
G22.3033007
Spring 2008
CS Dept, New
York University
Probabilistically Checkable Proofs and

Assignment 1: Due Monday, May 5^{th}, 3:30pm.
Many optimization problems of theoretical and practical interest are NPcomplete, meaning it is impossible to compute exact solutions to these problems in polynomial time unless P = NP. A natural way to cope with this curse of NPcompleteness is to seek approximate solutions instead of exact solutions. An algorithm with approximation ratio C computes, for every problem instance, a solution whose cost is within a factor C of the optimum. Optimization problems exhibit a wide range of behavior in their approximability. It is wellknown that BinPacking has an approximation algorithm with ratio 1+\epsilon for every \epsilon > 0. In theory jargon, we say that BinPacking has a polynomialtime approximation scheme (PTAS). However, it wasn't known till the early 90s whether problems like MAX3SAT, Vertex Cover, and MAXCUT have a PTAS. A celebrated result called the PCP Theorem finally showed that these problems have no PTAS unless P = NP. Such results that rule out the possibility of good approximation algorithms (under complexity theoretic assumptions like P != NP) are called inapproximability results or hardness of approximation results.
The PCP Theorem has an equivalent formulation from the point of view of proof checking. The PCP theorem states that every NPstatement has a probabilistically checkable proof, i.e. a proof which can be "spotchecked" by reading only a constant number of bits from the proof. These bits are selected by a randomized process using a very limited amount of randomness. The checking process always accepts a correct proof of a correct statement and rejects any cheating proof of an incorrect statement with high probability. The term "holographic proof" is sometimes used to highlight this feature that a cheating proof must be wrong everywhere and therefore, can be detected by a spotcheck. The discovery of the connection between proof checking and inapproximability results is one of the most exciting theoretical developments in the last decade. Since then, PCPs have led to several breakthrough results in inapproximability theory, e.g. tight hardness results for Clique, MAX3SAT, and Set Cover.
This course will cover many of the inapproximability results and PCPs used to prove them.
No prior knowledge will be assumed, except the basic theory of NPcompleteness.
Participants are expected to scribe notes for one lecture, but this is optional (let this not deter you from taking the course). No assignments/exams !
Professor: Subhash Khot – Off 821, Ph: 2129984859
Template
latex files for scribenotes can be found here (stolen
from Sanjeev Arora's course at Princeton).
Course Syllabus
Here is a tentative list of topics, not necessarily in the order of presentation.
·
PCP
Theorem: Original proof. Low degree testing, Linearity testing
·
PCP
Theorem: Dinur’s proof.
·
Long
codes, Hastad's 3bit PCP, Hardness of MAX3SAT
·
Hardness
of Set Cover, Closest Vector Problem
·
Hardness
of Clique, FGLSS Reduction
·
Hardness
of Edge Disjoint Paths
·
Hardness
of Shortest Vector Problem
·
Hardness
of Minimum Distance of Code
·
Hardness
of Asymmetric kCenter Problem
·
Hardness
of Hypergraph Vertex Cover
·
Unique
Games Conjecture and its
consequences
·
Integrality
Ratio (for MAXCUT, Asymmetric TSP, Sparsest Cut,....)
Lecture notes for the same course I
taught at Georgia Tech (first few):
Lecture 1 (Basic definitions)
Lecture 2 (Equivalence of PCP Theorem to inapproximability
of MAX3SAT)
Lecture 3 (Hardness of Set Cover)
Lecture 4 (Hastad’s 3bit PCP)
Lecture 5 (Hastad’s 3bit PCP
continued)
Lecture 6 (Hardness of Minimum distance of
code)
Lecture 7 (Hardness of Clique, FGLSS)
Lecture notes for similar course
taught by Venkatesan Guruswami and
Ryan O’donnell at U. Washington can be found here. Check
out Dinur’s proof of PCP Theorem.
References
PCP
literature is extensive and often very technical. Here are good places to check
out.