Showing papers by "University of Cambridge published in 1989"

State-space solutions to standard H/sub 2/ and H/sub infinity / control problems

Abstract: Simple state-space formulas are derived for all controllers solving the following standard H/sub infinity / problem: For a given number gamma >0, find all controllers such that the H/sub infinity / norm of the closed-loop transfer function is (strictly) less than gamma . It is known that a controller exists if and only if the unique stabilizing solutions to two algebraic Riccati equations are positive definite and the spectral radius of their product is less than gamma /sup 2/. Under these conditions, a parameterization of all controllers solving the problem is given as a linear fractional transformation (LFT) on a contractive, stable, free parameter. The state dimension of the coefficient matrix for the LFT, constructed using the two Riccati solutions, equals that of the plant and has a separation structure reminiscent of classical LQG (i.e. H/sub 2/) theory. This paper is intended to be of tutorial value, so a standard H/sub 2/ solution is developed in parallel. >

Inositol phosphates and cell signalling

Abstract: Inositol 1,4,5-trisphosphate is a second messenger which regulates intracellular calcium both by mobilizing calcium from internal stores and, perhaps indirectly, by stimulating calcium entry. In these actions it may function with its phosphorylated metabolite, inositol 1,3,4,5-tetrakisphosphate. The subtlety of calcium regulation by inositol phosphates is emphasized by recent studies that have revealed oscillations in calcium concentration which are perhaps part of a frequency-encoded second-messenger system.

A logic of authentication

Abstract: Authentication protocols are the basis of security in many distributed systems, and it is therefore essential to ensure that these protocols function correctly. Unfortunately, their design has been extremely error prone. Most of the protocols found in the literature contain redundancies or security flaws.A simple logic has allowed us to describe the beliefs of trustworthy parties involved in authentication protocols and the evolution of these beliefs as a consequence of communication. We have been able to explain a variety of authentication protocols formally, to discover subtleties and errors in them, and to suggest improvements. In this paper, we present the logic and then give the results of our analysis of four published protocols, chosen either because of their practical importance or because they serve to illustrate our method.

On q-analogues of the quantum harmonic oscillator and the quantum group SU(2)q

Abstract: The quantum group SU(2)q is discussed by a method analogous to that used by Schwinger to develop the quantum theory of angular momentum Such theory of the q-analogue of the quantum harmonic oscillator, as is required for this purpose, is developed

X-ray fluorescence from the inner disc in Cygnus X-1

Abstract: Les spectres RX de nombreuses sources de trous noirs en accretion suggerent la presence de matiere relativement froide entouree d'un plasma emetteur de RX dur. Les raies fluorescentes du fer sont produites par l'irradiation RX du gaz froid. On montre que la raie d'emission large du fer trouvee dans Cyg X-1 par Barr P., White N.E. et Page C.G. (1985, Mon. Not. R. astr. Soc., 216) est bien modelisee par une emission fluorescente emanant des parties internes d'un disque d'accretion

Basal Lipid Peroxidation in Substantia Nigra Is Increased in Parkinson's Disease

Abstract: Polyunsaturated fatty acid (PUFA) levels (an index of the amount of substrate available for lipid peroxidation) were measured in several brain regions from patients who died with Parkinson's disease and age-matched control human postmortem brains. PUFA levels were reduced in parkinsonian substantia nigra compared to other brain regions and to control tissue. However, basal malondialdehyde (MDA; an intermediate in the lipid peroxidation process) levels were increased in parkinsonian nigra compared with other parkinsonian brain regions and control tissue. Expressing basal MDA levels in terms of PUFA content, the difference between parkinsonian and control substantia nigra was even more pronounced. Stimulating MDA production by incubating tissue with FeSO4 plus ascorbic acid, FeSO4 plus H2O2, or air alone produced lower MDA levels in the parkinsonian substantia nigra, probably reflecting the lower PUFA content. These results may indicate that an increased level of lipid peroxidation continues to occur in the parkinsonian nigra up to the time of death, perhaps because of continued exposure to excess free radicals derived from some endogenous or exogenous neurotoxic species.

Mammalian mating systems.

Abstract: Male mammals show a diverse array of mating bonds, including obligate monogamy, unimale and group polygyny and promiscuity. These are associated with a wide variety of different forms of mate guarding, including the defence of feeding and mating territories, the defence of female groups and the defence of individual receptive females. Female mating bonds include long-term monogamy, serial monogamy, polyandry and promiscuity. Both male and female mating behaviour varies widely within species. Variation in male mating behaviour is related to the effect of male assistance in rearing young and to the defensibility of females by males. The latter is, in turn, related to female ranging behaviour and to the size and stability of female groups. Much of the variation in mammalian mating bonds and systems of mate guarding can be attributed to differences in these three variables.

Increased Nigral Iron Content and Alterations in Other Metal Ions Occurring in Brain in Parkinson's Disease

Abstract: Levels of iron, copper, zinc, manganese, and lead were measured by inductively coupled plasma spectroscopy in parkinsonian and age-matched control brain tissue There was 31-35% increase in the total iron content of the parkinsonian substantia nigra when compared to control tissue In contrast, in the globus pallidus total iron levels were decreased by 29% in Parkinson's disease There was no change in the total iron levels in any other region of the parkinsonian brain Total copper levels were reduced by 34-45% in the substantia nigra in Parkinson's disease; no difference was found in the other brain areas examined Zinc levels were increased in substantia nigra in Parkinson's disease by 50-54%, and the zinc content of the caudate nucleus and lateral putamen was also raised by 18-35% Levels of manganese and lead were unchanged in all areas of the parkinsonian brain studied when compared to control brains, except for a small decrease (20%) in manganese content of the medial putamen Increased levels of total iron in the substantia nigra may cause the excessive formation of toxic oxygen radicals, leading to dopamine cell death

Early predictors of adolescent aggression and adult violence

Abstract: The Cambridge Study in Delinquent Development is a prospective longitudinal survey of 411 London males from ages 8 years old to 32 years old. This article investigates the prediction of adolescent aggression (ages 12-14 years old), teenage violence (ages 16-18 years old), adult violence (age 32 years old), and convictions for violence. Generally, the best predictors were measures of economic deprivation, family criminality, poor child-rearing, school failure, hyperactivity-impulsivity-attention deficit, and antisocial child behavior. Similar predictors applied to all four measures of aggression and violence. It is concluded that aggression and violence are elements of a more general antisocial tendency, and that the predictors of aggression and violence are similar to the predictors of antisocial and criminal behavior in general.

Robust stabilization of normalized coprime factor plant descriptions with H/sub infinity /-bounded uncertainty

Abstract: The problem of robustly stabilizing a family of linear systems is explicitly solved in the case where the family is characterized by H/sub infinity / bounded perturbations to the numerator and denominator of the normalized left coprime factorization of a nominal system. This problem can be reduced to a Nehari extension problem directly and gives an optimal stability margin. All controllers satisfying a suboptimal stability margin are characterized, and explicit state-space formulas are given. >

The thickness, volume and grainsize of tephra fall deposits

Abstract: An improved empirical method for the plotting of field data and the calculation of tephra fall volumes is presented. The widely used “area” plots of ln(thickness) against ln(isopach area) are curved, implying an exponential thinning law. Use of ln(thickness)−(area)1/2 diagrams confirm the exponential dependence of many parameters (e.g. thickness, maximum and median clast size) with distance from source, producing linear graphs and allowing volumes to be calculated without undue extrapolation of field data. The agreement between theoretical models of clast dispersion and observation is better than previously thought. Two new quantitative parameters are proposed which describe the rates of thinning of the deposit (bt the thickness half-distance) and the maximum clast size (bc the clast half-distance). Many deposits exhibit different grainsize and thickness thinning rates, with the maximum clast size diminishing 1–3 times slower than the thickness. This implies that the entrained grainsize population influences the morphologic and granulometric patterns of the resulting deposit, in addition to the effects of column height and wind-speed. The grainsize characteristics of a deposit are best described by reference to the half-distance ratio (bc/bt). A new classification scheme is proposed which plots the half-distance ratio against the thickness half-distance and may be contoured in terms of the column height.

Mapping the transition state and pathway of protein folding by protein engineering

Abstract: In the transition state for unfolding of barnase, the hydrophobic core between the major alpha-helix and beta-sheet is somewhat weakened, the C terminus of the major helix is largely intact but its N terminus is exposed and a major loop has been invaded by solvent.

Structure, biochemistry and mechanism of action of glycopeptide antibiotics.

Abstract: Glycopeptide antibiotics, including vancomycin and teicoplanin, are large, rigid molecules that inhibit a late stage in bacterial cell wall peptidoglycan synthesis. The three-dimensional structure contains a cleft into which peptides of highly specific configuration (L-aa-D-aa-D-aa) can fit: such sequences are found only in bacterial cell walls, hence glycopeptides are selectively toxic. Glycopeptides interact with peptides of this conformation by hydrogen bonding, forming stable complexes. As a result of binding to L-aa-D-Ala-D-Ala groups in wall intermediates, glycopeptides inhibit, apparently by steric hindrance, the formation of the backbone glycan chains (catalysed by peptidoglycan polymerase) from the simple wall subunits as they are extruded through the cytoplasmic membrane. The subsequent transpeptidation reaction that imparts rigidity to the cell wall is also thus inhibited. This unique mechanism of action, involving binding of the bulky inhibitor to the substrate outside the membrane so that the active sites of two enzymes cannot align themselves correctly, renders the acquisition of resistance to the glycopeptide antibiotics more difficult than that to the majority of the other antibiotic groups.

The validity of the eating disorder examination and its subscales.

Abstract: The EDE is a semistructured interview which has been developed as a measure of the specific psychopathology of anorexia nervosa and bulimia nervosa. To establish its discriminant validity it was administered to 100 patients with anorexia nervosa or bulimia nervosa and to 42 controls. The two groups differed significantly on all items. Five subscales were derived on rational grounds and evaluated on the two populations. The alpha coefficients for each subscale indicated a satisfactory degree of internal consistency. The EDE provides clinicians and research workers with a detailed and comprehensive profile of the psychopathological features of patients with eating disorders.

The allometry of growth and reproduction

Abstract: Preface 1. Introduction 2. The scaling of average daily metabolic rate and energy intake 3. Why do larger species invest relatively less in their offspring? 4. The intraspecific relationship of parental investment to female body weight 5. Growth and productivity 6. Quantitative models of body size 7. Sexual dimorphism in body size 8. Are larger species more dimorphic in body size? 9. Surface area/volume arguments in biology 10. Prospectus Concluding discussion Glossary of mathematical terms References Index.

The carbonate environment in bone mineral: a resolution-enhanced Fourier Transform Infrared Spectroscopy Study.

Abstract: The environment of carbonate ions in bones of different species (rat, rabbit, chicken, cow, human) was investigated by Fourier Transform Infrared Spectroscopy (FTIR) associated with a self-deconvolution technique. The carbonate bands in thev 2 CO 3 2− domain show three components which were identified by using synthetic standards and different properties of the apatitic structure (ionic affinity for crystallographic locations, ionic exchange). The major component at 871 cm−1 is due to carbonate ions located in PO 4 3− sites (type B carbonate). A band at 878 cm−1 was exclusively assigned to carbonate ions substituting for OH− ions in the apatitic structure (type A carbonate). A band at 866 cm−1 not previously observed was shown to correspond to a labile carbonate environment. The intensity ratio of type A to type B carbonate appears remarkably constant in all bone samples. The 866 cm−1 carbonate band varies in its relative intensity in different species.

Fitness costs of gestation and lactation in wild mammals.

Abstract: Like a number of plants1,2, some mammals commonly produce more progeny than they can afford to rear, terminating investment in some or even all of their offspring once the resources available for breeding are known3–5. Adaptive interpretations of juvenile wastage rely on the argument that the costs of gestation are small compared to those of feeding offspring. Though energetic evidence supports this conclusion6, it is unsafe to assume that the relative costs of gestation and lactation to the mother's survival and future reproductive success follow the same pattern because lactation commonly coincides with the period of maximum food availability. Controlling for individual variation, we show that in wild red deer (Cervus elaphus L) any costs of gestation to the mother's subsequent survival and reproductive success are slight compared to those of lactation.

“Tho' she kneel'd in that place where they grew…” The uses and origins of primate colour vision

Abstract: The disabilities experienced by colour-blind people show us the biological advantages of colour vision in detecting targets, in segregating the visual field and in identifying particular objects or states. Human dichromats have especial difficulty in detecting coloured fruit against dappled foliage that varies randomly in luminosity; it is suggested that yellow and orange tropical fruits have co-evolved with the trichromatic colour vision of Old World monkeys. It is argued that the colour vision of man and of the Old World monkeys depends on two subsystems that remain parallel and independent at early stages of the visual pathway. The primordial subsystem, which is shared with most mammals, depends on a comparison of the rates of quantum catch in the short- and middle-wave cones; this system exists almost exclusively for colour vision, although the chromatic signals carry with them a local sign that allows them to sustain several of the functions of spatiochromatic vision. The second subsystem arose from the phylogenetically recent duplication of a gene on the X-chromosome, and depends on a comparison of the rates of quantum catch in the long- and middle-wave receptors. At the early stages of the visual pathway, this chromatic information is carried by a channel that is also sensitive to spatial contrast. The New World monkeys have taken a different route to trichromacy: in species that are basically dichromatic, heterozygous females gain trichromacy as a result of X-chromosome inactivation, which ensures that different photopigments are expressed in two subsets of retinal photoreceptor.

Thapsigargin, a novel molecular probe for studying intracellular calcium release and storage.

Abstract: 1 Department of Clinical Chemistry, University Hospital, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen, Denmark; 2 School of Biological Sciences, University of East Anglia, Norwich NR4 7T J, UK; 3 Department of Clinical Physiology and Nuclear Medicine, University Hospital, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen, Denmark; 4 Department of Chemistry BC, Royal Danish School of Pharmacy, Universitetsparken 2, DK-2100 Copenhagen, Denmark; s MRC Molecular Neurobiology Unit, University of Cambridge Medical School, Hills Road, Cambridge CB2 2QH, UK; and 6 Thrombosis Group, Copenhagen Science Park Symbion, Haraldsgade 68, DK-2100 Copenhagen, Denmark

Tight-binding models and density-functional theory

Abstract: Semiempirical tight-binding models have been widely used but the details of their relationship to more fundamental theories have never been clear and so they have usually been treated as fitting and interpolation schemes rather than as quantitative calculational tools. In this paper, we show how simple tight-binding models can be understood as stationary approximations to self-consistent density-functional theory and give prescriptions for calculating all the required potentials and matrix elements. Some preliminary applications have given encouraging results.