2,601 research outputs found
Three flavors of extremal Betti tables
We discuss extremal Betti tables of resolutions in three different contexts.
We begin over the graded polynomial ring, where extremal Betti tables
correspond to pure resolutions. We then contrast this behavior with that of
extremal Betti tables over regular local rings and over a bigraded ring.Comment: 20 page
Description of stochastic and chaotic series using visibility graphs
Nonlinear time series analysis is an active field of research that studies
the structure of complex signals in order to derive information of the process
that generated those series, for understanding, modeling and forecasting
purposes. In the last years, some methods mapping time series to network
representations have been proposed. The purpose is to investigate on the
properties of the series through graph theoretical tools recently developed in
the core of the celebrated complex network theory. Among some other methods,
the so-called visibility algorithm has received much attention, since it has
been shown that series correlations are captured by the algorithm and
translated in the associated graph, opening the possibility of building
fruitful connections between time series analysis, nonlinear dynamics, and
graph theory. Here we use the horizontal visibility algorithm to characterize
and distinguish between correlated stochastic, uncorrelated and chaotic
processes. We show that in every case the series maps into a graph with
exponential degree distribution P (k) ~ exp(-{\lambda}k), where the value of
{\lambda} characterizes the specific process. The frontier between chaotic and
correlated stochastic processes, {\lambda} = ln(3/2), can be calculated
exactly, and some other analytical developments confirm the results provided by
extensive numerical simulations and (short) experimental time series
Robot-assisted Optical Ultrasound Scanning
Optical ultrasound, where ultrasound is both generated and received using light, can be integrated in very small diameter instruments making it ideally suited to minimally invasive interventions. One-dimensional information can be obtained using a single pair of optical fibres comprising of a source and detector but this can be difficult to interpret clinically. In this paper, we present a robotic-assisted scanning solution where a concentric tube robot manipulates an optical ultrasound probe along a consistent trajectory. A torque coil is utilised as a buffer between the curved nitinol tube and the probe to prevent torsion on the probe and maintain the axial orientation of the probe while the tube is rotating. The design and control of the scanning mechanism are presented along with the integration of the mechanism with a fibre-based imaging probe. Trajectory repeatability is assessed using electromagnetic tracking and a technique to calibrate the transformation between imaging and robot coordinates using a known model is presented. Finally, we show example images of 3D printed phantoms generated by collecting multiple OpUS A-scans within the same 3D scene to illustrate how robot-assisted scanning can expand the field of view
Patterns of Individual Shopping Behavior
Much of economic theory is built on observations of aggregate, rather than
individual, behavior. Here, we present novel findings on human shopping
patterns at the resolution of a single purchase. Our results suggest that much
of our seemingly elective activity is actually driven by simple routines. While
the interleaving of shopping events creates randomness at the small scale, on
the whole consumer behavior is largely predictable. We also examine
income-dependent differences in how people shop, and find that wealthy
individuals are more likely to bundle shopping trips. These results validate
previous work on mobility from cell phone data, while describing the
unpredictability of behavior at higher resolution.Comment: 4 pages, 5 figure
Conductance Through Graphene Bends and Polygons
We investigate the transmission of electrons between conducting nanoribbon
leads oriented at multiples of 60 degrees with respect to one another,
connected either directly or through graphene polygons. A mode-matching
analysis suggests that the transmission at low-energies is sensitive to the
precise way in which the ribbons are joined. Most strikingly, we find that
armchair leads forming 120-degree angles can support either a large
transmission or a highly suppressed transmission, depending on the specific
geometry. Tight-binding calculations demonstrate the effects in detail, and are
also used to study transmission at higher energies as well as for zigzag ribbon
leads.Comment: 14 pages, 21 figure
Absolutely Koszul algebras and the Backelin-Roos property
We study absolutely Koszul algebras, Koszul algebras with the Backelin-Roos
property and their behavior under standard algebraic operations. In particular,
we identify some Veronese subrings of polynomial rings that have the
Backelin-Roos property and conjecture that the list is indeed complete. Among
other things, we prove that every universally Koszul ring defined by monomials
has the Backelin-Roos property
Discrete Model of Ideological Struggle Accounting for Migration
A discrete in time model of ideological competition is formulated taking into
account population migration. The model is based on interactions between global
populations of non-believers and followers of different ideologies. The complex
dynamics of the attracting manifolds is investigated.
Conversion from one ideology to another by means of (i) mass media influence
and (ii) interpersonal relations is considered. Moreover a different birth rate
is assumed for different ideologies, the rate being assumed to be positive for
the reference population, made of initially non-believers. Ideological
competition can happen in one or several regions in space. In the latter case,
migration of non-believers and adepts is allowed; this leads to an enrichment
of the ideological dynamics. Finally, the current ideological situation in the
Arab countries and China is commented upon from the point of view of the
presently developed mathematical model. The massive forced conversion by
Ottoman Turks in the Balkans is briefly discussed.Comment: 24 pages, with 5 figures and 52 refs.; prepared for a Special issue
of Advances in Complex System
First-principles data for solid-solution strengthening of magnesium: From geometry and chemistry to properties
Solid-solution strengthening results from solutes impeding the glide of
dislocations. Existing theories of strength rely on solute-dislocation
interactions, but do not consider dislocation core structures, which need an
accurate treatment of chemical bonding. Here, we focus on strengthening of Mg,
the lightest of all structural metals and a promising replacement for heavier
steel and aluminum alloys. Elasticity theory, which is commonly used to predict
the requisite solute-dislocation interaction energetics, is replaced with
quantum-mechanical first-principles calculations to construct a predictive
mesoscale model for solute strengthening of Mg. Results for 29 different
solutes are displayed in a "strengthening design map" as a function of solute
misfits that quantify volumetric strain and slip effects. Our strengthening
model is validated with available experimental data for several solutes,
including Al and Zn, the two most common solutes in Mg. These new results
highlight the ability of quantum-mechanical first-principles calculations to
predict complex material properties such as strength.Comment: 9 pages, 7 figures, 2 table
Assessment of Marine Weather forecasts over the Indian Sector of Southern Ocean
The Southern Ocean (SO) is one of the important regions where significant processes and feedbacks of the Earth\u27s climate take place. Expeditions to the SO provide useful data for improving global weather/climate simulations and understanding many processes. Some of the uncertainties in these weather/climate models arise during the first few days of simulation/forecast and do not grow much further. NCMRWF issued real-time five day weather forecasts of mean sea level pressure, surface winds, winds at 500 hPa & 850 hPa and rainfall, daily to NCAOR to provide guidance for their expedition to Indian sector of SO during the austral summer of 2014–2015. Evaluation of the skill of these forecasts indicates possible error growth in the atmospheric model at shorter time scales. The error growth is assessed using the model analysis/reanalysis, satellite data and observations made during the expedition. The observed variability of sub-seasonal rainfall associated with mid-latitude systems is seen to exhibit eastward propagations and are well reproduced in the model forecasts. All cyclonic disturbances including the sub-polar lows and tropical cyclones that occurred during this period were well captured in the model forecasts. Overall, this model performs reasonably well over the Indian sector of the SO in medium range time scale
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