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考虑下述造血模型dN(t)/dt=-δN(t)-βθ^nN(t)/θ^n N^n(t) 2βθ^nN(t-τ)/θ^n N^n(t-τ)e^γτ,t≥0,其中δ、β、θ、γ、τ∈(0, ∞),n∈(1, ∞),得到持久生存的充分条件。 相似文献
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The existence and stability of an internal (i.e., completely polymorphic) equilibrium for viability selection at a single multiallelic locus is investigated. Generations are discrete and nonoverlapping; the population is panmictic, monoecious, and diploid. Various necessary and sufficient conditions for the existence of an internal equilibrium are established and applied to the loss of alleles. Some necessary conditions for the existence of an asymptotically stable internal equilibrium are also established. All these conditions are simpler and yield general biological conclusions more easily than the classical necessary and sufficient conditions. 相似文献
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H. L. Smith 《Journal of mathematical biology》1991,30(1):31-48
The distribution of concentrations of two competing microbial species among the vessels of an n-vessel gradostat at equilibrium is studied for the standard mathematical model of the gradostat. As the equilibrium concentrations cannot be explicitly computed, a continuum limit, as the number of vessels becomes large, is considered which yields a singularly perturbed boundary value problem. Standard singular perturbation techniques yield information on equilibrium species concentration distributions which agree well with numerical calculations for even moderate values of n.Research supported by NSF Grant DMS 8922654 相似文献
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Spatial patterns in a discrete-time SIS patch model 总被引:1,自引:0,他引:1
How do spatial heterogeneity, habitat connectivity, and different movement rates among subpopulations combine to influence the observed spatial patterns of an infectious disease? To find out, we formulated and analyzed a discrete-time SIS patch model. Patch differences in local disease transmission and recovery rates characterize whether patches are low-risk or high-risk, and these differences collectively determine whether the spatial domain, or habitat, is low-risk or high-risk. In low-risk habitats, the disease persists only when the mobility of infected individuals lies below some threshold value, but for high-risk habitats, the disease always persists. When the disease does persist, then there exists an endemic equilibrium (EE) which is unique and positive everywhere. This EE tends to a spatially inhomogeneous disease-free equilibrium (DFE) as the mobility of susceptible individuals tends to zero. The limiting DFE is nonempty on all low-risk patches and it is empty on at least one high-risk patch. Sufficient conditions for the limiting DFE to be empty on other high-risk patches are given in terms of disease transmission and recovery rates, habitat connectivity, and the infected movement rate. These conditions are also illustrated using numerical examples. 相似文献
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本文运用常微分方程稳定性理论及方法讨论了一类具有相互干扰的捕食与被捕食模型的稳定性,得到了正平衡点的局部稳定性条件,正平衡点在第一象限的全局稳定性条件及解的有界性. 相似文献
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An algorithm developed for pH computation has been tested to calculate the theoretical pH changes in a culture medium during the course of a fermentation. A divergence between the computed pH value and the value measured with the electrode allows us to highlight the presence of undetected ionic products. The calculation with the algorithm by means of a computer requires only the knowledge of the ionic properties of the substrates and detected products and existing thermodynamic constants. (c) 1993 Wiley & Sons, Inc. 相似文献
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《Journal of biological dynamics》2013,7(5):447-462
This paper deals with the problem of non-selective harvesting of a prey–predator system by using a reasonable catch-rate function instead of usual catch-per-unit-efforthypothesis. Here both the prey and the predator species obey the law of logistic growth. We have taken the predator functional response to prey density in such a form that each predator's functional response to the prey density approaches a constant as the prey population increases. Boundedness of the exploited system is examined. The existence of its steady states and their stability (local and global) are studied using Eigenvalue analysis. The existence of bionomic equilibria has been illustrated using a numerical example. The problem of determining the optimal harvesting policy is then solved by using Pontryagin's maximum principle. 相似文献
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一类具常数接触率传染病模型的稳定性分析 总被引:1,自引:0,他引:1
本文研究了一类具常数接触率的传染病模型,用上下解方法和Liapnuov泛函讨论了地方病平衡点及无病平衡点的渐近行为,得到了各自全局稳定的充分条件. 相似文献
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本文是《两捕食-食饵自治扩散系统的持续生存》一文的续篇.主要证明了系统正平衡点的存在性与全局稳定性. 相似文献
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da Silveira Costa MI 《Mathematical biosciences》2007,205(1):77-82
In this work, it is shown that in a deterministic context, a threshold policy can induce cyclic behavior in an otherwise exploited stable population. These dynamics ensue as a result of the combination of the degree of harvesting pressure and more protective threshold densities. Virtual equilibrium in variable structure systems plays a determinant role in this dynamical outcome. 相似文献
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一类捕食者与被捕食者模型的持久性与稳定性 总被引:3,自引:1,他引:2
研究了一类捕食者与被捕食者模型,该生态系统是一个食饵种群被一个捕食种群捕食.当给定参数满足一定条件下,利用比较原理和构造Lyapunov函数的方法,证明了系统的持久性和全局渐近稳定性,并讨论了正平衡点的渐近稳定性. 相似文献
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Maxwell KL Wildes D Zarrine-Afsar A De Los Rios MA Brown AG Friel CT Hedberg L Horng JC Bona D Miller EJ Vallée-Bélisle A Main ER Bemporad F Qiu L Teilum K Vu ND Edwards AM Ruczinski I Poulsen FM Kragelund BB Michnick SW Chiti F Bai Y Hagen SJ Serrano L Oliveberg M Raleigh DP Wittung-Stafshede P Radford SE Jackson SE Sosnick TR Marqusee S Davidson AR Plaxco KW 《Protein science : a publication of the Protein Society》2005,14(3):602-616
Recent years have seen the publication of both empirical and theoretical relationships predicting the rates with which proteins fold. Our ability to test and refine these relationships has been limited, however, by a variety of difficulties associated with the comparison of folding and unfolding rates, thermodynamics, and structure across diverse sets of proteins. These difficulties include the wide, potentially confounding range of experimental conditions and methods employed to date and the difficulty of obtaining correct and complete sequence and structural details for the characterized constructs. The lack of a single approach to data analysis and error estimation, or even of a common set of units and reporting standards, further hinders comparative studies of folding. In an effort to overcome these problems, we define here a "consensus" set of experimental conditions (25 degrees C at pH 7.0, 50 mM buffer), data analysis methods, and data reporting standards that we hope will provide a benchmark for experimental studies. We take the first step in this initiative by describing the folding kinetics of 30 apparently two-state proteins or protein domains under the consensus conditions. The goal of our efforts is to set uniform standards for the experimental community and to initiate an accumulating, self-consistent data set that will aid ongoing efforts to understand the folding process. 相似文献
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Walters C Clarke A Cliff MJ Lund PA Harding SE 《European biophysics journal : EBJ》2000,29(6):420-428
A combination of sedimentation equilibrium and sedimentation velocity in the analytical ultracentrifuge is used to investigate
the hydrodynamic integrity and increased self-association interactions of the mutant GroEL Y203W when compared to the wild-type
GroEL molecule, which may be derived from increased hydrophobic exposure caused by the mutation. Sedimentation velocity has
revealed that three distinct species were present throughout the concentration ranges used, corresponding to 14-mer (GroEL
“super monomer”) and 28-mer (“super dimer”) subunit compositions with a small amount of 42-mer (“super trimer”), which, from
the relative concentration of each species, would give an estimated weight average molecular weight of (1.0 ± 0.1) × 106 Da. Sedimentation equilibrium gave an apparent weight average molecular weight (M
w,app) of (910,000 ± 5000) Da, which is in agreement with these findings. These results are in contrast to wild-type GroEL which,
in excellent agreement with the previous findings of Behlke and co-workers, revealed a single species with an M
w,app of (805,000 ± 5200) Da and a sedimentation coefficient s
0
20,w of (21.6 ± 0.3) S. We therefore conclude that the tryptophan mutation at the Y203 location causes a significant degree of
self-association of the GroEL 14-mer assembly (with dimer and trimer present). These findings would appear to correlate well
with the findings of Gibbons et al., who showed an increase in hydrophobic exposure due to this mutation.
Received: 4 January 2000 / Revised version: 5 April 2000 / Accepted: 5 April 2000 相似文献
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Role of pericellular matrix in development of a mechanically functional neocartilage 总被引:4,自引:0,他引:4
The role of the chondrocyte pericellular matrix (PCM) was examined in a three-dimensional chondrocyte culture system to determine whether retention of the native pericellular matrix could stimulate collagen and proteoglycan accumulation and also promote the formation of a mechanically functional hyaline-like neocartilage. Porcine chondrocytes and chondrons, consisting of the chondrocyte with its intact pericellular matrix, were maintained in pellet culture for up to 12 weeks. Sulfated glycosaminoclycans and type II collagen were measured biochemically. Immunocytochemistry was used to examine collagen localization as well as cell distribution within the pellets. In addition, the equilibrium compressive moduli of developing pellets were measured to determine whether matrix deposition contributed to the mechanical stiffness of the cartilage constructs. Pellets increased in size and weight over a 6-week period without apparent cell proliferation. Although chondrocytes quickly rebuilt a PCM rich in type VI collagen, chondron pellets accumulated significantly more proteoglycan and type II collagen than did chondrocyte pellets, indicating a greater positive effect of the native PCM. After 5 weeks in chondron pellets, matrix remodeling was evident by microscopy. Cells that had been uniformly distributed throughout the pellets began to cluster between large areas of interterritorial matrix rich in type II collagen. After 12 weeks, clusters were stacked in columns. A rapid increase in compressive strength was observed between 1 and 3 weeks in culture for both chondron and chondrocyte pellets and, by 6 weeks, both had achieved 25% of the equilibrium compressive stiffness of cartilage explants. Retention of the in vivo PCM during chondrocyte isolation promotes the formation of a mechanically functional neocartilage construct, suitable for modeling the responses of articular cartilage to chemical stimuli or mechanical compression. 相似文献