List of Records in School of Physical Sciences Publications - UQ eSpace

A basin system and fluid-flow analysis of the Zn-Pb-Ag Mount Isa-type deposits of Northern Australia: Identifying metal source, basinal brine reservoirs, times of fluid expulsion, and organic matter reactions

2007-08-15T10:48:18Z

A Bethe ansatz solvable model for superpositions of Cooper pairs and condensed molecular bosons

Hibberd, K. E. og Dunning, C. og Links, J. — 2007-08-15T10:14:56Z

We introduce a general Hamiltonian describing coherent superpositions of Cooper pairs and condensed molecular bosons. For particular choices of the coupling parameters, the model is integrable. One integrable manifold, as well as the Bethe ansatz solution, was found by Dukelsky et al. [J. Dukelsky, G.G. Dussel, C. Esebbag, S. Pittel, Phys. Rev. Lett. 93 (2004) 050403]. Here we show that there is a second integrable manifold, established using the boundary quantum inverse scattering method. In this manner we obtain the exact solution by means of the algebraic Bethe ansatz. In the case where the Cooper pair energies are degenerate we examine the relationship between the spectrum of these integrable Hamiltonians and the quasi-exactly solvable spectrum of particular Schrodinger operators. For the solution we derive here the potential of the Schrodinger operator is given in terms of hyperbolic functions. For the solution derived by Dukelsky et al., loc. cit. the potential is sextic and the wavefunctions obey PT-symmetric boundary conditions. This latter case provides a novel example of an integrable Hermitian Hamiltonian acting on a Fock space whose states map into a Hilbert space of PE-symmetric wavefunctions defined on a contour in the complex plane. (c) 2006 Elsevier B.V. All rights reserved.

A brave new world of information

Bahr, M. — 2007-08-14T14:57:47Z

Absolute stability of positive systems with differential constraints

Diamond, P. M. og Opoitsev, V. — 2008-06-10T10:39:34Z

We consider discrete-time systems x(k + 1) = Ax(k) + f(x(k)) where the matrix A of the linear part is known and positive, the non-linearity f is unknown but belongs to a class for which A + f'(x) is positive with spectral radius < I for all x is an element of R-n. This, with the additional property that x - Ax - f(x) is proper, is sufficient for global stability of the system. The results are applied to the continuous system x = Ax + B phi(C(T)x) by considering the translation operator along trajectories and studying the resulting discrete system.

Absorption and dispersion by a multiple driven two-level atom

2007-08-14T17:53:20Z

We investigate the absorption and dispersion properties of a two-level atom driven by a polychromatic field. The driving field is composed of a strong resonant (carrier) frequency component and a large number of symmetrically detuned sideband fields (modulators). A rapid increase in the absorption at the central frequency and the collapse of the response of the system from multiple frequencies to a single frequency are predicted to occur when the Rabi frequency of the modulating fields is equal to the Rabi frequency of the carrier field. These are manifestations of the undressing or a disentanglement of the atomic and driving field states, that leads to a collapse of the atom to its ground state. Our calculation permits consideration of the question of the undressing of the driven atom by a multiple-modulated field and the predicted spectra offer a method of observing undressing. Moreover, we find that the absorption and dispersion spectra split into multiplets whose structures depend on the Rabi frequency of the modulating fields. The spectral features can jump between different resonance frequencies by changing the Rabi frequency of the modulating fields or their initial phases, which can have potential applications as a quantum frequency filter.

Absorption and fluorescence spectroscopy of rhodamine 6G in titanium dioxide nanocomposites

2007-08-15T04:21:28Z

A comparison has been made between the spectroscopic properties of the laser dye rhodamine 6G (R6G) in mesostructured titanium dioxide (TiO2) and in ethanol. Steady-state excitation and emission techniques have been used to probe the dye-matrix interactions. We show that the TiO2-nanocomposite studied is a good host for R6G, as it allows high dye concentrations, while keeping dye molecules isolated, and preventing aggregation. Our findings have important implications in the context of solid state dye-lasers and microphotonic device applications. (C) 2003 Elsevier B.V. All rights reserved.

Absorption spectrum of a strongly driven atom in a detuned squeezed vacuum

Bosticky, M. og Ficek, Z. og Dalton, B. J. — 2007-08-13T11:19:53Z

We present numerical and analytical results for the Mollow probe absorption spectrum of a coherently driven two-level system in a narrow bandwidth squeezed vacuum field. The spectra are calculated for the case where the Rabi frequency of the driving field is much larger than the natural linewidth and the squeezed vacuum carrier frequency is detuned from the driving laser frequency. The driving laser is on resonance. We show that in a detuned squeezed vacuum the standard Mellow features are each split into triplets. The central components of each triplet are weakly dependent on the squeezing phase but the sidebands strongly depend on the phase and can have dispersive or absorptive/emissive profiles. We also derive approximate analytical expressions for the spectral features and find that the multi-peak structure of the spectrum can be interpreted either via the eigenfrequencies of a generalized Floquet Hamiltonian or in terms of three-photon transitions between dressed stales involving a probe field photon and a correlated photon pair from the squeezed vacuum field.

Absorptive quantum measurements via coherently coupled quantum dots

2007-08-13T12:07:58Z

We propose an absorptive measurement scheme via coupled quantum dots based on studies of the quantum dynamics of coherently coupled dots. The system is described through a Markov master equation that is related to a measurable quantity, the current. We analyse the measurement configuration and calculate the correlations and noise spectra beyond the adiabatic approximation.

Abyssal peridotites and associated basalts whole-rock trace element systematics: evidence for melt-solid interaction during melt ascent beneath ocean ridges

2008-06-06T16:45:01Z

A catalog of HI-selected galaxies from the south celestial cap region of sky

2007-08-13T13:04:03Z

The first deep catalog of the H I Parkes All Sky Survey (HIPASS) is presented, covering the south celestial cap (SCC) region. The SCC area is similar to2400 deg(2) and covers delta < -62&DEG;. The average rms noise for the survey is 13 mJy beam(-1). Five hundred thirty-six galaxies have been cataloged according to their neutral hydrogen content, including 114 galaxies that have no previous cataloged optical counterpart. This is the largest sample of galaxies from a blind H I survey to date. Most galaxies in optically unobscured regions of sky have a visible optical counterpart; however, there is a small population of low-velocity H I clouds without visible optical counterparts whose origins and significance are unclear. The rms accuracy of the HIPASS positions is found to be 1.'9. The H I mass range of galaxies detected is from &SIM;10(6) to &SIM;10(11) M-.. There are a large number of late-type spiral galaxies in the SCC sample (66%), compared with 30% for optically selected galaxies from the same region in the NASA Extragalactic Database. The average ratio of H I mass to B luminosity of the sample increases according to optical type, from 1.8 M-./L-. for early types to 3.2 M-./L-. for late-type galaxies. The H I-detected galaxies tend to follow the large-scale structure traced by galaxies found in optical surveys. From the number of galaxies detected in this region of sky, we predict the full HIPASS catalog will contain &SIM;5000 galaxies, to a peak flux density limit of &SIM;39 mJy (3 σ), although this may be a conservative estimate as two large voids are present in the region. The H I mass function for this catalog is presented in a subsequent paper.

Accounting for management costs in sensitivity analyses of matrix population models

2007-08-15T10:24:47Z

Traditional sensitivity and elasticity analyses of matrix population models have been used to p inform management decisions, but they ignore the economic costs of manipulating vital rates. For exam le, the growth rate of a population is often most sensitive to changes in adult survival rate, but this does not mean that increasing that rate is the best option for managing the population because it may be much more expensive than other options. To explore how managers should optimize their manipulation of vital rates, we incorporated the cost of changing those rates into matrix population models. We derived analytic expressions for locations in parameter space where managers should shift between management of fecundity and survival, for the balance between fecundity and survival management at those boundaries, and for the allocation of management resources to sustain that optimal balance. For simple matrices, the optimal budget allocation can often be expressed as simple functions of vital rates and the relative costs of changing them. We applied our method to management of the Helmeted Honeyeater (Lichenostomus melanops cassidix; an endangered Australian bird) and the koala (Phascolarctos cinereus) as examples. Our method showed that cost-efficient management of the Helmeted Honeyeater should focus on increasing fecundity via nest protection, whereas optimal koala management should focus on manipulating both fecundity and survival simultaneously, These findings are contrary to the cost-negligent recommendations of elasticity analysis, which would suggest focusing on managing survival in both cases. A further investigation of Helmeted Honeyeater management options, based on an individual-based model incorporating density dependence, spatial structure, and environmental stochasticity, confirmed that fecundity management was the most cost-effective strategy. Our results demonstrate that decisions that ignore economic factors will reduce management efficiency.

Accurate and precise isotope ratio measurements of NBS 981 lead by multiple collector inductively coupled plasma mass spectrometry. Why are the ratios different from TIMS measurements?

Collerson, K. D. og Palacz, Z. — 2008-06-06T17:05:21Z

Accurate switching intensities and length scales in quasi-phase-matched materials

2007-09-19T16:31:45Z

We consider unseeded type I second-harmonic generation in quasi-phase-matched quadratic nonlinear materials and derive an accurate analytical expression for the evolution of the average intensity. The intensity dependent nonlinear phase mismatch that is due to the cubic nonlinearity induced by quasi phase matching is found. The equivalent formula for the intensity of maximum conversion, the crossing of which changes the one-period nonlinear phase shift of the fundamental abruptly by rr, corrects earlier estimates [Opt. Lett. 23, 506 (1998)] by a factor of 5.3. We find the crystal lengths that are necessary to obtain an optimal flat phase versus intensity response on either side of this separatrix intensity. (C) 2001 Optical Society of America.

A census of critical sets in the Latin squares of order at most six

Adams, P og Bean, R og Khodkar, A — 2007-08-15T02:41:14Z

A critical set in a Latin square of order n is a set of entries from the square which can be embedded in precisely one Latin square of order n, Such that if any element of the critical set. is deleted, the remaining set can be embedded, in more than one Latin square of order n.. In this paper we find all the critical sets of different sizes in the Latin squares of order at most six. We count the number of main and isotopy classes of these critical sets and classify critical sets from the main classes into various strengths. Some observations are made about the relationship between the numbers of classes, particularly in the 6 x 6 case. Finally some examples are given of each type of critical set.

A characteristic of strong separation property for convex sets

Yang, G. og Yuan, X. — 2008-06-10T15:21:59Z

A checklist for ecological management of landscapes for conservation

2008-04-14T09:55:36Z

The management of landscapes for biological conservation and ecologically sustainable natural resource use are crucial global issues. Research for over two decades has resulted in a large literature, yet there is little consensus on the applicability or even the existence of general principles or broad considerations that could guide landscape conservation. We assess six major themes in the ecology and conservation of landscapes. We identify 13 important issues that need to be considered in developing approaches to landscape conservation. They include recognizing the importance of landscape mosaics (including the integration of terrestrial and aquatic areas), recognizing interactions between vegetation cover and vegetation configuration, using an appropriate landscape conceptual model, maintaining the capacity to recover from disturbance and managing landscapes in an adaptive framework. These considerations are influenced by landscape context, species assemblages and management goals and do not translate directly into on-the-ground management guidelines but they should be recognized by researchers and resource managers when developing guidelines for specific cases. Two crucial overarching issues are: (i) a clearly articulated vision for landscape conservation and (ii) quantifiable objectives that offer unambiguous signposts for measuring progress

Achieving Peak Brightness in an Atom Laser

2009-02-11T16:08:55Z

In this Letter we present experimental results and a simple analytic theory on the first continuous (long pulse) Raman atom laser. We analyze the flux and brightness of a generic two state atom laser with an analytic model that shows excellent agreement with our experiments. We show that, for the same source size, the brightness achievable with a Raman atom laser is at least 3 orders of magnitude greater than achievable in any other demonstrated continuously outcoupled atom laser.

Achromatic optical phase shifter/modulator

Zvyagin, Andrei V. og Sampson, David D. — 2007-04-11T09:20:00Z

We propose and demonstrate, theoretically and experimentally, a novel achromatic optical phase shifter modulator based on a frequency-domain optical delay line configured to maintain zero group delay as variable phase delay is generated by means of tilting a mirror. Compared with previously reported phase shifter modulators, e.g., based on the Pancharatnam (geometric) phase, our device is high speed and polarization insensitive and produces a large, bounded phase delay that, uniquely, is one-to-one mapped to a measurable parameter, the tilt angle.

Acid mine drainage at Mount Morgan, Queensland (Australia): Experimental simulation and geochemical modelling of buffering reactions.

2008-04-02T13:35:14Z

A class of compact dwarf galaxies from disruptive processes in galaxy clusters

2007-04-02T16:55:27Z

Dwarf galaxies have attracted increased attention in recent years, because of their susceptibility to galaxy transformation processes within rich galaxy clusters. Direct evidence for these processes, however, has been difficult to obtain, with a small number of diffuse light trails and intra-cluster stars, being the only signs of galaxy disruption. Furthermore, our current knowledge of dwarf galaxy populations may be very incomplete, because traditional galaxy surveys are insensitive to extremely diffuse or compact galaxies. Aware of these concerns, we recently undertook an all-object survey of the Fornax galaxy cluster. This revealed a new population of compact members, overlooked in previous conventional surveys. Here we demonstrate that these 'ultra-compact' dwarf galaxies are structurally and dynamically distinct from both globular star clusters and known types of dwarf galaxy, and thus represent a new class of dwarf galaxy. Our data are consistent with the interpretation that these are the remnant nuclei of disrupted dwarf galaxies, making them an easily observed tracer of galaxy disruption.

A class of human exons with predicted distant branch points revealed by analysis of AG dinucleotide exclusion zones

2007-08-16T10:07:00Z

A comparison of bichromatic beam splitters for atoms

2007-08-13T10:26:09Z

We analyse and compare various aspects of the performance of atomic beam splitters fur two- and three-level atoms, both of which use bichromatic optical fields. We calculate the extent to which spontaneous emission degrades the sharpness of the splitting, and how it might degrade the visibility of an idealised atom interferometer which includes either beam splitting mechanism. (C) 1998 Elsevier Science B.V.

A comparison of breakage behaviour for New Zealand and Australian coals

2008-08-11T09:40:35Z

A comparison of models for predicting population persistence

Cairns, B. J. og Ross, J. V. og Taimre, T. — 2008-03-27T13:55:57Z

A Comparison of Surface Brightness Profiles for Ultracompact Dwarfs and Dwarf Elliptical Nuclei: Implications for the “Threshing” Scenario

2007-08-15T06:16:49Z

Using imaging from the Hubble Space Telescope, we derive surface brightness profiles for ultracompact dwarfs in the Fornax Cluster and for the nuclei of dwarf elliptical galaxies in the Virgo Cluster. Ultracompact dwarfs are more extended and have higher surface brightnesses than typical dwarf nuclei, while the luminosities, colors, and sizes of the nuclei are closer to those of Galactic globular clusters. This calls into question the production of ultracompact dwarfs via threshing, whereby the lower surface brightness envelope of a dwarf elliptical galaxy is removed by tidal processes, leaving behind a bare nucleus. Threshing may still be a viable model if the relatively bright Fornax ultracompact dwarfs considered here are descended from dwarf elliptical galaxies whose nuclei are at the upper end of their luminosity and size distributions.

A comparison of two multi-component wavefield separation techniques

Hendrick, N. og Hearn, S. J. — 2008-06-06T13:25:48Z

A complex change of variables for geometrical reasoning

Stokes, T. og Bulmer, M. — 2007-08-23T23:21:41Z

A conjecture on small embeddings of partial Steiner triple systems

Bryant, D — 2007-08-14T17:34:29Z

A well-known, and unresolved, conjecture states that every partial Steiner triple system of order u can be embedded in a Steiner triple system of order v for all v equivalent to 1 or 3 (mod 6), v greater than or equal to 2u + 1. However, some partial Steiner triple systems of order u can be embedded in Steiner triple systems of order v < 2u + 1. A more general conjecture that considers these small embeddings is presented and verified for some cases. (C) 2002 Wiley Periodicals, Inc.

A Constant Torque Micro-Viscometer

2007-08-23T21:23:38Z

We present a technique to measure the viscosity of microscopic volumes of liquid using rotating optical tweezers. The technique can be used when only microlitre (or less) sample volumes are available, for example biological or medical samples, or to make local measurements in complicated micro-structures such as cells. The rotation of the optical tweezers is achieved using the polarisation of the trapping light to rotate a trapped birefringent spherical crystal, called vaterite. Transfer of angular momentum from a circularly polarised beam to the particle causes the rotation. The transmitted light can then be analysed to determine the applied torque to the particle and its rotation rate. The applied torque is determined from the change in the circular polarisation of the beam caused by the vaterite and the rotation rate is used to find the viscous drag on the rotating spherical particle. The viscosity of the surrounding liquid can then be determined. Using this technique we measured the viscosity of liquids at room temperature, which agree well with tabulated values. We also study the local heating effects due to absorption of the trapping laser beam. We report heating of 50-70 K/W in the region of liquid surrounding the particle.

A construction for R-matrices without difference property in the spectral parameter

Links, J — 2008-06-10T10:22:01Z

A new construction is given for obtaining R-matrices which solve the McGuire-Yang-Baxter equation in such a way that the spectral parameters do not possess the difference property. A discussion of the derivation of the supersymmetric U model for correlated electrons is given in this context, such that applied chemical potential and magentic field terms can be coupled arbitrarily. As a limiting case the Bariev model is obtained. (C) 2000 Elsevier Science B.V. All rights reserved.

Acritarch palynoflora of the Coolibah Formation (Lower Ordovician), Georgina Basin, Queensland

Playford, G. og Wicander, R. — 2009-07-13T15:20:08Z

Acritarchs and prasinophyte phycomata: a short course

Playford, G. — 2007-08-14T19:20:57Z

Acritarchs and spores from the Upper Devonian Lime Creek Formation, Iowa, U.S.A.

Wicander, R. og Playford, G. — 2009-07-13T12:11:57Z

Adaptive homodyne measurement of optical phase

2007-09-19T15:38:33Z

We present an experimental demonstration of the power of feedback in quantum metrology, confirming the predicted [H. M. Wiseman, Phys. Rev. Lett. 75, 4587 (1995)] superior performance of an adaptive homodyne technique for single-shot measurement of optical phase. For measurements performed on weak coherent states with no prior knowledge of the signal phase, adaptive homodyne estimation approaches closer to the intrinsic quantum uncertainty than any previous technique. Our results underscore the importance of real-time feedback for reaching quantum limits in measurement and control.

Adaptive measurements and optimal states for quantum interferometry

Berry, D. og Wiseman, H. M. — 2008-06-06T15:05:26Z

Adaptive Phase estimation is more accurate than nonadaptive phase estimation for continuous beams of light

2007-08-15T04:17:31Z

We consider the task of estimating the randomly fluctuating phase of a continuous-wave beam of light. Using the theory of quantum parameter estimation, we show that this can be done more accurately when feedback is used (adaptive phase estimation) than by any scheme not involving feedback (nonadaptive phase estimation) in which the beam is measured as it arrives at the detector. Such schemes not involving feedback include all those based on heterodyne detection or instantaneous canonical phase measurements. We also demonstrate that the superior accuracy of adaptive phase estimation is present in a regime conducive to observing it experimentally.

Adaptive phase measurements in linear optical quantum computation

2007-08-15T05:28:40Z

Photon counting induces an effective non-linear optical phase shift in certain states derived by linear optics from single photons. Although this non-linearity is non-deterministic, it is sufficient in principle to allow scalable linear optics quantum computation (LOQC). The most obvious way to encode a qubit optically is as a superposition of the vacuum and a single photon in one mode-so-called 'single-rail' logic. Until now this approach was thought to be prohibitively expensive (in resources) compared to 'dual-rail' logic where a qubit is stored by a photon across two modes. Here we attack this problem with real-time feedback control, which can realize a quantum-limited phase measurement on a single mode, as has been recently demonstrated experimentally. We show that with this added measurement resource, the resource requirements for single-rail LOQC are not substantially different from those of dual-rail LOQC. In particular, with adaptive phase measurements an arbitrary qubit state a alpha/0 > + beta/1 > can be prepared deterministically.

Adiabatic elimination in compound quantum systems with feedback

Warszawski, P. og Wiseman, H. M. — 2008-06-10T16:08:12Z

Feedback in compound quantum systems is effected by using the output from one subsystem (“the system”) to control the evolution of a second subsystem (“the ancilla”) that is reversibly coupled to the system. In the limit where the ancilla responds to fluctuations on a much shorter time scale than does the system, we show that it can be adiabatically eliminated, yielding a master equation for the system alone. This is very significant as it decreases the necessary basis size for numerical simulation and allows the effect of the ancilla to be understood more easily. We consider two types of ancilla: a two-level ancilla (e.g., a two-level atom) and an infinite-level ancilla (e.g., an optical mode). For each, we consider two forms of feedback: coherent (for which a quantum-mechanical description of the feedback loop is required) and incoherent (for which a classical description is sufficient). We test the master equations we obtain using numerical simulation of the full dynamics of the compound system. For the system (a parametric oscillator) and feedback (intensity-dependent detuning) we choose, good agreement is found in the limit of heavy damping of the ancilla. We discuss the relation of our work to previous work on feedback in compound quantum systems, and also to previous work on adiabatic elimination in general.

A diffusion approach to approximating preservation probabilities for gene duplicates

O'Hely, Martin — 2007-08-15T09:40:51Z

Consider a haploid population and, within its genome, a gene whose presence is vital for the survival of any individual. Each copy of this gene is subject to mutations which destroy its function. Suppose one member of the population somehow acquires a duplicate copy of the gene, where the duplicate is fully linked to the original gene's locus. Preservation is said to occur if eventually the entire population consists of individuals descended from this one which initially carried the duplicate. The system is modelled by a finite state-space Markov process which in turn is approximated by a diffusion process, whence an explicit expression for the probability of preservation is derived. The event of preservation can be compared to the fixation of a selectively neutral gene variant initially present in a single individual, the probability of which is the reciprocal of the population size. For very weak mutation, this and the probability of preservation are equal, while as mutation becomes stronger, the preservation probability tends to double this reciprocal. This is in excellent agreement with simulation studies.

A diffusion model for the fate of tandem gene duplicates in diploids

O'Hely, M. F. — 2008-03-27T10:47:19Z

Suppose one chromosome in one member of a population somehow acquires a duplicate copy of the gene, fully linked to the original gene's locus. Preservation is the event that eventually every chromosome in the population is a descendant of the one which initially carried the duplicate. For a haploid population in which the absence of all copies of the gene is lethal, the probability of preservation has recently been estimated via a diffusion approximation. That approximation is shown to carry over to the case of diploids and arbitrary strong selection against the absence of the gene. The techniques used lead to some new results. In the large population limit, it is shown that the relative probability that descendants of a small number of individuals carrying multiple copies of the gene fix in the population is proportional to the number of copies carried. The probability of preservation is approximated when chromosomes carrying two copies of the gene are subject to additional, fully non-functionalizing mutations, thereby modelling either an additional cost of replicating a longer genome, or a partial duplication of the gene. In the latter case the preservation probability depends only on the mutation rate to null for the duplicated portion of the gene. (C) 2007 Elsevier Inc. All rights reserved.

A discussion of constrained binary embeddings with applicatins to cryptanalysis of irregularly clocked stream ciphers

Donovan, D M og Lefevre, J G og SIMPSON, L — 2007-08-23T22:04:18Z

A discussion of 2-critical sets in Abelian 2-groups

Donovan, D. M. og Fu, C.-M. og Khodkar, A. — 2007-08-23T23:38:12Z

Adjusted likelihood methods for modelling dispersion in generalized linear models

Smyth, G. og Verbyla, A. P. — 2008-06-10T13:35:26Z

Adsorption dynamics measured by permeation and batch adsorption methods

Rutherford, S. W. og Do, D. D. — 2007-08-19T15:27:01Z

We have measured the adsorption equilibrium and kinetics of carbon dioxide on a commercially available activated carbon by two methods; permeation and batch adsorption. The two methods are compared and found to yield consistent results. All experiments are performed at low pressure (<5 torr) and in this range the isotherm was found to be reversible and non-linear Because of the observed non-linearity, the batch adsorption experiments were conducted differentially in order to obtain the adsorbed phase diffusivity at local conditions. The diffusion process was described by gas phase diffusion, adsorption and adsorbed phase diffusion which was modelled using the Darken relation. Results of adsorption equilibria and kinetics will be discussed in detail in this investigation. (C) 2000 Elsevier Science S.A. All rights reserved.

Advantages of flexible delivery for introductory physics

Rayner, A. — 2008-06-06T15:05:59Z

A dynamic quantitative approach for predicting the shape of phytoplankton profiles in the ocean

2007-09-19T18:01:52Z

Estimation of primary production over large areas of the ocean requires information on the shape of phytoplankton profiles. In this study we develop a generic quantitative approach to describe the continuous variation of profile shape within a region. We illustrate this approach by application to the dynamic southern Benguela upwelling system. First, we describe profile shape by fitting a four-parameter shifted Gaussian model. We then use a model-building approach to relate each parameter to a suite of environmental variables that are either known for each point of the ocean in time and space (depth of the water column, season, and area) or are easily obtained from remote sensing (sea surface temperature and surface chlorophyll a). As these variables are highly correlated and non-linearly related to profile shape, we use generalised additive models to visualise the non-linear relationships between each parameter and all environmental variables simultaneously. These relationships are then parameterised using generalised linear models to obtain a predictive equation for each profile parameter. Relationships identified made intuitive sense in terms of the evolution of phytoplankton blooms in upwelling systems. We found strong predictive relationships for the depth of maximum chlorophyll (r(2) = 0.70) and the total chlorophyll in the peak (r(2) = 0.74), the two most important parameters for estimating primary production. Predictive relationships were weaker for the width of the peak (r(2) = 0.21) and the background chlorophyll (r(2) = 0.15). The predictive equations identified can be applied on a pixel-by-pixel basis to concurrent sea surface temperature and ocean colour images to estimate profile shape, and can be imbedded within local algorithms to provide regional primary production estimates. This approach can easily be applied to other biogeo-chemical provinces. (C) 2003 Elsevier Ltd. All rights reserved.

Aether theories: A Physics fairytale re-told

Hajek, K. M. og Nieminen, T A — 2007-08-23T22:10:49Z

A family of perfect factorisations of complete bipartite graphs

2007-08-13T12:58:39Z

A 1-factorisation of a graph is perfect if the union of any two of its 1-factors is a Hamiltonian cycle. Let n = p(2) for an odd prime p. We construct a family of (p-1)/2 non-isomorphic perfect 1-factorisations of K-n,K-n. Equivalently, we construct pan-Hamiltonian Latin squares of order n. A Latin square is pan-Hamiltoilian if the permutation defined by any row relative to any other row is a single Cycle. (C) 2002 Elsevier Science (USA).

A Fast Cross-Entropy Method for Estimating Buffer Overflows in Queueing Networks

2004-06-02T00:00:00Z

In this paper we propose a fast adaptive Importance Sampling method for the efficient simulation of buffer overflow probabilities in queueing networks. The method comprises three stages. First we estimate the minimum Cross-Entropy tilting parameter for a small buffer level; next, we use this as a starting value for the estimation of the optimal tilting parameter for the actual (large) buffer level; finally, the tilting parameter just found is used to estimate the overflow probability of interest. We study various properties of the method in more detail for the M/M/1 queue and conjecture that similar properties also hold for quite general queueing networks. Numerical results support this conjecture and demonstrate the high efficiency of the proposed algorithm.

A fast cross-entropy method for estimating buffer overflows in queuing networks

2007-05-23T16:55:45Z

In this paper, we propose a fast adaptive importance sampling method for the efficient simulation of buffer overflow probabilities in queueing networks. The method comprises three stages. First, we estimate the minimum cross-entropy tilting parameter for a small buffer level; next, we use this as a starting value for the estimation of the optimal tilting parameter for the actual (large) buffer level. Finally, the tilting parameter just found is used to estimate the overflow probability of interest. We study various properties of the method in more detail for the M/M/1 queue and conjecture that similar properties also hold for quite general queueing networks. Numerical results support this conjecture and demonstrate the high efficiency of the proposed algorithm.