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Search Results: 1 - 10 of 544813 matches for " M. De Vries "
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A property of algebraic univoque numbers
M. de Vries
Mathematics , 2006,
Abstract: Consider the set $\uu$ of real numbers $q \ge 1$ for which only one sequence $(c_i)$ of integers $0 \le c_i \le q$ satisfies the equality $\sum_{i=1}^{\infty} c_i q^{-i} = 1$. In this note we show that the set of algebraic numbers in $\uu$ is dense in the closure $\uuu$ of $\uu$.
M.R. De Vries
South African Journal of Industrial Engineering , 2012,
Abstract: ENGLISH ABSTRACT: Information system development methodologies have been applied by numerous organisations since the mid-1980s in an attempt to improve the efficiency and effectiveness of designing and developing new information systems. Despite advances in methodologies, tools and techniques, productivity is still low. High quality products are seldom produced and at high cost. The advantages and disadvantages of using a methodological approach is discussed. The author identifies the key drivers for applying an information system development methodology successfully and provides a method for selecting or evaluating a methodology tailored to an organisation’s unique set of organisational, cultural and environmental variables. The framework has been applied to Waymark Infotech, a South African information technology organisation. AFRIKAANSE OPSOMMING: Ontwikkelingsmetodologie vir inligtingstelsels word sedert die middel van die 1980’s deur vele organisasies aangewend in hul poging om die effektiwiteit en doeltreffendheid van die ontwikkelingsproses van inligtingstelsels te verbeter. Ten spyte daarvan dat vooruitgang en ontwikkeling plaasgevind het ten opsigte van metodologie , hulpmiddels en tegnieke, is produktiwiteit steeds laag. Duur en lae-gehalte inligtingstelselprodukte word gelewer. Die voor- en nadele van ‘n metodologiese benadering word beredeneer. Die outeur identifiseer die kernaspekte nodig vir die suksesvolle aanwending van ‘n ontwikkelingsmedodologie vir inligtingstelsels. ‘n Metode word aangebied aan die hand waarvan ’n bepaalde metodologie gekies of ontwikkel kan word. Die metode neem verskeie veranderlikes (organisatories, kultureel en omgewingsverwant) in ag.
Invariant densities for random $β$-expansions
K. Dajani,M. de Vries
Mathematics , 2006,
Abstract: Let $\beta >1$ be a non-integer. We consider expansions of the form $\sum_{i=1}^{\infty} d_i \beta^{-i}$, where the digits $(d_i)_{i \geq 1}$ are generated by means of a Borel map $K_{\beta}$ defined on $\{0,1\}^{\N}\times [ 0, \lfloor \beta \rfloor /(\beta -1)]$. We show existence and uniqueness of an absolutely continuous $K_{\beta}$-invariant probability measure w.r.t. $m_p \otimes \lambda$, where $m_p$ is the Bernoulli measure on $\{0,1\}^{\N}$ with parameter $p$ $(0 < p < 1)$ and $\lambda$ is the normalized Lebesgue measure on $[0 ,\lfloor \beta \rfloor /(\beta -1)]$. Furthermore, this measure is of the form $m_p \otimes \mu_{\beta,p}$, where $\mu_{\beta,p}$ is equivalent with $\lambda$. We establish the fact that the measure of maximal entropy and $m_p \otimes \lambda$ are mutually singular. In case the number 1 has a finite greedy expansion with positive coefficients, the measure $m_p \otimes \mu_{\beta,p}$ is Markov. In the last section we answer a question concerning the number of universal expansions, a notion introduced in [EK].
Modelled and measured effects of clouds on UV Aerosol Indices on a local, regional, and global scale
M. Penning de Vries ,T. Wagner
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2011,
Abstract: The UV Aerosol Indices (UVAI) form one of very few available tools in satellite remote sensing that provide information on aerosol absorption. The UVAI are also quite insensitive to surface type and are determined in the presence of clouds – situations where most aerosol retrieval algorithms do not work. The UVAI are most sensitive to elevated layers of absorbing aerosols, such as mineral dust and smoke, but they can also be used to study non-absorbing aerosols, such as sulphate and secondary organic aerosols. Although UVAI are determined for cloud-contaminated pixels, clouds do affect the value of UVAI in several ways: (1) they shield the underlying scene (potentially containing aerosols) from view, (2) they enhance the apparent surface albedo of an elevated aerosol layer, and (3) clouds unpolluted by aerosols also yield non-zero UVAI, here referred to as "cloudUVAI". The main purpose of this paper is to demonstrate that clouds can cause significant UVAI and that this cloudUVAI can be well modelled using simple assumptions on cloud properties. To this aim, we modelled cloudUVAI by using measured cloud optical parameters – either with low spatial resolution from SCIAMACHY, or high resolution from MERIS – as input. The modelled cloudUVAI were compared with UVAI determined from SCIAMACHY reflectances on different spatial (local, regional and global) and temporal scales (single measurement, daily means and seasonal means). The general dependencies of UVAI on cloud parameters were quite well reproduced, but several issues remain unclear: compared to the modelled cloudUVAI, measured UVAI show a bias, in particular for large cloud fractions. Also, the spread in measured UVAI is larger than in modelled cloudUVAI. In addition to the original, Lambert Equivalent Reflector (LER)-based UVAI algorithm, we have also investigated the effects of clouds on UVAI determined using the so-called Modified LER (MLER) algorithm (currently applied to TOMS and OMI data). For medium-sized clouds the MLER algorithm performs better (UVAI are closer to 0), but like for LER UVAI, MLER UVAI can become as large as 1.2 for small clouds and deviate significantly from zero for cloud fractions near 1. The effects of clouds should therefore also be taken into account when MLER UVAI data are used. Because the effects of clouds and aerosols on UVAI are not independent, a simple subtraction of modelled cloudUVAI from measured UVAI does not yield a UVAI representative of a cloud-free scene when aerosols are present. We here propose a first, simple approach for the correction of cloud effects on UVAI. The method is shown to work reasonably well for small to medium-sized clouds located above aerosols.
Audit on breast biopsies of non-palpable and difficult-to-access lesions
M Naude, CS de Vries, EF Nel
South African Journal of Radiology , 2006,
Abstract: Fine-needle aspiration biopsy (FNAB) is done regularly at our mammography unit on lesions that are non-palpable and difficult to access. Studies done at other hospitals on palpable and non-palpable lesions show a wide variety of results. Therefore we wanted to develop a database of information regarding FNAB results at our own unit. A retrospective descriptive study was done from the reports of all patients who had FNABs at our unit over a 1-year period (15 December 2004 – 1 December 2005). A convenient sampling of 48 women of all ages was used. Four patients did not fit the study criteria and were excluded. Cytological data were retrieved from the original reports and no standardised criteria were used to determine adequacy. Of the available cytology results 23.9% (11 of 46) were positive for malignancy, 17.4% (8 of 46) were negative for malignancy, 4.4% (2 of 46) showed atypical cells and 54.3% (25 of 46) of reported results were inconclusive. In the group of fine-needle aspirations that showed atypical cells (2 patients), neither had a follow-up ultrasound after 3 or 6 months, but both patients were followed up with a mammogram after 6 months. In patients with fine-needle aspiration results that were inconclusive, 20% were followed up with ultrasound after 3 months. After 6 months 8% had a follow-up ultrasound and 36% had a follow-up mammogram. Four per cent of the patients had a routine mammogram at 1 year and 56% were referred to the surgery department. (Some patients had more than one follow-up examination.) The aim of our audit was to develop baseline statistics regarding FNAB results for our unit. The high number of inconclusive results in our study (56.82%) could be due to the fact that FNABs of non-palpable lesions are more difficult to perform. The level of experience of each radiologist performing the aspiration as well as the skill of the cytologist should also be considered. FNABs will be continued at our unit and a follow-up study for comparison of statistics is planned. Such a comparison between studies will assist us in setting a standard for future FNAB results at our unit. South African Journal of Radiology Vol. 10 (2) 2006: pp. 4-5
M. De Vries,P.J. Conradie
South African Journal of Industrial Engineering , 2012,
Abstract: ENGLISH ABSTRACT: Management consultancy organisations of the 21st century realise that they need to move away from balance sheet accounting systems as the primary tool of management. Intangible assets, such as knowledge, skills and process assets, may be worth more than their physical assets, and require effective management to gain a competitive advantage. Many models for process-improvement and knowledge management that currently exist could be used in leveraging organisational intangible assets. Though the intricate interaction between the domains of process improvement management and knowledge management is clear in current models, a fully integrated model does not exist. The aim of this article is to demonstrate the integration possibilities of process improvement and knowledge management in an attempt to improve the practices of both during the same exercise, using an integrated model. AFRIKAANSE OPSOMMING: Bestuurskonsultasie-organisasies van die 21ste eeu besef dat hul moet wegbeweeg van balansstaat gebaseerde rekenkundige stelsels as die primêre gereedskapstuk vir bestuur. Ontasbare bates, soos kennis, vaardighede en prosesbates, mag meer waardevol wees as fisiese bates, en moet effektief bestuur word om ‘n mededingende voordeel te behou. Talle modelle vir prosesverbeteringsbestuur en kennisbestuur bestaan tans en word ingespan om optimale benutting van die ontasbare bates te verseker. Alhoewel die ingewikkelde interaksie tussen die velde van prosesverbeteringsbestuur en kennisbestuur duidelik is in die bestaande modelle, bestaan daar nie ‘n ge ntegreerde model nie. Hierdie artikel demonstreer die integrasie moontlikhede van prosesverbeteringsbestuur en kennisbestuur deur gebruik te maak van ‘n ge ntegreerde model, sodat praktyke van beide velde gedurende dieselfde oefening verbeter word.
R. de Jong, Van standspolitiek naar partijloyaliteit. Verkiezingen voor de Tweede Kamer 1848-1887
B.M.A. de Vries
BMGN : Low Countries Historical Review , 2000,
H. Gras, Geschiedenis van Assen
B.M.A. de Vries
BMGN : Low Countries Historical Review , 2002,
R.J. de Vries, Enkhuizen 1650-1850. Bloei en achteruitgang van een Zuiderzeestad
B.M.A. de Vries
BMGN : Low Countries Historical Review , 1990,
D.J. Wolffram, Vrij van wat neerdrukt en beklemt. Staat, gemeenschap en sociale politiek (1870-1918)
B.M.A. de Vries
BMGN : Low Countries Historical Review , 2005,
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