3,644 research outputs found
Oscillations in a maturation model of blood cell production.
We present a mathematical model of blood cell production which describes both the development of cells through the cell cycle, and the maturation of these cells as they differentiate to form the various mature blood cell types. The model differs from earlier similar ones by considering primitive stem cells as a separate population from the differentiating cells, and this formulation removes an apparent inconsistency in these earlier models. Three different controls are included in the model: proliferative control of stem cells, proliferative control of differentiating cells, and peripheral control of stem cell committal rate. It is shown that an increase in sensitivity of these controls can cause oscillations to occur through their interaction with time delays associated with proliferation and differentiation, respectively. We show that the characters of these oscillations are quite distinct and suggest that the model may explain an apparent superposition of fast and slow oscillations which can occur in cyclical neutropenia. © 2006 Society for Industrial and Applied Mathematics
On the Exponentials of Some Structured Matrices
In this note explicit algorithms for calculating the exponentials of
important structured 4 x 4 matrices are provided. These lead to closed form
formulae for these exponentials. The techniques rely on one particular Clifford
Algebra isomorphism and basic Lie theory. When used in conjunction with
structure preserving similarities, such as Givens rotations, these techniques
extend to dimensions bigger than four.Comment: 19 page
Chemical evolution of star clusters
I discuss the chemical evolution of star clusters, with emphasis on old
globular clusters, in relation to their formation histories. Globular clusters
clearly formed in a complex fashion, under markedly different conditions from
any younger clusters presently known. Those special conditions must be linked
to the early formation epoch of the Galaxy and must not have occurred since.
While a link to the formation of globular clusters in dwarf galaxies has been
suggested, present-day dwarf galaxies are not representative of the
gravitational potential wells within which the globular clusters formed.
Instead, a formation deep within the proto-Galaxy or within dark-matter
minihaloes might be favoured. Not all globular clusters may have formed and
evolved similarly. In particular, we may need to distinguish Galactic halo from
Galactic bulge clusters.Comment: 27 pages, 2 figures. To appear as invited review article in a special
issue of the Phil. Trans. Royal Soc. A: Ch. 6 "Star clusters as tracers of
galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed.
LaTeX, requires rspublic.cls style fil
Quantising on a category
We review the problem of finding a general framework within which one can
construct quantum theories of non-standard models for space, or space-time. The
starting point is the observation that entities of this type can typically be
regarded as objects in a category whose arrows are structure-preserving maps.
This motivates investigating the general problem of quantising a system whose
`configuration space' (or history-theory analogue) is the set of objects
\Ob\Q in a category \Q.
We develop a scheme based on constructing an analogue of the group that is
used in the canonical quantisation of a system whose configuration space is a
manifold , where and are Lie groups. In particular, we
choose as the analogue of the monoid of `arrow fields' on \Q. Physically,
this means that an arrow between two objects in the category is viewed as an
analogue of momentum. After finding the `category quantisation monoid', we show
how suitable representations can be constructed using a bundle (or, more
precisely, presheaf) of Hilbert spaces over \Ob\Q. For the example of a
category of finite sets, we construct an explicit representation structure of
this type.Comment: To appear in a volume dedicated to the memory of James Cushin
On positive solutions and the Omega limit set for a class of delay differential equations
This paper studies the positive solutions of a class of delay differential
equations with two delays. These equations originate from the modeling of
hematopoietic cell populations. We give a sufficient condition on the initial
function for such that the solution is positive for all time .
The condition is "optimal". We also discuss the long time behavior of these
positive solutions through a dynamical system on the space of continuous
functions. We give a characteristic description of the limit set of
this dynamical system, which can provide informations about the long time
behavior of positive solutions of the delay differential equation.Comment: 15 pages, 2 figure
Particle Impact Analysis of Bulk Powder During Pneumatic Conveyance
Fragmentation of powders during transportation is a common problem for manufacturers of food and pharmaceutical products. We illustrate that the primary cause of breakage is due to inter-particle collisions, rather than particle-wall impacts, and provide a statistical mechanics model giving the number of collisions resulting in fragmentation
The adrenocortical response to synthetic ACTH following a trek to high altitude
Background: Gradual ascent to high altitude is typically associated with reduced resting aldosterone and unchanged cortisol, features that may facilitate acclimatization but are poorly understood. Aims: To investigate the cortisol and aldosterone response to adrenocorticotrophic hormone at altitude. Methods: Eleven subjects underwent a 250 µg short synacthen test at sea-level and again after trekking to 3600 m in Nepal. Cortisol and aldosterone were measured by conventional assay from blood samples taken immediately prior to the administration of synacthen (T0) and then 30 (T30) and 60 (T60) minutes later. Results: At 3600 m resting basal cortisol and aldosterone levels were both significantly lower than they were at sea-level (p=0.004, p=0.003 respectively). Cortisol values at T30 and T60 were no different between sea-level and 3600 m but the increment after synacthen was significantly (p=0.041) greater at 3600 m due to a lower basal value. Aldosterone at T30 and T60 was significantly lower (p=0.003 for both) at 3600 m than at sea-level and the increment following synacthen was also significantly (p=0.003) less at 3600 m. Conclusions: At 3600 m there appears to be a divergent adrenal response to synthetic adrenocorticotrophic hormone with an intact cortisol response but a reduced aldosterone response, relative to sea-level. This may reflect a specific effect of hypoxia on aldosterone synthesis and may be beneficial to acclimatization
Optimal Resource Allocation in Random Networks with Transportation Bandwidths
We apply statistical physics to study the task of resource allocation in
random sparse networks with limited bandwidths for the transportation of
resources along the links. Useful algorithms are obtained from recursive
relations. Bottlenecks emerge when the bandwidths are small, causing an
increase in the fraction of idle links. For a given total bandwidth per node,
the efficiency of allocation increases with the network connectivity. In the
high connectivity limit, we find a phase transition at a critical bandwidth,
above which clusters of balanced nodes appear, characterised by a profile of
homogenized resource allocation similar to the Maxwell's construction.Comment: 28 pages, 11 figure
Entropy production and the arrow of time
We present an exact relationship between the entropy production and the
distinguishability of a process from its time-reverse, quantified by the
relative entropy between forward and backward states. The relationship is shown
to remain valid for a wide family of initial conditions, such as canonical,
constrained canonical, multi-canonical and grand canonical distributions, as
well as both for classical and quantum systems.Comment: 15 pages, no figure
Newly-Discovered Globular Clusters in NGC 147 and NGC 185 from PAndAS
Using data from the Pan-Andromeda Archaeological Survey (PAndAS), we have
discovered four new globular clusters (GCs) associated with the M31 dwarf
elliptical (dE) satellites NGC 147 and NGC 185. Three of these are associated
with NGC 147 and one with NGC 185. All lie beyond the main optical boundaries
of the galaxies and are the most remote clusters yet known in these systems.
Radial velocities derived from low resolution spectra are used to argue that
the GCs are bound to the dwarfs and are not part of the M31 halo population.
Combining PAndAS with UKIRT/WFCAM data, we present the first homogeneous
optical and near-IR photometry for the entire GC systems of these dEs.
Colour-colour plots and published colour-metallicity relations are employed to
constrain GC ages and metallicities. It is demonstrated that the clusters are
in general metal poor ([Fe/H] < -1.25 dex), while the ages are more difficult
to constrain. The mean (V-I) colours of the two GC systems are very similar
to those of the GC systems of dEs in the Virgo and Fornax clusters, as well as
the extended halo GC population in M31. The new clusters bring the GC specific
frequency (S_N) to ~9 in NGC 147 and ~5 in NGC 185, consistent with values
found for dEs of similar luminosity residing in a range of environments.Comment: 14 pages, 6 figures, 6 tables, accepted for publication in MNRA
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