Computational Complexity

1. Computational Complexity CPSC 468/568, Fall 2008 Time: Tu & Th, 1:00-2:15 pm Room: AKW 500 Satisfies the QR requirement. http://zoo.cs.yale.edu/classes/cs468

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. Schedule Sept. 25: First HW Assignment Due Oct. 14: Second HW Assignment Due Oct. 16: First In-Class Exam Oct. 24: Fall Semester Drop Date Oct. 30: Third HW Assignment Due Nov. 13: Fourth HW Assignment Due Dec. 2: Fifth HW Assignment Due Dec. 4: Second In-Class Exam

4. Requirements • Modest reading assignments • 5 Written HW Assignments, each of which will be worth 10% of the course grade • 2 In-Class Exams, each worth 25% of the course grade • No final exam during exam week

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: 203-432-6432 Assistant: Judi Paige (judi.paige@yale.edu, 203-436-1267, AKW 507a, 8:30 am – 4:30 pm M-F) Note: Do not send email to Professor Feigenbaum, who suffers from RSI. Contact her through Ms. Paige or the TA.

7. TA: Nicholas Ruozzi Office: AKW 202 Office Hours: By appointment Email: nicholas.ruozzi@yale.edu

8. If you’re undecided … Check out: • zoo.cs.yale.edu/classes/cs468/spr07/ and …/fall07/ • www.cs.princeton.edu/theory/complexity/ (a new complexity-theory text by Sanjeev Arora and Boaz Barak of Princeton) • www.cs.berkeley.edu/~luca/cs278-02/ (a complexity-theory course taught by Luca Trevisan at Berkeley in 2002) • www.cs.lth.se/home/Rolf_Karlsson/bk/retro.pdf (“NP-Completeness: A Retrospective,” by Christos Papadimitriou, 1997 International Colloquium on Automata, Languages, and Programming)

9. Questions?

10. Introduction to Complexity Classes

11. Computational ComplexityThemes • “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. Distinctions • Matching: • HC: Fundamentally Different

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

19. NP-completeness • P-time reduction • Cook’s theorem If B εNP, then B≤ P-timeSAT • 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. NP coNP P

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

23. Probabilistic Classes RP 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 coRP (Outdated) Nontrivial Result PRIMES ε ZPP ( = RP ∩ coRP)

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

25. NP coNP RP coRP ZPP P

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

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

28. PSPACE NP coNP RP coRP ZPP P

29. EXP PSPACE P#P PH iP 2P NP P