A dispersion minimizing scheme for the 3-D Helmholtz equation based on ray theory

We develop a new dispersion minimizing compact finite difference scheme for the Helmholtz equation in 2 and 3 dimensions. The scheme is based on a newly developed ray theory for difference equations. A discrete Helmholtz operator and a discrete operator to be applied to the source and the wavefields are constructed. Their coefficients are piecewise polynomial functions of $hk$, chosen such that phase and amplitude errors are minimal. The phase errors of the scheme are very small, approximately as small as those of the 2-D quasi-stabilized FEM method and substantially smaller than those of alternatives in 3-D, assuming the same number of gridpoints per wavelength is used. In numerical experiments, accurate solutions are obtained in constant and smoothly varying media using meshes with only five to six points per wavelength and wave propagation over hundreds of wavelengths. When used as a coarse level discretization in a multigrid method the scheme can even be used with downto three points per wavelength. Tests on 3-D examples with up to $10^8$ degrees of freedom show that with a recently developed hybrid solver, the use of coarser meshes can lead to corresponding savings in computation time, resulting in good simulation times compared to the literature.Comment: 33 pages, 12 figures, 6 table

The state of change : analysing policy change in Dutch and English higher education

The general claim in comparative politics that majoritarian democracies are capable of faster policy making than consensus democracies was only partly substantiated in this study. Looking at the short term, England has indeed been capable of swift policy making. On several occasions during the period from 1980 to 1995, it has designed policies and decided on them in a fast, centralised and isolated style that would be impossible in the Dutch consensus democracy. Paradoxically, when comparing the two systems over this longer period both countries have shown very comparable levels of policy change

A pseudodifferential equation with damping for one-way wave propagation in inhomogeneous acoustic media

A one-way wave equation is an evolution equation in one of the space directions that describes (approximately) a wave field. The exact wave field is approximated in a high frequency, microlocal sense. Here we derive the pseudodifferential one-way wave equation for an inhomogeneous acoustic medium using a known factorization argument. We give explicitly the two highest order terms, that are necessary for approximating the solution. A wave front (singularity) whose propagation velocity has non-zero component in the special direction is correctly described. The equation can't describe singularities propagating along turning rays, i.e. rays along which the velocity component in the special direction changes sign. We show that incorrectly propagated singularities are suppressed if a suitable dissipative term is added to the equation.Comment: 15 page

Excessive(?) Entry of National Telecom Networks, 1990-2001

We document entry and capacity expansion in US long-distance fiber-optic networks before and during the “telecom boom.” We disentangle the many swaps and leases between networks in order to measure owned route miles versus route miles shared with other carriers. Entry appears much more moderate when these shared miles are not counted. Strategic behavior can lead to excessive entry, and we find evidence of such behavior regarding total miles (including swaps and leases) but not regarding owned miles. We conclude that entry was excessive only with regard to swaps and leases, but not with regard to the physical building of the networks.telecommunications, investment, preemption

An organizational change approach for enterprise system implementations

Enterprise Information Systems or just Enterprise Systems (ES) have become\ud increasingly popular since the last decade of the previous century. Many organizations\ud have deployed an ES implementation and the current adoption rate of these systems is\ud still rising. The implementation of an ES, especially when driven by a BPR rationale, has\ud a large impact on the adopting organization. Extensive research has shown that the\ud resulting organizational change process requires profound support, which most\ud implementations currently lack, causing a substantial number of implementation\ud failures. A literature overview in chapter 1 focuses on the diverse contributions dealing\ud with this organizational change problem in the domain of ES implementations. The ES\ud implementation process is researched from various perspectives, which leads to\ud fragmented knowledge and mostly explorative or descriptive research results.\ud Prescriptive research is carried out less often. This kind of research leads to conceptual\ud change frameworks or guidelines. On the other hand, consistent creation of a\ud profound and applicable ES specific change approach is lacking. This dissertation\ud focuses on this omission and designs an organizational change approach, which it will\ud then deploy in a longitudinal case study

Tacit Collusion in Capacity Investment: The Role of Capacity Exchanges

In many capacity-intensive industries (e.g. electricity, bandwidth), exchanges allow firms to buy and sell wholesale capacity before selling on the retail market. This allows firms to smooth demand shocks, but it also raises suspicions that exchanges facilitate tacit collusion to limit capacity investment. This paper models investment and exchange in a one-shot game and in a repeated game with tacit collusion. It finds that the presence of the exchange does not reduce total capacity investment, and thus does not raise consumer prices. In fact, the exchange may make it more difficult to sustain tacit collusion.capacity investment; capacity exchanges; business to business exchanges; tacit collusion

How useful are integrated household survey data for policy-oriented analysis of poverty? Lessons from the Cote d'Ivoire living standards survey

The author reflects on the pros and cons of using integrated household survey data in empirical analysis aimed at providing a quantitative basis for policy decisions affecting welfare, poverty, and the fulfillment of basic needs. The experience examined is that of using four years of data from the Cote d'Ivoire Living Standards Survey (1985-88) to link changes in poverty and welfare to macroeconomic trends. The author groups the lessons learned from this work around four themes. Survey content: when survey data are rich, transparency of methodology is important. It is essential that analysts provide explicit information about how their income and spending aggregates were constructed. These aggregates must be deflated with a regional price index, but prices should be collected separately from household survey data. Data on household spending and basic needs fulfillment are the key information for poverty analysis. Sample size and design: bigger and simpler is better. The author recommends increasing (at least double) sample size in future living standards surveys; this could be done without increasing the cost of the survey by reducing or eliminating the income modules of the questionnaire. It is important to involve analysts and policymakers in survey design. They need to identify up front, using current knowledge, the important socioeconomic and target groups on which the survey must be able to report. The sample designer can then compose the sample in such a way that certain groups will be undersampled and others oversampled, to make the analysis of the resulting sample as useful as possible. Frequency of data collection: the author recommends that an integrated survey of the CILSS type be undertaken every four or five years, to provide benchmark data and to permit in-depth analysis of household behavior and response to policy, if the country has the capability to fully analyze the data. In the intervening years, a much simpler collection of household spending and basic needs data can be used to monitor changes in welfare and poverty. The role of panel data: to be really useful, panel data collection should be extended over longer periods than two years, although this increases the costs and difficulties of finding the same households. If a country undertakes an integrated survey every four to five years and alighter monitoring survey in between, a small, parallel, panel survey could be conducted. The monitoring sample and the panel sample should be drawn from the same master sample.Health Economics&Finance,Poverty Assessment,Poverty Monitoring&Analysis,Poverty Lines,Environmental Economics&Policies