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1.
Ernesto Trucco 《Bulletin of mathematical biology》1963,25(3):303-323
This paper, consisting of two parts, gives all the mathematical details that were omitted in a previous work by G. A. Sacher
and E. Trucco (“The Stochastic Theory of Mortality.”Ann. N. Y. Acad. Sci.,96, 985–1007, cited here as ST). We assume that the reader is familiar with ST, where the stochastic theory of mortality, originally
proposed by Sacher, is discussed at length. We recall that the basic model presented there refers to an ensemble of particles
performing Brownian motion in one dimension, with the added constraint of two absorbing barriers. These two points, collectively,
are designated as the “lethal bound.” Part I (section 1 to 4) deals with the special case in which the two absorbing barriers
are symmetrically located at a finite distance from the origin. The solution of the Fokker-Planck equation is obtained from
the theory of eigenvalue problems. Quite generally, the eigenfunctions functions belong to the family of Kummer's confluent
hypergeometric functions, but the symmetry condition imposed here results in considerable simplification and makes it possible
to estimate the first few eigenvalues by a graphical procedure. In section 3 we show how perturbation theory can be applied
in the limiting case of “weak homeostasis,” and section 4 deals with the opposite extreme of “strong homeostasis.” A rigorous
proof is given for the result corresponding to equation (28) of ST (asymptotic or quasi-static approximation for the “force
of mortality”).
This work was performed under the auspices of the U.S. Atomic Energy Commission. 相似文献
2.
Analysis of a mathematical model for the growth of tumors 总被引:13,自引:0,他引:13
In this paper we study a recently proposed model for the growth of a nonnecrotic, vascularized tumor. The model is in the form of a free-boundary problem whereby the tumor grows (or shrinks) due to cell proliferation or death according to the level of a diffusing nutrient concentration.
The tumor is assumed to be spherically symmetric, and its boundary is an unknown function r=s(t). We concentrate on the case where at the boundary of the tumor the birth rate of cells exceeds their death rate, a necessary
condition for the existence of a unique stationary solution with radius r=R
0 (which depends on the various parameters of the problem). Denoting by c the quotient of the diffusion time scale to the tumor doubling time scale, so that c is small, we rigorously prove that
(i) lim inf
t→∞
s(t)>0, i.e. once engendered, tumors persist in time. Indeed, we further show that
(ii) If c is sufficiently small then s(t)→R
0 exponentially fast as t→∞, i.e. the steady state solution is globally asymptotically stable. Further,
(iii) If c is not “sufficiently small” but is smaller than some constant γ determined explicitly by the parameters of the problem, then
lim sup
t→∞
s(t)<∞; if however c is “somewhat” larger than γ then generally s(t) does not remain bounded and, in fact, s(t)→∞ exponentially fast as t→∞.
Received: 25 February 1998 / Revised version: 30 April 1998 相似文献
3.
Gerald Rosen 《Bulletin of mathematical biology》1980,42(1):95-106
For precise boundary conditions of biological relevance, it is proved that the steadily propagating plane-wave solution to
the Fisher equation requires the unique (eigenvalue) velocity of advance 2(Df)1/2, whereD is the diffusivity of the mutant species andf is the frequency of selection in favor of the mutant. This rigorous result shows that a so-called “wrong equation”, i.e.
one which differs from Fisher's by a term that is seemingly inconsequential for certain initial conditions, cannot be employed
readily to obtain approximate solutions to Fisher's, for the two equations will often have qualitatively different manifolds
of exact solutions. It is noted that the Fisher equation itself may be inappropriate in certain biological contexts owing
to the manifest instability of the lowerconcentration uniform equilibrium state (UES). Depicting the persistence of a mutantdeficient
spatial pocket, an exact steady-state solution to the Fisher equation is presented. As an alternative and perhaps more faithful
model equation for the propagation of certain species properties through a homogeneous population, we consider a reaction-diffusion
equation that features a cubic-polynomial rate expression in the species concentration, with two stable UES and one intermediate
unstable UES. This equation admits a remarkably simple exact analytical solution to the steadily propagating plane-wave eigenvalue
problem. In the latter solution, the sign of the eigenvelocity is such that the wave propagates to yield the “preferred” stable
UES (namely, the one further removed from the unstable intermediate UES) at all spatial points ast→∞. The cubic-polynomial equation also admits an exact steady-state solution for a mutant-deficient or mutant-isolated spatial
pocket. Finally, the perpetuating growth of a mutant population from an arbitrary localized initial distribution, a mathematical
problem analogous to that for ignition in laminar flame theory, is studied by applying differential inequality analysis, and
rigorous sufficient conditions for extinction are derived here. 相似文献
4.
H. D. Landahl 《Bulletin of mathematical biology》1953,15(3):367-381
The transmission of some information or behavior pattern is treated as a flow of “particles” which execute random motions
over a population of individuals and which may multiply or disappear. Equations are derived for the number density of these
“particles” and from this is calculated the number of individuals through which the “particles” have passed. The results are
applied to a number of situations such as 1) uniform spatial distribution with multiplication factor decreasing with time
because of loss of interest or confusion of the information, 2) multiplication factor constant but the rate of spreal decreasing
with multiple hearings, 3) one-dimensional region with a small starting region with or without an absorbing barrier 4) two-dimensional
region with absorbing barrier, 5) continous sources of information within a small region in one dimension, 6) uniform spatial
distribution in which individuals do not respond to more than one hearing. 相似文献
5.
We propose a simple experiment to study delocalization and extinction in inhomogeneous biological systems. The nonlinear
steady state for, say, a bacteria colony living on and near a patch of nutrient or favorable illumination (“oasis”) in the
presence of a drift term (“wind”) is computed. The bacteria, described by a simple generalization of the Fisher equation,
diffuse, divide A→A + A, die A→ 0, and annihilate A + A→ 0. At high wind velocities all bacteria are blown into an unfavorable region (“desert”), and the colony dies out. At low
velocity a steady state concentration survives near the oasis. In between these two regimes there is a critical velocity at
which bacteria first survive. If the “desert” supports a small nonzero population, this extinction transition is replaced
by a delocalization transition with increasing velocity. Predictions for the behavior as a function of wind velocity are made
for one and two dimensions.
Received: 3 August 1998 / Revised version: 17 July 1999 / Published online: 4 July 2000 相似文献
6.
Zwanzig and Mori's projection-operator method is used in order to derive a generalized nonlinear Fokker-Planck equation for
one “relevant” species in the many species conservative Volterra model. The deterministic, autonomous, Markovian equations
of motion, when averaged over a suitable ensemble of initial conditions in general give rise to a non-autonomous, non-Markovian
stochastic process for the evolution of this relevant species. Moreover, this relevant species may show irreversible damping,
although self-interaction terms are absent in the many species model. 相似文献
7.
This article attempts to convey the joys and frustrations of skimming the Internet trying to find relevant information concerning
an academic’s work as a scientist, a student or an instructor. A brief overview of the Internet and the “do’s and don’ts”
for the neophyte as well for the more seasoned “navigator” are given. Some guidelines of “what works and what does not” and
“what is out there” are provided for the scientist with specific emphasis for biologists, as well as for all others having
an interest in science but with little interest in spending countless hours “surfing the net”. An extensive but not exhaustive
list of related websites is provided. 相似文献
8.
S. Yamamoto Hiroki Abe Toshiaki Kohgo Atushi Ogawa Akira Ohtake Hidemasa Hayashibe Hitoshi Sakuraba Yoshiyuki Suzuki Shuichi Aramaki Masaki Takayanagi Shuji Hasegawa Hiroo Niimi 《Human genetics》1996,98(1):116-118
Carnitine palmitoyltransferase II (CPT II) deficiency has two different clinical forms, one with “hepatic” and the other
with “muscular” symptoms. We studied the molecular basis of the “hepatic” form in two Japanese siblings. Their CPT II activity
in lymphoblasts was reduced to 3% of the level observed in normal controls. cDNA analysis showed that the proband was a compound
heterozygote. One allele carried a new mutation, G621→A (Glu174→Lys). The other carried three single-base substitutions; a
new mutation, T1249→A (Phe383→Tyr), and two previously reported polymorphisms. The brother had the same four substitutions.
Neither of the two new mutations in this study was detected in the 60 alleles of 30 Japanese control subjects. Secondary structure
prediction analysis of the mutated CPT II protein was different from that of the normal protein. We concluded that these mutations
caused the “hepatic” form of CPT II deficiency in the probands.
Received: 16 October 1995 相似文献
9.
10.
In the information processing procedure of stereo vision, the uniqueness constraint has been used as one of the constraints
to solve the “correspondence problem”. While the uniqueness constraint is valid in most cases, whether it is still valid in
some particular stimulus configuration (such as Panum’s limiting case) has been a problem of widespread debate for a long
time. To investigate the problem, we adopted the Panum’s limiting case as its basic stimulus configuration, and delved into
the phenomenon of binocular fusion from two distinct aspects: visual direction and orientation disparity. The results show
that in Panum’s limiting case binocular fusion does not comply with the rules governing regular binocular fusion as far as
visual direction and orientation disparity are concerned. This indicates that double fusion does not happen in Panum’s limiting
case and that the uniqueness constraint is still valid. 相似文献
11.
Nicholas R. Rhodes Tatyana Konovalova Qiaoli Liang Carolyn J. Cassady John B. Vincent 《Biological trace element research》2009,130(2):114-130
Despite chromium nicotinate’s popular use as a chromium nutritional supplement, the structure and composition of chromium
nicotinate have only been poorly described. As solid chromium nicotinate is intractable, being insoluble or unstable in common
solvents, studies on the solid have been limited, and studies of the solution from which the “compound” precipitates have
additionally provided little additional data. The results of mass spectrometric and spectroscopic investigations designed
to further elucidate the structure and composition of chromium nicotinate are described. The results demonstrated that the
three common methods for producing “chromium nicotinate” all yield different compounds, all of which are polymers of Cr(III),
oxygen-bound nicotinate, hydroxide, and water. Implications for interpreting results of nutritional studies of “chromium nicotinate”
are discussed. 相似文献
12.
Osamu Gotoh 《Bulletin of mathematical biology》1990,52(4):509-525
Pairwise optimal alignments between three or more sequences are not necessarily consistent as a whole, but consistent and
inconsistent residues are usually distributed in clusters. An efficient method has been developed for locating consistent
regions when each pairwise alignment is given in the form of a “skeletal representation” (Bull. math. Biol.
52, 359–373). This method is further extended so that the combination of pairwise alignments that gives the greatest consistency
is found when possibly many alignments are equally optimal for each pairwise comparison. A method for acceleration of simultaneous
multiple sequence alignment is proposed in which consistent regions serve as “anchor points” limiting application of direct
multi-way alignment to the rest of “inconsistent” regions.
Dedicated to Prof. Akiyoshi Wada on the occasion of his 60th birthday. 相似文献
13.
Gorban AN Pokidysheva LI Smirnova EV Tyukina TA 《Bulletin of mathematical biology》2011,73(9):2013-2044
The “Law of the Minimum” states that growth is controlled by the scarcest resource (limiting factor). This concept was originally
applied to plant or crop growth (Justus von Liebig, 1840, Salisbury, Plant physiology, 4th edn., Wadsworth, Belmont, 1992) and quantitatively supported by many experiments. Some generalizations based on more complicated “dose-response” curves
were proposed. Violations of this law in natural and experimental ecosystems were also reported. We study models of adaptation
in ensembles of similar organisms under load of environmental factors and prove that violation of Liebig’s law follows from
adaptation effects. If the fitness of an organism in a fixed environment satisfies the Law of the Minimum then adaptation
equalizes the pressure of essential factors and, therefore, acts against the Liebig’s law. This is the the Law of the Minimum paradox: if for a randomly chosen pair “organism–environment” the Law of the Minimum typically holds, then in a well-adapted system,
we have to expect violations of this law. 相似文献
14.
The exponential decay model of a neuron has been analyzed using the “random walk” approach of stochastic processes and an
“absorbing barrier” solution is obtained forg
T
(s)—the Laplace transform of the output pulse interval density function. An expression for the mean output frequency is derived
from this and a variety of input-output curves plotted which show frequency threshold effects in single neurons. Our results
are compared with those of other authors obtained by computer simulation techniques, and the significance of these results
discussed with reference to the possible behavior of networks constructed of such neuron units. 相似文献
15.
An important aspect of systems biology research is the so-called “reverse engineering” of cellular metabolic dynamics from
measured input-output data. This allows researchers to estimate and validate both the pathway’s structure as well as the kinetic
constants. In this paper, the recently published ‘Proximate Parameter Tuning’ (PPT) method for the identification of biochemical
networks is analysed. In particular, it is shown that the described PPT algorithm is essentially equivalent to a sequential
linear programming implementation of a constrained optimization problem. The corresponding objective function consists of
two parts, the first emphasises the data fitting where a residual 1-norm is used, and the second emphasises the proximity
of the calculated parameters to the specified nominal values, using an ∞-norm. The optimality properties of PPT algorithm
solution as well as its geometric interpretation are analyzed. The concept of optimal parameter locus is applied for the exploration
of the entire family of optimal solutions. An efficient implementation of the parameter locus is also developed. Parallels
are drawn with 1-norm parameter deviation regularization which attempt to fit the data with a minimal number of parameters.
Finally, a small example is used to illustrate all of these properties. 相似文献
16.
In the information processing procedure of stereo vision, the uniqueness constraint has been used as one of the constraints to solve the “correspondence problem”. While the uniqueness constraint is valid in most cases, whether it is still valid in some particular stimulus configuration (such as Panum’s limiting case) has been a problem of widespread debate for a long time. To investigate the problem, we adopted the Panum’s limiting case as its basic stimulus configuration, and delved into the phenomenon of binocular fusion from two distinct aspects: visual direction and orientation disparity. The results show that in Panum’s limiting case binocular fusion does not comply with the rules governing regular binocular fusion as far as visual direction and orientation disparity are concerned. This indicates that double fusion does not happen in Panum’s limiting case and that the uniqueness constraint is still valid. 相似文献
17.
Brada B 《Culture, medicine and psychiatry》2011,35(2):285-312
This essay argues that what makes “global health” “global” has more to do with configurations of space and time, and the claims
to expertise and moral stances these configurations make possible, than with the geographical distribution of medical experts
or the universal, if also uneven, distribution of threats to health. Drawing on a study of public–private partnerships supporting
Botswana’s HIV/AIDS treatment program, this essay demonstrates ethnographically the processes by which “global health” and
its quintessential spaces, namely “resource-limited” or “resource-poor settings,” are constituted, reinforced, and contested
in the context of medical education and medical practice in Botswana’s largest hospital. Using Silverstein’s work on orders
of indexicality, I argue that the terms of “global health” are best understood as chronotopic, and demonstrate how actors
orient themselves and others spatio-temporally, morally, and professionally by using or refuting those terms. I conclude by
arguing that taking “global health” on its own terms obscures the powerful forces by which it becomes intelligible. At stake
are the frames within which medical anthropologists understand their objects of study, as well as the potential for the spaces
of “global health” intervention to expand ever outward as American medical personnel attempt to calibrate their experiences
to their expectations. 相似文献
18.
A population of initially synchronized cells is considered wherein each cell grows according to a dispersionless growth law
and the probability of cell division is determined by cell age. The first and second moments of the distribution of birth
volumes are considered as functions of time and it is shown that it is impossible for both moments to approach finite, nonzero
limits ast→∞. This implies that the volume distribution of the population will not approach a limiting distribution on any finite, nonzero
volume interval and that the population will not attain balanced exponential growth. An illustrative example is worked out
in detail. The distribution of birth volumes is also analyzed as a function of generation number and it is found that the
logarithm of the birth volume in thejth generation is normally distributed asj→∞, with an unbounded variance. Generalizations and implications of these results are briefly discussed.
Work supported by the U.S. Atomic Energy Commission. 相似文献
19.
Morris H. Baslow 《Biosemiotics》2011,4(1):39-53
The human brain is a complex organ made up of neurons and several other cell types, and whose role is processing information
for use in elicitation of behaviors. To accomplish this, the brain requires large amounts of energy, and this energy is obtained
by the oxidation of glucose (Glc). However, the question of how the oxidation of Glc by individual neurons in brain results
in their collective ability to rapidly generate feats of cognition that allow them to recognize the nature of the universe
in which they live and to communicate this information remains unclear. In this article, insights into this process are provided
by first considering the brain’ s homeostatic “operating system” for supply of energy to stimulated neurons, and how this
system defines the basic unit of brain “structure”. This is followed by consideration of the brain’s “two-cell” neuronal communication
mechanism which defines the basic unit of brain “function”. Finally, an analysis of the nature of frequency-encoded “neuronal
languages” that enable ensembles of neurons to translate energy derived from the oxidation of Glc into a collective “mind”,
the aggregate of all brain processes including those involving perception, thought, insight, foresight, imagination and behavior. 相似文献
20.
Desmond A 《Journal of the history of biology》2001,34(1):3-50
A summary of revisionist accounts of the contextual meaning of`“professional” and “amateur,” as applied to the mid-Victorian
X Club, is followed by an analysis of the liberal goals and inner tensions of this coalition of gentlemen specialists and
government teachers. The changing status of amateurs is appraised, as are the new sites for the emerging laboratory discipline
of “biology.” Various historiographical strategies for recovering the women’s role are considered. The relationship of science
journalism to professionalization, and the constructive engagement of X Club publicists with their empowering audiences, are
discussed. Finally, the article assesses how far the content and boundary closure of ``biology,' forged by Thomas Henry Huxley,
were related to `professional' and political goals. Purebiology’s social and medical roots are examined, and the way inter-professional
and wider Darwinian conflicts resulted in anew lexicon of words for the X Clubbers around 1870, including“evolution” and “agnosticism,”
as well as “biology.” Biology’srole in the forging of British national identity is discussed, as are its relationship to the
social strategies of liberal, Dissenting, and industrial groups in the country, whose authority sustained the new laboratory
rhetoric.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献