24,985 research outputs found
On the extreme eigenvalues of regular graphs
In this paper, we present an elementary proof of a theorem of Serre
concerning the greatest eigenvalues of -regular graphs. We also prove an
analogue of Serre's theorem regarding the least eigenvalues of -regular
graphs: given , there exist a positive constant
and a nonnegative integer such that for any -regular graph
with no odd cycles of length less than , the number of eigenvalues
of such that is at least . This
implies a result of Winnie Li.Comment: accepted to J.Combin.Theory, Series B. added 5 new references, some
comments on the constant c in Section
How would GW150914 look with future GW detector networks?
The first detected gravitational wave signal, GW150914, was produced by the
coalescence of a stellar-mass binary black hole. Along with the subsequent
detection of GW151226, GW170104 and the candidate event LVT151012, this gives
us evidence for a population of black hole binaries with component masses in
the tens of solar masses. As detector sensitivity improves, this type of source
is expected to make a large contribution to the overall number of detections,
but has received little attention compared to binary neutron star systems in
studies of projected network performance. We simulate the observation of a
system like GW150914 with different proposed network configurations, and study
the precision of parameter estimates, particularly source location, orientation
and masses. We find that the improvements to low frequency sensitivity that are
expected with continued commissioning will improve the precision of chirp mass
estimates by an order of magnitude, whereas the improvements in sky location
and orientation are driven by the expanded network configuration. This
demonstrates that both sensitivity and number of detectors will be important
factors in the scientific potential of second generation detector networks.Comment: 18 pages, 5 figures, 2 table
SS433's circumbinary ring and accretion disc viewed through its attenuating disc wind
We present optical spectroscopy of the microquasar SS433 covering a
significant fraction of a precessional cycle of its jet axis. The components of
the prominent stationary H-alpha and H-beta lines are mainly identified as
arising from three emitting regions: (i) a super-Eddington accretion disc wind,
in the form of a broad component accounting for most of the mass loss from the
system, (ii) a circumbinary disc of material that we presume is being excreted
through the binary's L2 point, and (iii) the accretion disc itself as two
remarkably persistent components. The accretion disc components move with a
Keplerian velocity of ~600 km/s in the outer region of the disc. A direct
result of this decomposition is the determination of the accretion disc size,
whose outer radius attains ~8 R_sun in the case of Keplerian orbits around a
black hole mass of 10 M_sun. We determine an upper limit for the accretion disc
inner to outer radius ratio in SS433, R_in/R_out ~ 0.2, independent of the mass
of the compact object. The Balmer decrements, H-alpha/H-beta, are extracted
from the appropriate stationary emission lines for each component of the
system. The physical parameters of the gaseous components are derived. The
circumbinary ring decrement seems to be quite constant throughout precessional
phase, implying a constant electron density of log N_e(cm^-3) ~ 11.5 for the
circumbinary disc. The accretion disc wind shows a larger change in its
decrements exhibiting a clear dependence on precessional phase, implying a
sinusoid variation in its electron density log N_e(cm^-3) along our
line-of-sight between 10 and 13. This dependence of density on direction
suggests that the accretion disc wind is polloidal in nature.Comment: 7 pages, 5 figures. Accepted for publication in MNRAS Main Journal
Using GWAS Data to Identify Copy Number Variants Contributing to Common Complex Diseases
Copy number variants (CNVs) account for more polymorphic base pairs in the
human genome than do single nucleotide polymorphisms (SNPs). CNVs encompass
genes as well as noncoding DNA, making these polymorphisms good candidates for
functional variation. Consequently, most modern genome-wide association studies
test CNVs along with SNPs, after inferring copy number status from the data
generated by high-throughput genotyping platforms. Here we give an overview of
CNV genomics in humans, highlighting patterns that inform methods for
identifying CNVs. We describe how genotyping signals are used to identify CNVs
and provide an overview of existing statistical models and methods used to
infer location and carrier status from such data, especially the most commonly
used methods exploring hybridization intensity. We compare the power of such
methods with the alternative method of using tag SNPs to identify CNV carriers.
As such methods are only powerful when applied to common CNVs, we describe two
alternative approaches that can be informative for identifying rare CNVs
contributing to disease risk. We focus particularly on methods identifying de
novo CNVs and show that such methods can be more powerful than case-control
designs. Finally we present some recommendations for identifying CNVs
contributing to common complex disorders.Comment: Published in at http://dx.doi.org/10.1214/09-STS304 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Optimal redundancy against disjoint vulnerabilities in networks
Redundancy is commonly used to guarantee continued functionality in networked
systems. However, often many nodes are vulnerable to the same failure or
adversary. A "backup" path is not sufficient if both paths depend on nodes
which share a vulnerability.For example, if two nodes of the Internet cannot be
connected without using routers belonging to a given untrusted entity, then all
of their communication-regardless of the specific paths utilized-will be
intercepted by the controlling entity.In this and many other cases, the
vulnerabilities affecting the network are disjoint: each node has exactly one
vulnerability but the same vulnerability can affect many nodes. To discover
optimal redundancy in this scenario, we describe each vulnerability as a color
and develop a "color-avoiding percolation" which uncovers a hidden
color-avoiding connectivity. We present algorithms for color-avoiding
percolation of general networks and an analytic theory for random graphs with
uniformly distributed colors including critical phenomena. We demonstrate our
theory by uncovering the hidden color-avoiding connectivity of the Internet. We
find that less well-connected countries are more likely able to communicate
securely through optimally redundant paths than highly connected countries like
the US. Our results reveal a new layer of hidden structure in complex systems
and can enhance security and robustness through optimal redundancy in a wide
range of systems including biological, economic and communications networks.Comment: 15 page
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