REBOUND: An open-source multi-purpose N-body code for collisional dynamics

REBOUND is a new multi-purpose N-body code which is freely available under an open-source license. It was designed for collisional dynamics such as planetary rings but can also solve the classical N-body problem. It is highly modular and can be customized easily to work on a wide variety of different problems in astrophysics and beyond. REBOUND comes with three symplectic integrators: leap-frog, the symplectic epicycle integrator (SEI) and a Wisdom-Holman mapping (WH). It supports open, periodic and shearing-sheet boundary conditions. REBOUND can use a Barnes-Hut tree to calculate both self-gravity and collisions. These modules are fully parallelized with MPI as well as OpenMP. The former makes use of a static domain decomposition and a distributed essential tree. Two new collision detection modules based on a plane-sweep algorithm are also implemented. The performance of the plane-sweep algorithm is superior to a tree code for simulations in which one dimension is much longer than the other two and in simulations which are quasi-two dimensional with less than one million particles. In this work, we discuss the different algorithms implemented in REBOUND, the philosophy behind the code's structure as well as implementation specific details of the different modules. We present results of accuracy and scaling tests which show that the code can run efficiently on both desktop machines and large computing clusters.Comment: 10 pages, 9 figures, accepted by A&A, source code available at https://github.com/hannorein/reboun

Sheltering the Dream and Protecting the Dreamer : The Role of Place and Space in the Online Interactions of Fiction Authors and Readers

Fiction authors and readers have traditionally interacted through the mediation of a 3rd party (e.g. a publisher or agent), at events such as book signings or author readings. Held in physical spaces (e.g. bookshops or libraries), these events enable authors to discuss their book, and readers to ask them questions. In recent years, online social networking sites have introduced a new environment for direct, two-way interactions without this traditional mediation. Our understanding of how this change impacts authors and readers, and the role technology now plays as mediator, is currently limited. This paper describes a qualitative interview study held with six authors and six readers of Genre Fiction. The study revealed that neither party sees great benefit to interacting directly online - a finding partially explainable by the differences in how physical places and online spaces are structured to support their interactions. We drew on space and place research to develop an HCI perspective of the impact of this change. This paper contributes an enriched understanding of fiction author and reader interactions; in particular why they do not often interact directly - or wish to. We also demonstrate the usefulness of space and place theory in understanding the boundaries which divide author and reader

Asteroids in the Inner Solar System I - Existence

Ensembles of in-plane and inclined orbits in the vicinity of the Lagrange points of the terrestrial planets are integrated for up to 100 million years. The integrations incorporate the gravitational effects of Sun and the eight planets (Pluto is neglected). Mercury is the least likely planet, as it is unable to retain tadpole orbits over 100 million year timescales. Both Venus and the Earth are much more promising, as they possess rich families of stable tadpole and horseshoe orbits. Our survey of Trojans in the orbital plane of Venus is undertaken for 25 million years. Some 40% of the survivors are on tadpole orbits. For the Earth, the integrations are pursued for 50 million years. The stable zones in the orbital plane are larger for the Earth than for Venus, but fewer of the survivors are tadpoles. Both Venus and the Earth also have regions in which inclined test particles can endure near the Lagrange points. For Venus, only test particles close to the orbital plane are stable. For the Earth, there are two bands of stability, one at low inclinations (i < 16 degrees) and one at moderate inclinations (between 24 degrees and 34 degrees). The inclined test particles that evade close encounters are primarily moving on tadpole orbits. Our survey of in-plane test particles near the Martian Lagrange points shows no survivors after 60 million years. Low inclination test particles do not persist, as their inclinations are quickly increased until the effects of a secular resonance with Jupiter cause de-stabilisation. Numerical integrations of inclined test particles for timespans of 25 million years show stable zones for inclinations between 14 and 40 degrees.Comment: 20 pages, 21 figures, Monthly Notices (in press

The role of chaotic resonances in the solar system

Our understanding of the Solar System has been revolutionized over the past decade by the finding that the orbits of the planets are inherently chaotic. In extreme cases, chaotic motions can change the relative positions of the planets around stars, and even eject a planet from a system. Moreover, the spin axis of a planet-Earth's spin axis regulates our seasons-may evolve chaotically, with adverse effects on the climates of otherwise biologically interesting planets. Some of the recently discovered extrasolar planetary systems contain multiple planets, and it is likely that some of these are chaotic as well.Comment: 28 pages, 9 figure

Factors Influencing Nigeria’s Trade

This paper examined factor influencing Nigeria’s trade with the rest of the world using standard time series analysis technique on annual data spanning 1981 to 2012. Focus was on industrial and agricultural production. The stylized facts showed the increasing neglect the non-oil sector has suffered over the years. On average, oil trade dominates total trade and is about three times non-oil trade. Oil trade is also more volatile, about 2 times more volatile than non-oil trade. Also, oil balance of trade is comparatively two times more unstable. Nigeria’s export on average has been larger and more volatile than import due to the dominance of oil-export. All the variables were I(1) and cointegrated. The error correction results showed that in industrial output growth is more important in explaining balance of trade and total trade adjustments to equilibrium. This could be misleading, because the Nigerian industrial sector is dominated by activities in the petroleum (oil) sector which predominantly is an extractive industry. This implies primary product trade drives the Nigerian economy. This underscores why fluctuations in the crude oil market has had significant effect on the Nigerian economy in time past and in contemporary times. It was concluded that Nigeria’s trade pattern does not conform to the Hescher-Ohlin theory of factor endowment. Amongst others, it was recommended that the Nigerian state should come up with a consumption theory, where citizens and foreign nationals domiciled in the country are encouraged to consume more of made – in – Nigeria goods. Keywords: Factors, Nigeria, oil, non-oil, Trade JEL Classification: C22 F1

Asteroids in the Inner Solar System II - Observable Properties

This paper presents synthetic observations of long-lived, coorbiting asteroids of Mercury, Venus, the Earth and Mars. Our sample is constructed by taking the limiting semimajor axes, differential longitudes and inclinations for long-lived stability provided by simulations. The intervals are randomly populated with values to create initial conditions. These orbits are re-simulated to check that they are stable and then re-sampled every 2.5 years for 1 million years. The Mercurian sample contains only horseshoe orbits, the Martian sample only tadpoles. For both Venus and the Earth, the greatest concentration of objects on the sky occurs close to the classical Lagrange points at heliocentric ecliptic longitudes of 60 and 300 degrees. The distributions are broad especially if horseshoes are present in the sample. The full-width half maximum (FWHM) in heliocentric longitude for Venus is 325 degrees and for the Earth is 328 degrees. The mean and most common velocity of these coorbiting satellites coincides with the mean motion of the parent planet, but again the spread is wide with a FWHM for Venus of 27.8 arcsec/hr and for the Earth of 21.0 arcsec/hr. For Mars, the greatest concentration on the sky occurs at heliocentric ecliptic latitudes of 12 degrees. The peak of the velocity distribution occurs at 65 arcsec/hr, significantly less than the Martian mean motion, while its FWHM is 32.3 arcsec/hr. The case of Mercury is the hardest of all, as the greatest concentration occurs at heliocentric longitudes close to the Sun.Comment: 16 pages, 11 figures, Monthly Notices (in press). Higher quality figures available at http://www-thphys.physics.ox.ac.uk/users/WynEvans/home.htm

A Treatise of PD-LGD Correlation Modelling

The provision in Paragraph 468 of Basel II Framework Document for calculating loss given default (LGD) requires that parameters used in Pillar I of Basel II capital estimations must be reflective of economic downturn conditions so that relevant risks are accounted for. This provision is based on the fact that the probability of default (PD) and LGD correlations are not captured in the proposed formula for estimating economic capital. To help quantify economic downturn LGD, the Basel Committee proposed establishing a functional relationship between long-run and downturn LGD. To the best of our knowledge, the current proposed models that map out this relationship have the same underlying framework. This thesis presents a general factor PD-LGD correlation model within the conditional independence framework, where obligors’ defaults are conditional on a common state of affairs in the economy. We highlight a mistake that is frequently made in specifying loss given default, which is, current studies ignore the difference between account-level potential loss and LGD. By correcting this mistake and deriving the correct distribution of potential loss and LGD, sensitivity analysis is conducted to ascertain the impact of the defective model on economic capital and parameter estimates. The relationship between the account and portfolio level correlations are explored. Finally, an empirical analysis is conducted to validate the proposed estimation scheme of parameters in the model

Psychosocial Aspects of Physical Activity and Fitness In Special-Population, Minority Middle School Children

Special-population research predicting physical activity (PA) and fitness with minority middle school children from at-risk environments is rare. Hence, the purpose of our investigation was to evaluate the ability of important social cognitive and environment-based measures to predict PA and fitness with children with developmental delay, cognitive, and emotional impairments. Children (N = 89, ages 11-15) completed questionnaires assessing social cognitive and environment-based constructs, self report PA, and completed fitness testing. Correlational results supported some hypotheses. The descriptive and correlational results also indicated commonalities with similar research on non special-population minority middle school children from at-risk environments