87 research outputs found
Stiff Stability of the Hydrogen atom in dissipative Fokker electrodynamics
We introduce an ad-hoc electrodynamics with advanced and retarded
Lienard-Wiechert interactions plus the dissipative Lorentz-Dirac
self-interaction force. We study the covariant dynamical system of the
electromagnetic two-body problem, i.e., the hydrogen atom. We perform the
linear stability analysis of circular orbits for oscillations perpendicular to
the orbital plane. In particular we study the normal modes of the linearized
dynamics that have an arbitrarily large imaginary eigenvalue. These large
eigenvalues are fast frequencies that introduce a fast (stiff) timescale into
the dynamics. As an application, we study the phenomenon of resonant
dissipation, i.e., a motion where both particles recoil together in a drifting
circular orbit (a bound state), while the atom dissipates center-of-mass energy
only. This balancing of the stiff dynamics is established by the existence of a
quartic resonant constant that locks the dynamics to the neighborhood of the
recoiling circular orbit. The resonance condition quantizes the angular momenta
in reasonable agreement with the Bohr atom. The principal result is that the
emission lines of quantum electrodynamics (QED) agree with the prediction of
our resonance condition within one percent average deviation.Comment: 1 figure, Notice that Eq. (34) of the Phys. Rev. E paper has a typo;
it is missing the square Brackets of eq. (33), find here the correct e
Stationary probability density of stochastic search processes in global optimization
A method for the construction of approximate analytical expressions for the
stationary marginal densities of general stochastic search processes is
proposed. By the marginal densities, regions of the search space that with high
probability contain the global optima can be readily defined. The density
estimation procedure involves a controlled number of linear operations, with a
computational cost per iteration that grows linearly with problem size
A mathematical framework for critical transitions: normal forms, variance and applications
Critical transitions occur in a wide variety of applications including
mathematical biology, climate change, human physiology and economics. Therefore
it is highly desirable to find early-warning signs. We show that it is possible
to classify critical transitions by using bifurcation theory and normal forms
in the singular limit. Based on this elementary classification, we analyze
stochastic fluctuations and calculate scaling laws of the variance of
stochastic sample paths near critical transitions for fast subsystem
bifurcations up to codimension two. The theory is applied to several models:
the Stommel-Cessi box model for the thermohaline circulation from geoscience,
an epidemic-spreading model on an adaptive network, an activator-inhibitor
switch from systems biology, a predator-prey system from ecology and to the
Euler buckling problem from classical mechanics. For the Stommel-Cessi model we
compare different detrending techniques to calculate early-warning signs. In
the epidemics model we show that link densities could be better variables for
prediction than population densities. The activator-inhibitor switch
demonstrates effects in three time-scale systems and points out that excitable
cells and molecular units have information for subthreshold prediction. In the
predator-prey model explosive population growth near a codimension two
bifurcation is investigated and we show that early-warnings from normal forms
can be misleading in this context. In the biomechanical model we demonstrate
that early-warning signs for buckling depend crucially on the control strategy
near the instability which illustrates the effect of multiplicative noise.Comment: minor corrections to previous versio
Periodic Host Absence Can Select for Higher or Lower Parasite Transmission Rates
This paper explores the effect of discontinuous periodic host absence on the evolution of pathogen transmission rates by using Ro maximisation techniques. The physiological consequence of an increased transmission rate can be either an increased virulence, i.e. there is a transmission-virulence trade-off or ii) a reduced between season survival, i.e. there is a transmission-survival trade-off. The results reveal that the type of trade-off determines the direction of selection, with relatively longer periods of host absence selecting for higher transmission rates in the presence of a trade-off between transmission and virulence but lower transmission rates in the presence of a trade-of between transmission and between season survival. The fact that for the transmission-virulence trade-off both trade-off parameters operate during host presence whereas for the transmission-survival trade-off one operates during host presence (transmission) and the other (survival) during the period of host absence is the main cause for this difference in selection direction. Moreover, the period of host absence seems to be the key determinant of the pathogens transmission rate. Comparing plant patho-systems with contrasting biological features suggests that airborne plant pathogen respond differently to longer periods of host absence than soil-borne plant pathogens
3D Bioprinted Human Skeletal Muscle Constructs for Muscle Function Restoration
A bioengineered skeletal muscle tissue as an alternative for autologous tissue flaps, which mimics the structural and functional characteristics of the native tissue, is needed for reconstructive surgery. Rapid progress in the cell-based tissue engineering principle has enabled in vitro creation of cellularized muscle-like constructs; however, the current fabrication methods are still limited to build a three-dimensional (3D) muscle construct with a highly viable, organized cellular structure with the potential for a future human trial. Here, we applied 3D bioprinting strategy to fabricate an implantable, bioengineered skeletal muscle tissue composed of human primary muscle progenitor cells (hMPCs). The bioprinted skeletal muscle tissue showed a highly organized multi-layered muscle bundle made by viable, densely packed, and aligned myofiber-like structures. Our in vivo study presented that the bioprinted muscle constructs reached 82% of functional recovery in a rodent model of tibialis anterior (TA) muscle defect at 8 weeks of post-implantation. In addition, histological and immunohistological examinations indicated that the bioprinted muscle constructs were well integrated with host vascular and neural networks. We demonstrated the potential of the use of the 3D bioprinted skeletal muscle with a spatially organized structure that can reconstruct the extensive muscle defects
Scaling Effects and Spatio-Temporal Multilevel Dynamics in Epileptic Seizures
Epileptic seizures are one of the most well-known dysfunctions of the nervous system. During a seizure, a highly synchronized behavior of neural activity is observed that can cause symptoms ranging from mild sensual malfunctions to the complete loss of body control. In this paper, we aim to contribute towards a better understanding of the dynamical systems phenomena that cause seizures. Based on data analysis and modelling, seizure dynamics can be identified to possess multiple spatial scales and on each spatial scale also multiple time scales. At each scale, we reach several novel insights. On the smallest spatial scale we consider single model neurons and investigate early-warning signs of spiking. This introduces the theory of critical transitions to excitable systems. For clusters of neurons (or neuronal regions) we use patient data and find oscillatory behavior and new scaling laws near the seizure onset. These scalings lead to substantiate the conjecture obtained from mean-field models that a Hopf bifurcation could be involved near seizure onset. On the largest spatial scale we introduce a measure based on phase-locking intervals and wavelets into seizure modelling. It is used to resolve synchronization between different regions in the brain and identifies time-shifted scaling laws at different wavelet scales. We also compare our wavelet-based multiscale approach with maximum linear cross-correlation and mean-phase coherence measures
Is the astronomical forcing a reliable and unique pacemaker for climate? A conceptual model study
There is evidence that ice age cycles are paced by astronomical forcing,
suggesting some kind of synchronisation phenomenon. Here, we identify the type
of such synchronisation and explore systematically its uniqueness and
robustness using a simple paleoclimate model akin to the van der Pol relaxation
oscillator and dynamical system theory. As the insolation is quite a complex
quasiperiodic signal involving different frequencies, the traditional concepts
used to define synchronisation to periodic forcing are no longer applicable.
Instead, we explore a different concept of generalised synchronisation in terms
of (coexisting) synchronised solutions for the forced system, their basins of
attraction and instabilities. We propose a clustering technique to compute the
number of synchronised solutions, each of which corresponds to a different
paleoclimate history. In this way, we uncover multistable synchronisation
(reminiscent of phase- or frequency-locking to individual periodic components
of astronomical forcing) at low forcing strength, and monostable or unique
synchronisation at stronger forcing. In the multistable regime, different
initial conditions may lead to different paleoclimate histories. To study their
robustness, we analyse Lyapunov exponents that quantify the rate of convergence
towards each synchronised solution (local stability), and basins of attraction
that indicate critical levels of external perturbations (global stability). We
find that even though synchronised solutions are stable on a long term, there
exist short episodes of desynchronisation where nearby climate trajectories
diverge temporarily (for about 50 kyr). (...)Comment: 22 pages, 18 figure
Graz. Schloßberg.
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