1,250 research outputs found
Applications of incidence bounds in point covering problems
In the Line Cover problem a set of n points is given and the task is to cover
the points using either the minimum number of lines or at most k lines. In
Curve Cover, a generalization of Line Cover, the task is to cover the points
using curves with d degrees of freedom. Another generalization is the
Hyperplane Cover problem where points in d-dimensional space are to be covered
by hyperplanes. All these problems have kernels of polynomial size, where the
parameter is the minimum number of lines, curves, or hyperplanes needed. First
we give a non-parameterized algorithm for both problems in O*(2^n) (where the
O*(.) notation hides polynomial factors of n) time and polynomial space,
beating a previous exponential-space result. Combining this with incidence
bounds similar to the famous Szemeredi-Trotter bound, we present a Curve Cover
algorithm with running time O*((Ck/log k)^((d-1)k)), where C is some constant.
Our result improves the previous best times O*((k/1.35)^k) for Line Cover
(where d=2), O*(k^(dk)) for general Curve Cover, as well as a few other bounds
for covering points by parabolas or conics. We also present an algorithm for
Hyperplane Cover in R^3 with running time O*((Ck^2/log^(1/5) k)^k), improving
on the previous time of O*((k^2/1.3)^k).Comment: SoCG 201
Mixing times of lozenge tiling and card shuffling Markov chains
We show how to combine Fourier analysis with coupling arguments to bound the
mixing times of a variety of Markov chains. The mixing time is the number of
steps a Markov chain takes to approach its equilibrium distribution. One
application is to a class of Markov chains introduced by Luby, Randall, and
Sinclair to generate random tilings of regions by lozenges. For an L X L region
we bound the mixing time by O(L^4 log L), which improves on the previous bound
of O(L^7), and we show the new bound to be essentially tight. In another
application we resolve a few questions raised by Diaconis and Saloff-Coste, by
lower bounding the mixing time of various card-shuffling Markov chains. Our
lower bounds are within a constant factor of their upper bounds. When we use
our methods to modify a path-coupling analysis of Bubley and Dyer, we obtain an
O(n^3 log n) upper bound on the mixing time of the Karzanov-Khachiyan Markov
chain for linear extensions.Comment: 39 pages, 8 figure
LHC constraints on M_1/2 and m_0 in the semi-constrained NMSSM
Constraints from searches for squarks and gluinos at the LHC at sqrt{s}=8 TeV
are applied to the parameter space of the NMSSM with universal squark/slepton
and gaugino masses at the GUT scale, but allowing for non-universal soft Higgs
mass parameters (the sNMSSM). We confine ourselves to regions of the parameter
space compatible with a 125 GeV Higgs boson with diphoton signal rates at least
as large as the Standard Model ones, and a dark matter candidate compatible
with WMAP and XENON100 constraints. Following the simulation of numerous points
in the m_0-M_{1/2} plane, we compare the constraints on the sNMSSM from 3-5
jets + missing E_T channels as well as from multijet + missing E_T channels
with the corresponding cMSSM constraints. Due to the longer squark decay
cascades, lower bounds on M_{1/2} are alleviated by up to 50 GeV. For heavy
squarks at large m_0, the dominant constraints originate from multijet +
missing E_T channels due to gluino decays via stop pairs.Comment: 18 pages, 2 Tables, 3 Figure
The same-sign top signature of R-parity violation
Baryonic R-parity violation could explain why low-scale supersymmetry has not
yet been discovered at colliders: sparticles would be hidden in the intense
hadronic activity. However, if the known flavor structures are any guide, the
largest baryon number violating couplings are those involving the top/stop, so
a copious production of same-sign top-quark pairs is in principle possible.
Such a signal, with its low irreducible background and efficient identification
through same-sign dileptons, provides us with tell-tale signs of baryon number
violating supersymmetry. Interestingly, this statement is mostly independent of
the details of the supersymmetric mass spectrum. So, in this paper, after
analyzing the sparticle decay chains and lifetimes, we formulate a simplified
benchmark strategy that covers most supersymmetric scenarios. We then use this
information to interpret the same-sign dilepton searches of CMS, draw
approximate bounds on the gluino and squark masses, and extrapolate the reach
of the future 14 TeV runs.Comment: 32 pages, 12 figures, 3 tables, 1 appendi
Positive Gorenstein Ideals
We introduce positive Gorenstein ideals. These are Gorenstein ideals in the
graded ring \RR[x] with socle in degree 2d, which when viewed as a linear
functional on \RR[x]_{2d} is nonnegative on squares. Equivalently, positive
Gorenstein ideals are apolar ideals of forms whose differential operator is
nonnegative on squares. Positive Gorenstein ideals arise naturally in the
context of nonnegative polynomials and sums of squares, and they provide a
powerful framework for studying concrete aspects of sums of squares
representations. We present applications of positive Gorenstein ideals in real
algebraic geometry, analysis and optimization. In particular, we present a
simple proof of Hilbert's nearly forgotten result on representations of ternary
nonnegative forms as sums of squares of rational functions. Drawing on our
previous work, our main tools are Cayley-Bacharach duality and elementary
convex geometry
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