

1. Black-box (oracle) Feed me a weighted graph G and I will tell you the weight of the max-weight matching of G.

2. Black-box (oracle) 5 Feed me a weighted graph G and I will tell you the weight of the max-weight matching of G. 2 2

3. Black-box (oracle) 5 Feed me a weighted graph G and I will tell you the weight of the max-weight matching of G. 2 2 5

4. Black-box (oracle) Feed me a weighted graph G and I will tell you the weight of the max-weight matching of G. here is a graph G, find the max-weight matching G

5. Black-box (oracle) here is a graph G, find the max-weight matching Feed me a weighted graph G and I will tell you the weight of the max-weight matching of G. pick a vertex uV(G) for each edge {u,v}E(G) wundefined if oracle(G-u-v) + w(u,v) = oracle (G) then wv if w is undefined then recurse on (G-u) else print({u,w}); recurse on (G-u-v)

6. 3-SAT x = variable x = negation of a variable literals clause = disjunction of literals x  y  z x  z

7. 3-SAT INSTANCE: collection C of clauses, each clause has at most 3 literals QUESTION: does there exist an assignment of true/false to the variables which satisfies all the clauses in C

8. 3-SAT INSTANCE: collection C of clauses, each clause has at most 3 literals QUESTION: does there exist an assignment of true/false to the variables which satisfies all the clauses in C x  y  z x  y z x y x

9. Independent Set subset S of vertices such that no two vertices in S are connected

10. Independent Set subset S of vertices such that no two vertices in S are connected

11. Independent Set OPTIMIZATION VERSION: INSTANCE: graph G SOLUTION: independent set S in G MEASURE: maximize the size of S DECISION VERSION: INSTANCE: graph G, number K QUESTION: does G have independent set of size  K

12. Independent Set  3-SAT “is easier than” if we have a black-box for 3-SAT then we can solve Independent Set in polynomial time Independent Set reduces to 3-SAT

13. Independent Set  3-SAT if we have a black-box for 3-SAT then we can solve Independent Set in polynomial time Give me a 3-SAT formula and I will tell you if it is satisfiable We would like to solve the Independent Set problem using the black box in polynomial time.

14. Independent Set  3-SAT Give me a 3-SAT formula and I will tell you if it is satisfiable Graph G, K  3-SAT formula F efficient transformation (i.e., polynomial – time) G has independent set of size  K  F is satisfiable

15. Independent Set  3-SAT Give me a 3-SAT formula and I will tell you if it is satisfiable Graph G, K  3-SAT formula F V = {1,...,n} variables x1,....,xn E = edges  xi xj for ij E + we need to ensure that  K of the xi are TRUE

16. 3-SAT  Independent Set Give me a graph G and a number K and I will tell you if G has independent set of size  K 3-SAT formula F  graph G, number K

17. 3-SAT  Independent Set Give me a graph G and a number K and I will tell you if G has independent set of size  K 3-SAT formula F  graph G, number K x y  z w  y z y y z w z x

18. 3-SAT  Independent Set 3-SAT formula F  graph G, number K x y  z w  y z y y z w z x • efficiently computable • F satisfiable  IS of size  m •  IS of size  m  F satisfiable

19. 3-SAT  Independent Set Independent Set  3-SAT if 3-SAT is in P then Independent Set is in P if Independent Set is in P then 3-SAT is in P 3-SAT Independent Set

20. Many more reductions Clique Subset-Sum 3-COL Planar 3-COL Hamiltonian path 3-SAT Independent Set

21. P and NP P = decision problems that can be solved in polynomial time. NP = decision problems for which the YES answer can be certified and this certificate can be verified in polynomial time.

22. NP = decision problems for which the YES answer can be certified and this certificate can be verified in polynomial time. 3-SAT Independent Set NOT-3-SAT ?

23. NP = decision problems for which the YES answer can be certified and this certificate can be verified in polynomial time. COOK’S THEOREM Every problem A NP A  3-SAT

24. NP = decision problems for which the YES answer can be certified and this certificate can be verified in polynomial time. B is NP-hard if every problem A NP A  B B is NP-complete if B is NP-hard, and B is in NP

25. NP-hard NP-complete P NP

26. Some NP-complete problems Clique Subset-Sum 3-COL Planar 3-COL Hamiltonian path 3-SAT Independent Set

27. Clique subset S of vertices such that every two vertices in S are connected

28. Clique INSTANCE: graph G, number K QUESTION: does G have a clique of size  K?

29. Subset-Sum INSTANCE: numbers a1,...,an,B QUESTIONS: is there S {1,...,n} such that ai = B iS

30. 3-COL INSTANCE: graph G QUESTION: can the vertices of G be assigned colors red,green,blue so that no two neighboring vertices have the same color?

31. 3-SAT  3-COL x x R G B B G=true y x z G G G R G

32. Planar-3-COL INSTANCE: planar graph G QUESTION: can the vertices of G be assigned colors red,green,blue so that no two neighboring vertices have the same color?

33. 3-COL  Planar-3-COL

34. 4-COL INSTANCE: graph G QUESTION: can the vertices of G be assigned one of 4 colors so that no two neighboring vertices have the same color?

35. 3-COL  4-COL

36. 3-COL  4-COL  G G

37. planar 4-COL INSTANCE: planar graph G QUESTION: can the vertices of G be assigned one of 4 colors so that no two neighboring vertices have the same color? planar 3-COL  planar 4-COL ???

38. 4-COL  3-COL 4-COL  NP Cook  4-COL  3-SAT 3-SAT 3-COL Thus: 4-COL  3-COL

39. 2-COL  3-COL

40. 2-COL  3-COL  G G

41. 3-COL  2-COL ??? 2-COL in P