Suppose that every k points in a n point metric space X are
D-distortion embeddable into l1.
We give upper and lower bounds on the distortion required to
embed the entire space X into l1.
In this paper we present a new approximation algorithm
for the MAX ACYCLIC SUBGRAPH problem. Given an instance where
the maximum acyclic subgraph contains 1/2 + δ fraction of
all edges, our algorithm finds an acyclic subgraph with
1/2 + Ω(δ/ log n) fraction of all edges.
In this paper we present approximation algorithms for
the maximum constraint satisfaction problem with k
variables in each constraint (MAX k-CSP).
Given a (1 - ε) satisfiable 2CSP our first algorithm
finds an assignment of variables satisfying a 1 - O(sqrt{ε})
fraction of all constraints. The best previously known
result, due to Zwick, was 1 - O(ε1/3).
The second algorithm finds a ck/2k approximation
for the MAX k-CSP problem (where c > 0.44 is an absolute
constant). This result improves the previously best
known algorithm by Hast, which had an approximation
guarantee of Ω(k/(2k log k)).
Both results are optimal assuming the Unique
Games Conjecture and are based on rounding natural
semidefinite programming relaxations. We also believe
that our algorithms and their analysis are simpler than
those previously known.
"A Divide and Conquer Algorithm for d-Dimensional Arrangement,"
In this paper we present a new approximation algorithm for Unique
Games. For a Unique Game with n vertices and k states,
if a (1 - ε) fraction of all constraints is
satisfiable, the algorithm finds an assignment satisfying a
1-O(ε(log n log k)1/2)
fraction of all constraints.
To this end, we introduce new embedding techniques for
rounding semidefinite relaxations of problems with large
domain size.
We present approximation algorithms for unique games. For instances with domain size
k where the optimal solution satisfies 1 - ε fraction of all constraints, our algorithms
satisfy roughly k-ε/(2 - ε) and 1 - O(ε log k) fraction of all constraints. Our
algorithms are based on rounding a natural semidefinite programming relaxation for the problem
and their performance almost matches the integrality gap of this relaxation. Our results are
near-optimal if the Unique Games Conjecture is true, i.e. any improvement (beyond low order terms)
would refute the conjecture.
We give O(sqrt{log n})-approximation algorithms for the Min UnCut,
Min 2CNF Deletion, Directed Balanced Separator, and Directed Sparsest
Cut problems. The previously best known algorithms give an O(log
n)-approximation for Min UnCut, Directed Balanced Separator, Directed
Sparsest Cut, and an O(log n log log n)-approximation for Min 2CNF
Deletion.
We also show that the integrality gap of an SDP relaxation of the
Minimum Multicut problem is Ω(log n).
We introduce a new graph parameter, called the Grothendieck constant of a graph.
This parameter is a generalization of the classical Grothendieck constant; and it is
equal to an integrality gap of a certain SDP problem, which has various algorithmic applications.
Our results improve a recent result of Kashin and Szarek on Gram matrices of uniformly bounded functions,
and settle a problem of Megretski and of Charikar and Wirth.
In this paper we investigate the notion of conditional independence and prove several information
inequalities for conditionally independent random variables.
Communications in Information and Systems, vol. 2, no. 2, pp. 147-166, December 2002,
In this paper we prove a countable set of non-Shannon-type linear information inequalities for entropies of discrete random variables,
i.e., information inequalities which cannot be
reduced to the "basic" inequality I(X : Y |Z) > 0. Our results generalize the inequalities of Z. Zhang
and R. Yeung (1998) who found the first examples of non-Shannon-type information inequalities.
We prove the conjecture from Igor Pak's paper
"The Invariants: New Horizons".
The conjecture states that any tiling of a rectangle by T-tetrominoes
can be transformed to any other tiling by a series of "local
moves". We also show connections between this problem and the "six-vertex
model" (studied in mathematical physics).