Interferometer predictions with triangulated images: solving the multi-scale problem

Interferometers play an increasingly important role for spatially resolved observations. If employed at full potential, interferometry can probe an enormous dynamic range in spatial scale. Interpretation of the observed visibilities requires the numerical compu- tation of Fourier integrals over the synthetic model images. To get the correct values of these integrals, the model images must have the right size and resolution. Insufficient care in these choices can lead to wrong results. We present a new general-purpose scheme for the computation of visibilities of radiative transfer images. Our method requires a model image that is a list of intensities at arbitrarily placed positions on the image-plane. It creates a triangulated grid from these vertices, and assumes that the intensity inside each triangle of the grid is a linear function. The Fourier integral over each triangle is then evaluated with an analytic expression and the complex visibility of the entire image is then the sum of all triangles. The result is a robust Fourier trans- form that does not suffer from aliasing effects due to grid regularities. The method automatically ensures that all structure contained in the model gets reflected in the Fourier transform.Comment: 9 pages, 7 figures, accepted for publication in MNRA

Labour Supply Effects of an Early Retirement Programme

In 1988, an early retirement program (AFP) was introduced in Norway for the 66-years-old. Since then, AFP has gradually been extended and by now it covers workers aged 62-66. In this paper we employ a multinominal logit model to study the transition between states in the labour market. The model is estimated on a large panel data set covering the period 1988-2 to 1999-4. The estimated model tracks the development quite well, as also outside sample predictions do. The model is used to assess the future labour market impact of abolishing AFP. We find that by abolishing AFP may increase the labour force participation among older men (55-67) in 2005 from 72 percent in the baseline projection to 83 percent. For females the corresponding increase is from 62 to 67 percent.Early retirement, large panel data sets, econometric models

VALUING AGRICULTURAL MORTGAGE-BACKED SECURITIES

A model to value Federal Agricultural Mortgage Corporation (Farmer Mac) agricultural mortgage-backed securities (AMBS) is developed and numerically solved. The results suggest prepayment penalties currently being used by Farmer Mac reduce yields on AMBS considerably. Even with prepayment penalties, it can be advantageous for profit maximizing mortgagors to optimally prepay or even default on agricultural mortgages. The model is used to quantify prepayment and default risk by valuing the embedded options in the mortgages. Monte Carlo simulation is also used to determine the probability to optimal prepayment given the term structure assumption used to develop the model.agricultural mortgage-backed securities, default, dynamic programming, simulation, prepayment, Agricultural Finance, G13, G21,

Multiple-Length Division Revisited: A Tour of the Minefield

Long division of natural numbers plays a crucial role in Cobol arithmetic, cryptography, and primality testing. Only a handful of textbooks discuss the theory and practice of long division, and none of them do it satisfactorily. This tutorial attempts to fill this surprising gap in the literature on computer algorithms. We illustrate the subtleties of long division by examples, define the problem concisely, summarize the theory, and develop a complete Pascal algorithm using a consistent terminology

Analysis of a Parallel Mergesort

The paper describes a performance model of a fine-grained, parallel mergesort which sorts N elements in 0(N) time using 0(logN) processors. The model predicts both the communication time involved in merging the elements and the decomposition time required to activate and terminate the binary tree of processes. The parallel algorithm is written in Joyce and runs on an Encore Multimax

The face of the factory girl: Educational rhetoric in the mediation of an historical place

World heritage cities and sites convey culture and form understandings of the past as well as show a way to relate to it in a present context. These cities and sites are concrete examples of what to remember; and their mediated rhetoric, as found inexhibitions and on websites, gives a view into the management of our collective, cultural memory. This article will analyse key elements of the website for New Lanark, a Scottish heritage site, and in so doing will observe how rhetorical agency is shaped through the use of iconic photographs, visual rhetoric and the foregrounding of specific topics. Predominant on this website is the face of a young factory girl, smiling at the viewer. The heritage site thus has its identity formed, but so does the public addressed by this rhetoric. The institutional position of the historical place and its mediated rhetoric point to the fact that world heritage sites are rhetorical educational texts, far from being as neutral as might be expected

Interferometric view of the circumstellar envelopes of northern FU Orionis-type stars

FU Orionis-type objects are young, low-mass stars with large outbursts in visible light that last for several years or decades. They are thought to represent an evolutionary phase during the life of every young star when accretion from the circumstellar disk is enhanced during recurring time periods. These outbursts are able to rapidly build up the star while affecting the circumstellar disk and thus the ongoing or future planet formation. In many models infall from a circumstellar envelope seems to be necessary to trigger the outbursts. We observed the J=1$-$0 rotational transition of $^{13}$CO and C$^{18}$O towards eight northern FU Orionis-type stars (V1057 Cyg, V1515 Cyg, V2492 Cyg, V2493 Cyg, V1735 Cyg, V733 Cep, RNO 1B and RNO 1C) and derive temperatures and envelope masses and discuss the morphology and kinematics of the circumstellar material. We detected extended CO emission associated with all our targets. Smaller scale CO clumps were found to be associated with five objects with radii of 2000$-$5000 AU and masses of 0.02$-$0.5 $M_{\odot}$; these are clearly heated by the central stars. Three of these envelopes are also strongly detected in the 2.7 mm continuum. No central CO clumps were detected around V733 Cep and V710 Cas but there are many other clumps in their environments. Traces of outflow activity were observed towards V1735 Cyg, V733 Cep and V710 Cas. The diversity of the observed envelopes enables us to set up an evolutionary sequence between the objects. We find their evolutionary state to range from early, embedded Class I stage to late, Class II-type objects with very low-mass circumstellar material. The results reinforce the idea of FU Orionis-type stars as representatives of a transitory stage between embedded Class I young stellar objects and classical T-Tauri stars.Comment: 17 pages, 11 figures; accepted for publication in A&

HOUSEHOLDER REDUCTION

This tutorial discusses Householder reduction of n linear equations to a triangular form which can be solved by back substitution. The main strengths of the method are its numerical stability and suitability for parallel computing. We explain how Householder reduction can be derived from elementary matrix algebra. The method is illustrated by a numerical example and a Pascal algorithm. We assume that the reader has a general knowledge of vector and matrix algebra but is less familiar with linear transformation of a vector space