Heterogeneous-homogeneous effects in chemical reactors

Imperial Users onl

Tax Policy and Tax Research in Canada

In a survey of tax reform in recent years, Richard Bird and Michael Smart explore the relationship between tax policy and tax research. They conclude that there have been important examples of apparent influences of research on policy. For instance, they are encouraged that the downward pressure on personal and corporate taxes has certainly been supported, if not initiated, by the increasing evidence of distortions caused by high marginal tax rates. In their view, the adoption of the GST can be explained by the acceptance of the federal government of the economic argument that Canada had to switch to a value-added tax to reduce economic distortions. On the other hand, they are disappointed that the equally convincing economic studies of the damage done by poorly-designed excise, property and payroll taxes do not seem to have had any effect. Consequently, they believe that political economy factors were probably the more dominant explanation of the tax reforms than the simple acceptance of advice from economists. Their conclusion is that if economists want to have a greater influence on policy, they need to pay more attention to the issues that motivate policymakers, including, most notably, distributional issues, and they need to write in a way, and in a forum, that will most likely come to the notice of the policy-makers.Canada, Taxation, Income Tax, Value-Added Tax, Value Added Tax, VAT

Correlations in the three-dimensional Lyman-alpha forest contaminated by high column density absorbers

Correlations measured in three dimensions in the Lyman-alpha forest are contaminated by the presence of the damping wings of high column density (HCD) absorbing systems of neutral hydrogen (HI; having column densities $N(\mathrm{HI}) > 1.6\times10^{17}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$), which extend significantly beyond the redshift-space location of the absorber. We measure this effect as a function of the column density of the HCD absorbers and redshift by measuring 3D flux power spectra in cosmological hydrodynamical simulations from the Illustris project. Survey pipelines exclude regions containing the largest damping wings. We find that, even after this procedure, there is a scale-dependent correction to the 3D Lyman-alpha forest flux power spectrum from residual contamination. We model this residual using a simple physical model of the HCD absorbers as linearly biased tracers of the matter density distribution, convolved with their Voigt profiles and integrated over the column density distribution function. We recommend the use of this model over existing models used in data analysis, which approximate the damping wings as top-hats and so miss shape information in the extended wings. The simple 'linear Voigt model' is statistically consistent with our simulation results for a mock residual contamination up to small scales ($|k| < 1\,h\,\mathrm{Mpc}^{-1}$). It does not account for the effect of the highest column density absorbers on the smallest scales (e.g., $|k| > 0.4\,h\,\mathrm{Mpc}^{-1}$ for small damped Lyman-alpha absorbers; HCD absorbers with $N(\mathrm{HI}) \sim 10^{21}\,\mathrm{atoms}\,\mathrm{cm}^{-2}$). However, these systems are in any case preferentially removed from survey data. Our model is appropriate for an accurate analysis of the baryon acoustic oscillations feature. It is additionally essential for reconstructing the full shape of the 3D flux power spectrum.Comment: 13 pages, 11 figures. Minor changes to match version published in MNRA

An Emulator for the Lyman-alpha Forest

We present methods for interpolating between the 1-D flux power spectrum of the Lyman-$\alpha$ forest, as output by cosmological hydrodynamic simulations. Interpolation is necessary for cosmological parameter estimation due to the limited number of simulations possible. We construct an emulator for the Lyman-$\alpha$ forest flux power spectrum from $21$ small simulations using Latin hypercube sampling and Gaussian process interpolation. We show that this emulator has a typical accuracy of 1.5% and a worst-case accuracy of 4%, which compares well to the current statistical error of 3 - 5% at $z < 3$ from BOSS DR9. We compare to the previous state of the art, quadratic polynomial interpolation. The Latin hypercube samples the entire volume of parameter space, while quadratic polynomial emulation samples only lower-dimensional subspaces. The Gaussian process provides an estimate of the emulation error and we show using test simulations that this estimate is reasonable. We construct a likelihood function and use it to show that the posterior constraints generated using the emulator are unbiased. We show that our Gaussian process emulator has lower emulation error than quadratic polynomial interpolation and thus produces tighter posterior confidence intervals, which will be essential for future Lyman-$\alpha$ surveys such as DESI.Comment: 28 pages, 10 figures, accepted to JCAP with minor change

Malware in the Mobile Device Android Environment

exploit smartphone operating systems has exponentially expanded. Android has become the main target to exploit due to having the largest install base amongst the smartphone operating systems and owing to the open access nature in which application installations are permitted. Many Android users are unaware of the risks associated with a malware infection and to what level current malware scanners protect them. This paper tests how efficient the currently available malware scanners are. To achieve this, ten representative Android security products were selected and tested against a set of 5,560 known and categorized Android malware samples. The tests were carried out using a digital-forensically rigorous testing framework and methodology, which ensures the scientific validity of the results. The detection rates of the tested malware scanners varied widely with half unable to detect any samples at all during initial testing. The malware scanners that were able to detect the samples scored highly with the top four between 97-99% and a fifth scanner scoring 87%. The results emphasise the need for more complex detection mechanisms and protections in future versions of Android and the next generation of malware scanners. Keywords: malware, mobile forensics, Androi

Taxation and Executive Compensation: Evidence from Stock Options *

Abstract Understanding the effects of taxes on executive compensation provides insight into the process determining this compensation and is a key input to policymakers considering changes in top income tax rates. A recent tax reform in Canada, which greatly increased the effective tax rate on stock option compensation for a subset of firms, by removing a deduction previously available at the corporate level, provides a natural experiment with which to address this question. I collect a novel panel dataset of executive compensation for the period 2008-2011. Findings from a difference-in-differences strategy suggest that this tax increase resulted in a relatively immediate reduction in both stock option grants and the fraction of total compensation made up of stock options. There appears to have been limited, if any, substitution towards other components of compensation, such as cash bonuses or grants of shares. Hence, the burden of the tax must have been substantially borne by the affected executives. This finding is in contrast with much of the extant literature, which typically has not found evidence for tax incentives as important drivers of executive compensation. * I would like to thank Michael Smart, Robert McMillan, Laurence Booth and Alex Edwards for their guidance and support throughout this project, as well as several anonymous reviewers for helpful comments. I gratefully acknowledge financial support from the SSHRC CGS Doctoral Fellowship, the Dorothy J. Powel