10,532 research outputs found
Evolving small-world networks with geographical attachment preference
We introduce a minimal extended evolving model for small-world networks which
is controlled by a parameter. In this model the network growth is determined by
the attachment of new nodes to already existing nodes that are geographically
close. We analyze several topological properties for our model both
analytically and by numerical simulations. The resulting network shows some
important characteristics of real-life networks such as the small-world effect
and a high clustering.Comment: 11 pages, 4 figure
On the path of the Gulf Stream and the Atlantic meridional overturning circulation
Author Posting. © American Meteorological Society, 2010. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 23 (2010): 3146–3154, doi:10.1175/2010JCLI3310.1.The Atlantic meridional overturning circulation (AMOC) simulated in various ocean-only and coupled atmosphere–ocean numerical models often varies in time because of either forced or internal variability. The path of the Gulf Stream (GS) is one diagnostic variable that seems to be sensitive to the amplitude of the AMOC, yet previous modeling studies show a diametrically opposed relationship between the two variables. In this note this issue is revisited, bringing together ocean observations and comparisons with the GFDL Climate Model version 2.1 (CM2.1), both of which suggest a more southerly (northerly) GS path when the AMOC is relatively strong (weak). Also shown are some examples of possible diagnostics to compare various models and observations on the relationship between shifts in GS path and changes in AMOC strength in future studies.We wish to acknowledge support
(TJ) from WHOI’s Paul Fye Chair and NASA
(NNXZX09AF35G) and to NOAA/OAR (RZ) for this
work
Mathematical Modeling of Arterial Blood Pressure Using Photo-Plethysmography Signal in Breath-hold Maneuver
Recent research has shown that each apnea episode results in a significant
rise in the beat-to-beat blood pressure and by a drop to the pre-episode levels
when patient resumes normal breathing. While the physiological implications of
these repetitive and significant oscillations are still unknown, it is of
interest to quantify them. Since current array of instruments deployed for
polysomnography studies does not include beat-to-beat measurement of blood
pressure, but includes oximetry, it is both of clinical interest to estimate
the magnitude of BP oscillations from the photoplethysmography (PPG) signal
that is readily available from sleep lab oximeters. We have investigated a new
method for continuous estimation of systolic (SBP), diastolic (DBP), and mean
(MBP) blood pressure waveforms from PPG. Peaks and troughs of PPG waveform are
used as input to a 5th order autoregressive moving average model to construct
estimates of SBP, DBP, and MBP waveforms. Since breath hold maneuvers are shown
to simulate apnea episodes faithfully, we evaluated the performance of the
proposed method in 7 subjects (4 F; 32+-4 yrs., BMI 24.57+-3.87 kg/m2) in
supine position doing 5 breath maneuvers with 90s of normal breathing between
them. The modeling error ranges were (all units are in mmHg) -0.88+-4.87 to
-2.19+-5.73 (SBP); 0.29+-2.39 to -0.97+-3.83 (DBP); and -0.42+-2.64 to
-1.17+-3.82 (MBP). The cross validation error ranges were 0.28+-6.45 to
-1.74+-6.55 (SBP); 0.09+-3.37 to -0.97+-3.67 (DBP); and 0.33+-4.34 to
-0.87+-4.42 (MBP). The level of estimation error in, as measured by the root
mean squared of the model residuals, was less than 7 mmHgComment: 4 pages, published in 2018 40th Annual International Conference of
the IEEE Engineering in Medicine and Biology Society (EMBC
Long-term ionospheric cooling: Dependency on local time, season, solar activity, and geomagnetic activity
Ionospheric ion temperature Ti is an excellent approximation to neutral temperature Tn in the thermosphere, especially for altitudes below 300 km. This analysis of long‒term Ti trends in the F region over different local times is based on a database of incoherent scatter radar (ISR) observations spanning more than three solar cycles during 1968–2006 at Millstone Hill and represents an extended effort to a prior study focusing on noon‒time only. This study provides important information for understanding the difference between the ISR and other results. A gross average of the Ti trend at heights of Ti ∼ Tn (200–350 km) is ∼ −4 K/decade, a cooling trend close to the Tn estimation based on the satellite neutral density data. However, there exists considerable variability in the cooling: it is strong during the day and very weak during the night with a large apparent warming at low altitudes (200–350 km); it is strong at solar minimum for both daytime and nighttime. The strongest cooling for altitudes below 375 km occurs around 90–120 solar flux units of the 10.7 cm solar flux, not at the lowest solar flux. There appears more cooling toward high magnetic activity, but this dependency is very weak. No consistent and substantial seasonal dependency across different heights was found. We speculate that a fraction of the observed cooling trend may be contributed by a gradual shifting away from the sub‒auroral region at Millstone Hill, as part of the secular change in the Earth's magnetic field. In this 39 year long series of data record, two anomalous Ti drops were noticed, and we speculate on their connection to volcano eruptions in 1982 and 1991.National Science Foundation (U.S.) (Award AGS-1042569
Magnetic Field Effect on the Phase Transition in AdS Soliton Spacetime
We investigate the scalar perturbations in an AdS soliton background coupled
to a Maxwell field via marginally stable modes. In the probe limit, we study
the magnetic field effect on the holographic insulator/superconductor phase
transition numerically and analytically. The condensate will be localized in a
finite circular region for any finite constant magnetic field. Near the
critical point, we find that there exists a simple relation among the critical
chemical potential, magnetic field, the charge and mass of the scalar field.
This relation indicates that the presence of the magnetic field causes the
phase transition hard.Comment: 15 pages, 3 figures, 2 tables. contents improved and references adde
Holographic Superconductors with Ho\v{r}ava-Lifshitz Black Holes
We discuss the phase transition of planar black holes in Ho\v{r}ava-Lifshitz
gravity by introducing a Maxwell field and a complex scalar field. We calculate
the condensates of the charged operators in the dual CFTs when the mass square
of the complex scalar filed is and , respectively. We
compute the electrical conductivity of the \hl superconductor in the probe
approximation. In particular, it is found that there exists a spike in the
conductivity for the case of the operator with scaling dimension one. These
results are quite similar to those in the case of Schwarzschild-AdS black
holes, which demonstrates that the holographic superconductivity is a robust
phenomenon associated with asymptotic AdS black holes.Comment: 12 pages, 7 figures,refs adde
New molecular candidates: X(1910), X(2200), and X(2350)
Assuming the newly observed resonant structures X(1910), X(2200), and X(2350)
as , , and molecular states respectively,
we compute their mass values in the framework of QCD sum rules. The numerical
results are for state,
for state, and for state, which
coincide with the experimental values of X(1910), X(2200), and X(2350),
respectively. This supports the statement that X(1910), X(2200), and X(2350)
could be , , and molecular candidates
respectively.Comment: 9 pages, 9 eps figures; the name of X(2000) changed to X(1910)
according to the updated data of experiments; more references and discussions
added; accepted for publication in PRD. arXiv admin note: substantial text
overlap with arXiv:1211.2277, arXiv:1201.341
High Dimensional Apollonian Networks
We propose a simple algorithm which produces high dimensional Apollonian
networks with both small-world and scale-free characteristics. We derive
analytical expressions for the degree distribution, the clustering coefficient
and the diameter of the networks, which are determined by their dimension
Remarks on endomorphisms and rational points
Let X be a variety over a number field and let f: X --> X be an "interesting"
rational self-map with a fixed point q. We make some general remarks concerning
the possibility of using the behaviour of f near q to produce many rational
points on X. As an application, we give a simplified proof of the potential
density of rational points on the variety of lines of a cubic fourfold
(originally obtained by Claire Voisin and the first author in 2007).Comment: LaTeX, 22 pages. v2: some minor observations added, misprints
corrected, appendix modified
Transceiver Optimization for Wireless Powered Time-Division Duplex MU-MIMO Systems: Non-Robust and Robust Designs
Wireless powered communication (WPC) has been considered as one of the key technologies in the Internet of Things (IoT) applications. In this paper, we study a wireless powered time-division duplex (TDD) multiuser multiple-input multiple-output (MU-MIMO) system, where the base station (BS) has its own power supply and all users can harvest radio frequency (RF) energy from the BS. We aim to maximize the users' information rates by jointly optimizing the duration of users' time slots and the signal covariance matrices of the BS and users. Different to the commonly used sum rate and max-min rate criteria, the proportional fairness of users' rates is considered in the objective function. We first study the ideal case with the perfect channel state information (CSI), and show that the non-convex proportionally fair rate optimization problem can be transformed into an equivalent convex optimization problem. Then we consider practical systems with imperfect CSI, where the CSI mismatch follows a Gaussian distribution. A chance-constrained robust system design is proposed for this scenario, where the Bernstein inequality is applied to convert the chance constraints into the convex constraints. Finally, we consider a more general case where only partial knowledge of the CSI mismatch is available. In this case, the conditional value-at-risk (CVaR) method is applied to solve the distributionally robust system rate optimization problem. Simulation results are presented to show the effectiveness of the proposed algorithms
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