Making the Long Code Shorter

The long code is a central tool in hardness of approximation, especially in questions related to the unique games conjecture. We construct a new code that is exponentially more efficient, but can still be used in many of these applications. Using the new code we obtain exponential improvements over several known results, including the following: 1) For any ε >; 0, we show the existence of an n vertex graph G where every set of o(n) vertices has expansion 1 - ε, but G's adjacency matrix has more than exp(log δ n) eigenvalues larger than 1 - ε, where δ depends only on ε. This answers an open question of Arora, Barak and Steurer (FOCS 2010) who asked whether one can improve over the noise graph on the Boolean hypercube that has poly(log n) such eigenvalues. 2) A gadget that reduces unique games instances with linear constraints modulo K into instances with alphabet k with a blowup of K polylog(K), improving over the previously known gadget with blowup of 2 Ω(K). 3) An n variable integrality gap for Unique Games that survives exp(poly(log log n)) rounds of the SDP + Sherali Adams hierarchy, improving on the previously known bound of poly(log log n). We show a connection between the local testability of linear codes and small set expansion in certain related Cayley graphs, and use this connection to derandomize the noise graph on the Boolean hypercube.

Authors

• Boaz Barak
• Parikshit Gopalan
• Johan Håstad
• Raghu Meka
• Prasad Raghavendra
• David Steurer

Keywords

• Games
• Eigenvalues and eigenfunctions
• Polynomials
• Noise measurement
• Hypercubes
• Approximation methods
• Electronic mail
• Small Set
• Linear Constraints
• Unified Representation
• Small Expansion
• Central Tool
• Set Of Functions
• Linear Term
• Polynomial Of Degree
• Projection Operator
• Codeword
• Form Of Constraints
• Semidefinite Programming
• Affine Function
• Unique Problems
• Regular Graphs
• Relaxivity Values
• Detailed Version
• Large Eigenvalues
• Invariance Principle
• Short Codes
• Fraction Of Edges
• Efficient Version
• Max-Cut
• Unique games conjecture
• Small set expansion
• Long Code
• Locally Testable Codes