6,995 research outputs found
Two-Source Dispersers for Polylogarithmic Entropy and Improved Ramsey Graphs
In his 1947 paper that inaugurated the probabilistic method, Erd\H{o}s proved
the existence of -Ramsey graphs on vertices. Matching Erd\H{o}s'
result with a constructive proof is a central problem in combinatorics, that
has gained a significant attention in the literature. The state of the art
result was obtained in the celebrated paper by Barak, Rao, Shaltiel and
Wigderson [Ann. Math'12], who constructed a
-Ramsey graph, for some small universal
constant .
In this work, we significantly improve the result of Barak~\etal and
construct -Ramsey graphs, for some universal constant .
In the language of theoretical computer science, our work resolves the problem
of explicitly constructing two-source dispersers for polylogarithmic entropy
Intrinsic Optical and Electronic Properties from Quantitative Analysis of Plasmonic Semiconductor Nanocrystal Ensemble Optical Extinction
The optical extinction spectra arising from localized surface plasmon
resonance in doped semiconductor nanocrystals (NCs) have intensities and
lineshapes determined by free charge carrier concentrations and the various
mechanisms for damping the oscillation of those free carriers. However, these
intrinsic properties are convoluted by heterogeneous broadening when measuring
spectra of ensembles. We reveal that the traditional Drude approximation is not
equipped to fit spectra from a heterogeneous ensemble of doped semiconductor
NCs and produces fit results that violate Mie scattering theory. The
heterogeneous ensemble Drude approximation (HEDA) model rectifies this issue by
accounting for ensemble heterogeneity and near-surface depletion. The HEDA
model is applied to tin-doped indium oxide NCs for a range of sizes and doping
levels but we expect it can be employed for any isotropic plasmonic particles
in the quasistatic regime. It captures individual NC optical properties and
their contributions to the ensemble spectra thereby enabling the analysis of
intrinsic NC properties from an ensemble measurement. Quality factors for the
average NC in each ensemble are quantified and found to be notably higher than
those of the ensemble. Carrier mobility and conductivity derived from HEDA fits
matches that measured in the bulk thin film literature
Error-prone polymerase activity causes multinucleotide mutations in humans
About 2% of human genetic polymorphisms have been hypothesized to arise via
multinucleotide mutations (MNMs), complex events that generate SNPs at multiple
sites in a single generation. MNMs have the potential to accelerate the pace at
which single genes evolve and to confound studies of demography and selection
that assume all SNPs arise independently. In this paper, we examine clustered
mutations that are segregating in a set of 1,092 human genomes, demonstrating
that MNMs become enriched as large numbers of individuals are sampled. We
leverage the size of the dataset to deduce new information about the allelic
spectrum of MNMs, estimating the percentage of linked SNP pairs that were
generated by simultaneous mutation as a function of the distance between the
affected sites and showing that MNMs exhibit a high percentage of transversions
relative to transitions. These findings are reproducible in data from multiple
sequencing platforms. Among tandem mutations that occur simultaneously at
adjacent sites, we find an especially skewed distribution of ancestral and
derived dinucleotides, with , and their reverse complements making up 36% of the total. These
same mutations dominate the spectrum of tandem mutations produced by the
upregulation of low-fidelity Polymerase in mutator strains of S.
cerevisiae that have impaired DNA excision repair machinery. This suggests that
low-fidelity DNA replication by Pol is at least partly responsible for
the MNMs that are segregating in the human population, and that useful
information about the biochemistry of MNM can be extracted from ordinary
population genomic data. We incorporate our findings into a mathematical model
of the multinucleotide mutation process that can be used to correct
phylogenetic and population genetic methods for the presence of MNMs
Rounds vs Communication Tradeoffs for Maximal Independent Sets
We consider the problem of finding a maximal independent set (MIS) in the
shared blackboard communication model with vertex-partitioned inputs. There are
players corresponding to vertices of an undirected graph, and each player
sees the edges incident on its vertex -- this way, each edge is known by both
its endpoints and is thus shared by two players. The players communicate in
simultaneous rounds by posting their messages on a shared blackboard visible to
all players, with the goal of computing an MIS of the graph. While the MIS
problem is well studied in other distributed models, and while shared
blackboard is, perhaps, the simplest broadcast model, lower bounds for our
problem were only known against one-round protocols.
We present a lower bound on the round-communication tradeoff for computing an
MIS in this model. Specifically, we show that when rounds of interaction
are allowed, at least one player needs to communicate
bits. In particular, with logarithmic bandwidth, finding an MIS requires
rounds. This lower bound can be compared with the
algorithm of Ghaffari, Gouleakis, Konrad, Mitrovi\'c, and Rubinfeld [PODC 2018]
that solves MIS in rounds but with a logarithmic bandwidth for
an average player. Additionally, our lower bound further extends to the closely
related problem of maximal bipartite matching.
To prove our results, we devise a new round elimination framework, which we
call partial-input embedding, that may also be useful in future work for
proving round-sensitive lower bounds in the presence of edge-sharing between
players.
Finally, we discuss several implications of our results to multi-round
(adaptive) distributed sketching algorithms, broadcast congested clique, and to
the welfare maximization problem in two-sided matching markets.Comment: Full version of the paper in FOCS 2022, 44 page
Bayesian clustering in decomposable graphs
In this paper we propose a class of prior distributions on decomposable
graphs, allowing for improved modeling flexibility. While existing methods
solely penalize the number of edges, the proposed work empowers practitioners
to control clustering, level of separation, and other features of the graph.
Emphasis is placed on a particular prior distribution which derives its
motivation from the class of product partition models; the properties of this
prior relative to existing priors is examined through theory and simulation. We
then demonstrate the use of graphical models in the field of agriculture,
showing how the proposed prior distribution alleviates the inflexibility of
previous approaches in properly modeling the interactions between the yield of
different crop varieties.Comment: 3 figures, 1 tabl
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