1 Computational Complexity CPSC 468/568, Fall 2009 Time: Tu & Th, 2:30-3:45 pm Room: AKW 307 Satisfies the QR requirement.

2 Partial Topic Outline Complexity classes (P, NP, L, NL, etc.) Reductions and completeness The roles of, e.g., –Randomness –Interaction –Approximation ?Communication complexity

3 Requirements Modest reading assignments, mostly in Arora and Barak, Computational Complexity: A Modern Approach, Cambridge Univ. Press, Written HW Assignments, each worth 10% of the course grade 2 In-Class Exams, each worth 20% of the course grade No final exam during exam week

4 Schedule Sept. 17: First HW Assignment Due Oct. 1: Second HW Assignment Due Oct. 13: Third HW Assignment Due Oct. 15: First In-Class Exam Oct. 23: Fall Semester Drop Date Oct. 29: Fourth HW Assignment Due Nov. 17: Fifth HW Assignment Due Dec. 1: Sixth HW Assignment Due Dec. 3: Second In-Class Exam

5 Rules and Guidelines Deadlines are firm. Late penalty: 5% per day. Announcements and assignments will be posted on the class webpage (as well as conveyed in class). No “collaboration” on homeworks unless you are told otherwise. Pick up your graded homeworks and exams promptly, and tell the TA promptly if one is missing.

6 Instructor: Joan Feigenbaum Office: AKW 512 Office Hours: Thursdays 11:30 am - 12:30 pm and by appointment Phone: Assistant: Judi Paige , AKW 507a, 8:30 am – 4:30 pm M-F) Note: Do not send to Professor Feigenbaum, who suffers from RSI. Contact her through Ms. Paige or the TA.

7 TA: David Costanzo Office: AKW Office Hours: TBD

8 If you’re undecided … Check out: zoo.cs.yale.edu/classes/cs468/spr07/, …/fall07/, and …/fall08 (draft of textbook by Sanjeev Arora and Boaz Barak of Princeton) (a complexity-theory course taught by Luca Trevisan at Berkeley in 2002) (“NP-Completeness: A Retrospective,” by Christos Papadimitriou, 1997 International Colloquium on Automata, Languages, and Programming)

9 Questions?

10 Introduction to Complexity Classes

11 Computational Complexity Themes “Easy” vs. “Hard” Reductions (Equivalence) Provability Randomness

12 Poly-Time Solvable Nontrivial Example : Matching

13 Poly-Time Solvable Nontrivial Example : Matching

14 Poly-Time Verifiable Trivial Example : Hamiltonian Cycle

15 Poly-Time Verifiable Trivial Ex. : Hamiltonian Cycle

16 Is it Easier to Verify a Proof than to Find one? Fundamental Conjecture of Computational Complexity: P  NP

17 Matching: HC: Fundamentally Different Distinctions

18 Reduction of B to A If A is “Easy”, then B is, too. B Algorithm A “oracle” “black box”

19 NP-completeness P-time reduction Cook’s theorem If B ε NP, then B ≤ P-time SAT HC is NP-complete

20 Equivalence NP-complete problems are an equivalence Class under polynomial-time reductions. 10k’s problems Diverse fields Math, CS, Engineering, Economics, Physical Sci., Geography, Politics…

21 NPcoNP P

22 Random poly-time Solvable x ε L? poly-time Algorithm x r YES NO x ε {0,1} n r ε {0,1} poly(n)

23 Probabilistic Classes x ε L  “yes” w.p. ¾ x  L  “no” w.p. 1 x ε L  “yes” w.p. 1 x  L  “no” w.p. ¾ RP coRP (Outdated) Nontrivial Result PRIMES ε ZPP ( = RP ∩ coRP)

24 Two-sided Error BPP x  L  “yes”w.p. ¾ x  L  “no” w.p. ¾ Question to Audience: BPP set not known to be in RP or coRP?

25 RPcoRP ZPP NPcoNP P

26 Interactive Provability P V [PPT, ¢ ] x yes/no

27 L ε IP x ε L   P: “yes” w.p. ¾ x  L   P*: “no” w.p. ¾ Nontrivial Result Interactively Provable Poly-Space Solvable

28 PSPACE RPcoRP ZPP NPcoNP P

29 PSPACE EXP P #P PH iPiP 2P2P NP P