28 research outputs found
Certifying solution geometry in random CSPs: counts, clusters and balance
An active topic in the study of random constraint satisfaction problems
(CSPs) is the geometry of the space of satisfying or almost satisfying
assignments as the function of the density, for which a precise landscape of
predictions has been made via statistical physics-based heuristics. In
parallel, there has been a recent flurry of work on refuting random constraint
satisfaction problems, via nailing refutation thresholds for spectral and
semidefinite programming-based algorithms, and also on counting solutions to
CSPs. Inspired by this, the starting point for our work is the following
question: what does the solution space for a random CSP look like to an
efficient algorithm?
In pursuit of this inquiry, we focus on the following problems about random
Boolean CSPs at the densities where they are unsatisfiable but no refutation
algorithm is known.
1. Counts. For every Boolean CSP we give algorithms that with high
probability certify a subexponential upper bound on the number of solutions. We
also give algorithms to certify a bound on the number of large cuts in a
Gaussian-weighted graph, and the number of large independent sets in a random
-regular graph.
2. Clusters. For Boolean CSPs we give algorithms that with high
probability certify an upper bound on the number of clusters of solutions.
3. Balance. We also give algorithms that with high probability certify that
there are no "unbalanced" solutions, i.e., solutions where the fraction of
s deviates significantly from .
Finally, we also provide hardness evidence suggesting that our algorithms for
counting are optimal
New Notions and Constructions of Sparsification for Graphs and Hypergraphs
A sparsiļ¬er of a graph G (BenczuĀ“r and Karger; Spielman and Teng) is a sparse weighted subgraph Ė G that approximately retains the same cut structure of G. For general graphs, non-trivial sparsiļ¬cation is possible only by using weighted graphs in which diļ¬erent edges have diļ¬erent weights. Even for graphs that admit unweighted sparsiļ¬ers (that is, sparsiļ¬ers in which all the edge weights are equal to the same scaling factor), there are no known polynomial time algorithms that ļ¬nd such unweighted sparsiļ¬ers. We study a weaker notion of sparsiļ¬cation suggested by Oveis Gharan, in which the number of cut edges in each cut (S, ĀÆ S) is not approximated within a multiplicative factor (1 + Ē«), but is, instead, approximated up to an additive term bounded by Ē« times d Ā· |S| + vol(S), where d is the average
New notions and constructions of sparsification for graphs and hypergraphs
No abstract availabl
Combinatorics
Combinatorics is a fundamental mathematical discipline that focuses on the study of discrete objects and their
properties. The present workshop featured research in such diverse areas as Extremal, Probabilistic
and Algebraic Combinatorics, Graph Theory, Discrete Geometry, Combinatorial Optimization,
Theory of Computation and Statistical Mechanics. It provided current accounts of exciting developments and challenges in these fields and a stimulating venue for a variety of fruitful interactions.
This is a report on the meeting, containing extended abstracts of the presentations and a summary of the problem session
Metastability cascades and prewetting in the SOS model
We study Glauber dynamics for the low temperature D Solid-On-Solid
model on a box of side-length with a floor at height (inducing entropic
repulsion) and a competing bulk external field pointing down (the
prewetting problem). In 1996, Cesi and Martinelli showed that if the
inverse-temperature is large enough, then along a decreasing sequence
of critical points the dynamics is torpid:
its inverse spectral gap is when whereas it is at each
for each , due to a coexistence of rigid
phases at heights and . Our focus is understanding (a) the onset of
metastability as ; and (b) the effect of an
unbounded number of layers, as we remove the restriction , and
even allow for towards the case which has
layers and was studied by Caputo et al. (2014). We show that for
any , possibly growing with , the inverse gap is
as up to distance from this critical point, due to
a metastable layer at height on the way to forming the desired layer at
height . By taking (corresponding to ), this also interpolates down to the behavior of the dynamics when
. We compliment this by extending the fast mixing to all
uniformly bounded away from . Together, these
results provide a sharp understanding of the predicted infinite sequence of
dynamical phase transitions governed by the layering phenomenon.Comment: 49 pages, 3 figure