Showing papers by "University of the Witwatersrand published in 1973"
TL;DR: In this article, two stochastic optimal control problems are solved whose performance criteria are the expected values of exponential functions of quadratic forms, and the optimal controller is linear in both cases but depends upon the covariance matrix of the additive process noise so that the certainty equivalence principle does not hold.
Abstract: Two stochastic optimal control problems are solved whose performance criteria are the expected values of exponential functions of quadratic forms. The optimal controller is linear in both cases but depends upon the covariance matrix of the additive process noise so that the certainty equivalence principle does not hold. The controllers are shown to be equivalent to those obtained by solving a cooperative and a noncooperative quadratic (differential) game, and this leads to some interesting interpretations and observations. Finally, some stability properties of the asymptotic controllers are discussed.
692 citations
08 Feb 1973-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: The stratigraphy of the early greenstone occurrences in southern Africa is described with particular emphasis being given to the geological record established in the well-preserved Barberton Mountain Land of the Transvaal, and it is concluded that the early crust had analogies with the present-day abyssal regions of the oceans.
Abstract: The early Precambrian granite-greenstone basement complex of Rhodesia and South Africa constitutes one of the oldest known fragments of the Earth’s crust. Scattered within the expanse of granitic rocks are numerous greenstone belts some approaching, or even possibly exceeding, ages of about 3400 Ma. These ancient greenstone belt remnants represent the earliest clearly decipherable geological events on the Earth’s surface, and any evidence concerning the nature and evolutionary development of the primitive crust must be sought largely from an examination of their lithologies. The stratigraphy of the early greenstone occurrences in southern Africa is described with particular emphasis being given to the geological record established in the well-preserved Barberton Mountain Land of the Transvaal. The evidence for a primitive sialic versus simatic crust is reviewed and reference is made to the reported pre-greenstone belt basement remnants found in Rhodesia as well as in Swaziland. Comparisons are drawn between the early greenstone belts and modern island arcs, use being made of available geochemical data from the two environments. It is concluded that the early crust in southern Africa had analogies with the present-day abyssal regions of the oceans. Progressive transformation, involving island arc-like development and protocontinental nucleation, resulted from processes involving partial melting of the primitive lithosphere coupled with the secular mixing of mantle derived, granitic additions. The early thin, unstable, primordial crust underwent progressive thickening processes and, about 3000 Ma ago, stabilized sufficiently to permit the development of the first interior or cratonic-type sedimentary basins. Increases in the areal extent of the developing basins are briefly traced and the more significant events, from the earliest Precambrian to middle Proterozoic times (ca. 1700 Ma ago), are contrasted and summarized.
207 citations
TL;DR: The findings presented here do not support the theory that fat emboli are responsible for the subchondral bone changes and more credence is given to the theory of sub chondral microfracture in osteoporotic bone.
Abstract: 1. It is well known that the administration of corticosteroids may result in necrosis and progressive destruction of the femoral head. Identical changes have been found in chronic alcoholics, in South African negroes suffering from iron-overload osteoporosis and in patients with arthritis of the hip treated with non-steroid anti-inflammatory and analgesic preparations. The term "drug-induced arthropathy" is used to describe the common pathological lesion. 2. Seventy-two patients with this complication have been investigated and forty-two femoral heads were available for detailed study. The characteristic change is subchondral fragmentation and osteonecrosis, followed later by reactive bone formation and the typical increased radiographic density. 3. The frequent occurrence of fat occupying the Haversian canals in the affected femoral heads has not been adequately explained and its relationship to the destructive arthropathy remains obscure. The findings presented here do not support the theory that fat emboli are responsible for the subchondral bone changes. 4. More credence is given to the theory of subchondral microfracture in osteoporotic bone. The destructive arthropathy invariably follows the administration of some anti-inflammatory or analgesic preparation. It is postulated that a state of diminished sensibility predisposes to microtrauma in osteoporotic bone resulting in subarticular collapse of the femoral head.
133 citations
TL;DR: The Limpopo belt is an extensive ENE-trending linear zone of high-grade metamorphic tectonites which separates the Archaean nucleii of the Rhodesian craton to the north from the Kaapvaal craton in the south as mentioned in this paper.
Abstract: The Limpopo belt is an extensive ENE-trending linear zone of high-grade metamorphic tectonites which separates the Archaean nucleii of the Rhodesian craton to the north from the Kaapvaal craton to the south. The belt consists of reworked Archaean granite-greenstone terrain with an early Proterozoic cover sequence, the Messina Formation, infolded and metamorphosed with the basement. Two major zones of shearing and transcurrent dislocation separate marginal granulite zones from a central zone which consists of complexly infolded cover rocks and reworked basement. The northern granulite zone appears to grade transitionally into the Rhodesian craton to the north, whereas there is some evidence that the southern granulite zone is faulted against the Kaapvaal craton to the south. The whole belt has behaved as a zone of crustal weakness throughout geological time, and is characterized by repeated shear deformation, igneous intrusion and extrusion, despite the cessation of major regional tectono-thermal reactivation about 1900 Ma ago.
112 citations
TL;DR: In this article, the correlation matrix of a vector x is reduced to principal components and the components corresponding to low eigenvalues may be useful in suggesting possible alternative subregressions.
Abstract: SUMMARY In a multiple regression problem, let the p x 1 vector x consist of the dependent variable and p -1 predictor variables. The correlation matrix of x is reduced to principal components. The components corresponding to low eigenvalues may be useful in suggesting possible alternative subregressions. This possibility is analysed, and formulae derived for the derivation of subregressions from the principal components.
100 citations
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TL;DR: In this article, the correlation matrix of a vector x is reduced to principal components and the components corresponding to low eigenvalues may be useful in suggesting possible alternative subregressions.
Abstract: SUMMARY In a multiple regression problem, let the p x 1 vector x consist of the dependent variable and p -1 predictor variables. The correlation matrix of x is reduced to principal components. The components corresponding to low eigenvalues may be useful in suggesting possible alternative subregressions. This possibility is analysed, and formulae derived for the derivation of subregressions from the principal components.
99 citations
94 citations
TL;DR: Rose chafer beetle shells are composites of chitin fibres and proteinaceous matrix in an orthogonal plywood-like laminate as mentioned in this paper, which afford a novel solution to shear and crack-propagation resistance.
Abstract: Rose chafer beetle shells are composites of chitin fibres and proteinaceous matrix in an orthogonal plywood-like laminate. Fibre layers are connected by unique inter and intra-ply cross-links that afford a novel solution to shear and crack-propagation resistance.
89 citations
TL;DR: New stratigraphic evidence from Sterkfonteinand Makapansgat suggests a lengthy lapse of time between cave opening and deposition of dated fossil bones, and thus provides a basis for explaining the apparent discrepancies in faunal and geomorphological estimates of the age of several South African australopithecine sites.
Abstract: Divergences exist between recent faunal and geomorphological estimates of the age of several South African australopithecine sites. New stratigraphic evidence from Sterkfonteinand Makapansgat suggests a lengthy lapse of time between cave opening and deposition of dated fossil bones, and thus provides a basis for explaining the apparent discrepancies. The young date inferred for the Taung skull makes it up to 2 m.y. more recent than Australopithecus africanus from the Transvaal caves. As Taung is clearly not a member of Homo erectus, it is reasonable to hypothesise that it may be a later survivor of A. robustus.
83 citations
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59 citations
TL;DR: A basal section of the Lower Proterozoic Transvaal Dolomite from the northern Cape has been re-examined in this paper, and columns and columns, and associations built around spheroidal forms are also represented.
TL;DR: Day and night sleep differed with respect to both duration and pattern, and night duty seemed to affect the pattern of sleep stage distribution as well as the absolute amount of, not only total sleep, but also some sleep stages, such as Stage SS.
Abstract: The diurnal sleep patterns of female nurses working night duty were compared to their nocturnal sleep patterns while they were working regular hours during the day. Continuous EEG, EOG, and EMG recordings were made at the end of 2 month periods of night and day duty respectively. Day and night sleep differed with respect to both duration and pattern. Despite an earlier onset, the major sleep period was shorter during the day than the night and seemed to be more interrupted later in the session. This finding is in keeping with the increased amount of Stage 1 and decreased amount of slow wave sleep during the day than the night. Although no differences were evident with respect to overall percent REM, differences in the distribution of REM did occur. REM sleep occurred sooner during day than night sleep and there was more of it during the first part of day sleep. Thus night duty seemed to affect the pattern of sleep stage distribution as well as the absolute amount of, not only total sleep, but also some sleep stages, such as Stage SS. It is an open question how the naps of extended duration taken while on night duty influence the pattern of sleep during the day.
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TL;DR: In this paper, a comparison of plume traces with palaeomagnetic data from lithospheric plates for the past 50 m.y.d. suggests that the mantle has rolled about its own independent axis within an outer shell which as a whole is fixed in respect to the Earth's spin axis.
Abstract: Comparison of plume traces with palaeomagnetic data from lithospheric plates for the past 50 m.y. suggests that the mantle has rolled about its own independent axis within an outer lithospheric shell which as a whole is fixed in respect to the Earth's spin axis.
TL;DR: In this article, the authors apply the technique d'analyse par activation neutronique non destructive, utilisant un detecteur Ge(Li) a haute resolution for determiner des traces d'impuretes dans des diamants.
Abstract: On applique la technique d'analyse par activation neutronique non destructive, utilisant un detecteur Ge(Li) a haute resolution pour determiner des traces d'impuretes dans des diamants. La presence de 40 elements est mise en evidence dans les diamants naturels. On donne des resultats quantitatifs portant sur 13 elements dans 25 echantillons de diamant representatifs d'une meme origine. Un rapport a ete etabli entre la chimie des inclusions et la couleur des diamants d'une part, et les traces des elements trouves d'autre part.
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TL;DR: In this article, the valuation of diamondiferous deposits depends on the density distribution, the size distribution of stones and the selling price structure prevailing at a particular time, and confidence limits are calculated which allow for inferences from sampling to recovery.
Abstract: The valuation of diamondiferous deposits depends on (i) the density distribution, (ii) the size distribution of stones and (iii) the selling price structure prevailing at a particular time. In the density distribution the stochastic variable is defined as the number of individual stones found per sample unit irrespective of the stone size. The density distribution is discrete and allows for barren sample units. As diamonds often occur in clusters, a Poisson mixture distribution is derived which is entirely new, as existing discrete distributions cannot represent the stone number frequencies found in nature. For the sizes of diamonds, which are measured by weight, a new compound logarithmic normal distribution is developed. Combination of (i), (il) and (iii) leads to a meaningful appraisal of diamondiferous ground expressed in monetary value per production unit of mining. Finally, confidence limits are calculated which allow for inferences from sampling to recovery.
TL;DR: The airborne substance which is given off by locust faeces has been extracted and submitted to ultraviolet, infrared, mass, and nuclear magnetic resonance spectroscopy, and results indicated a derivative of guaiacol, which is a degradation product of the metabolism of the lignin of plants.
TL;DR: Ketamine is compared with thiopentone with special reference to the occurrence of intra-operative awareness in Caesarean sections at Baragwanath Hospital on account of a high incidence of awareness during operation.
TL;DR: Isotope-exchange experiments revealed fundamental differences in the partial reactions catalysed by the rat and Xenopus laevis synthetases, and the results for the rat liver enzyme are consistent with a Tri Tri sequential mechanism.
Abstract: 1. An improved radioassay for glutathione synthetase and gamma-glutamylcysteine synthetase was developed. 2. Xenopus laevis liver gamma-glutamylcysteine synthetase was purified 324-fold by saline-bicarbonate extraction, protamine sulphate precipitation, CM-cellulose and DEAE-cellulose column chromatography, and gel filtration. 3. Rat liver gamma-glutamylcysteine synthetase was purified 11400-fold by a procedure similar to that employed for the Xenopus laevis enzyme. 4. Rat liver gamma-glutamylcysteine synthetase activity was inhibited by GSH and activated by glycine. These effects, which were not found in the enzyme from Xenopus laevis, may have a regulatory significance. 5. Isotope-exchange experiments revealed fundamental differences in the partial reactions catalysed by the rat and Xenopus laevis synthetases. The enzyme from Xenopus laevis appears to follow a Bi Bi Uni Uni Ping Pong mechanism, with glutamyl-enzyme as intermediate before the addition of cysteine and the release of gamma-glutamylcysteine. The results for the rat liver enzyme are consistent with a Tri Tri sequential mechanism.
TL;DR: The outcome of acute cholecystitis depends on the obstruction of the gallbladder, and rapid resolution enabling an accurate diagnosis to be made and appropriate treatment to be given to bring the patient into optimal condition before definitive operation is performed.
TL;DR: A nursing sister developed Australia-antigen-positive hepatitis 101 days after the blood of an Australia- antigens-positive patient was accidentally introduced into her eye.
Abstract: A nursing sister developed Australia-antigen-positive hepatitis 101 days after the blood of an Australia-antigen-positive patient was accidentally introduced into her eye. There may be important implications for those caring for patients with Australia-antigenemia or undertaking laboratory investigations on their blood.
TL;DR: In this paper, a selection was made of nine basic and ultrabasic samples that are considered to have undergone only isochemical metamorphism, and of seven felsic rocks although these are characteristically more altered.
Abstract: Previous geological, geochemical and geochronological data relating to the Onverwacht Group, Swaziland Sequence are summarised. A selection was made of nine basic and ultrabasic samples that are considered to have undergone only isochemical metamorphism, and of seven felsic rocks although these are characteristically more altered. Brief petrographic descriptions are given and sample localities are shown on the accompanying maps.
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TL;DR: The use of astatine-211 as an alternative to iodine isotopes in radioactive antigen suicide experiments was extended and inhibition was specific in that responsiveness to unrelated antigens was retained.
Abstract: This is a study of the use of astatine-211 as an alternative to iodine isotopes in radioactive antigen suicide experiments. The theoretical advantages of 211At are that it is a high energy (5·87 MeV), short half-life (7·2 hr)α-emitter with a short path length of 60 μm. Its destructive effect, measured in terms of degree of ionization per micron is 300 times that of 125I and 131I. Streptokinase (SK), tuberculin (PPD) and phytohaemagglutinin (PHA) were labelled electrolytically with 211At (3–9% incorporation). These antigens were not destroyed by the labelling technique or by self-irradiation during radioactive decay. Autoradiography showed that only a small fraction of lymphocytes bound 211At-labelled SK and PPD whilst most lymphocytes bound 211At-labelled PHA. Lymphocytes exposed for 40 min to 0·35–1·40 μCi of labelled antigens lost 35–83% of their ability to transform upon subsequent exposure to the unlabelled antigens. The inhibition was specific in that responsiveness to unrelated antigens was retained. These results extend our previous findings with the use of 211At in in vivo antigen suicide experiments.
TL;DR: The ESR spectra of several brown diamonds measured at 77 K show a central group of three equally strong lines, with two groups of six closely spaced anisotropic lines of equal intensity at about 30-40 G on either side of the central group as discussed by the authors.
Abstract: The ESR spectra of several brown diamonds measured at 77 K show a central group of three equally strong lines, with two groups of six closely spaced anisotropic lines of equal intensity at about 30-40 G on either side of the central group. The central group of lines has double the intensity of the side lines. Spacings within the groups are about 5 G. These lines are interpreted as arising from a centre with two substitutional nitrogen atoms in next-neighbour positions. The hyperfine and superhyperfine couplings, which have axial symmetry, are due to a single electron coupling with both nitrogens, and their respective values are Aparallel=413±03 × 10−4 cm−1, Aperpendicular=292±03 × 10−4 cm−1 and A prime parallel=52±03 × 10−4 cm−1, A prime perpendicular=44±03 × 10−4 cm−1. The centre has an isotropic g of 20028±00002. Measurements of the angular dependence of the lines at 77 K and at higher temperatures showed that the N-C-N centre was not randomly distributed between all possible orientations but confined to sets of parallel {110} planes in the diamond. Above 200 K, averaging effects were observed and, from measurements of the broadening of the lines with increase of temperature when slow hopping was taking place, and the narrowing of new lines that appeared at high temperatures, the activation energy of the electron hopping between the two N-C bonds was found to be 024±001 eV.
TL;DR: In this paper, the Thomasson analysis was used to find diatom associations formed by 2% or more of species, found by the Thomasso analysis method, in relation to the physical and chemical vicissitudes of a stream in which the head waters become cut off from those of the middle reaches by a barren zone caused by mineral and acid pollution from dumps resulting from the gold mining industry on the Witwatersrand.
Abstract: Diatom associations formed by 2% or more of species, found by the Thomasson analysis method, are discussed in relation to the physical and chemical vicissitudes of a stream in which the head waters become cut off from those of the middle reaches by a barren zone caused by mineral and acid pollution from dumps resulting from the gold-mining industry on the Witwatersrand. Associations are found indicative of the originating waters, of regions of instability due to the pollution, whether it be its onset or recovery therefrom and of the lower recovered middle reaches. In each reach there are found to be associations indicative of pH, nitrogen content, dissolved oxygen and alkalinity. Thus the associations enable one to "read" the conditions of the river. An additional association occurs at the very turbid mouth of the river which is indicative of low light requirement. The work supports the findings of CHOLNOKY (1958 et seq.) in his researches into the diatom ecology of South African streams.
TL;DR: In this paper, the exact distribution of the test-statistic u-test was derived and the approach to normality was shown that the normality is slow, and a practical question dealt with the extrapolation of Ractliffe's results to Poisson expectations with A < 5.
Abstract: A further practical question deals with the extrapolation of Ractliffe's results to Poisson expectations with A < 5. In this paper, the exact distribution of the test-statistic u will be derived. It will be shown that the approach to normality is slow. Formulae will be given for numerical calculation of individual and cumulative probabilities. Particularly for a probability level of 5 per cent (double-tail), the u-test performs much better than the customary "binomial" test, with the added advantage of simplicity and non-requirement of tabulated values. The u-test may safely be applied from A = 2 5 upwards. 50
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TL;DR: Two patients, aged 41/2 and 5 months, with respiratory distress and obstructive emphysema, were found to have mediastinal bronchogenic cysts.
Abstract: Two patients, aged 41/2 and 5 months, with respiratory distress and obstructive emphysema, were found to have mediastinal bronchogenic cysts.