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Biomedical subjects

I Ispolatov

Publications and source records attributed to I Ispolatov.

13 recordsLinked to original sources
PubMed2005

Binaries and core-ring structures in self-gravitating systems.

Low-energy states of self-gravitating systems with finite angular momentum are considered. A constraint is introduced to confine cores and other condensed objects within the system boundaries by gravity alone. This excludes previously observed astrophysically irrelevant asymmetric configurations with a single core. We show that, for an intermediate range of a short-distance cutoff and small angular momentum, the equilibrium configuration is an asymmetric binary. For larger angular momentum or for a smaller range of the short-distance cutoff, the equilibrium configuration consists of a central core and an equatorial ring. The mass of the ring varies between zero for vanishing rotation and the full system mass for the maximum angular momentum L(max) a localized gravitationally bound system can have. The value of L(max) scales as square root of ((ln(1/x0)), where x0 is a ratio of a short-distance cutoff range to the system size. An example of the soft gravitational potential is considered; the conclusions are shown to be valid for other forms of short-distance regularization.

Journal Article↗
PubMed2005

Duplication-divergence model of protein interaction network.

We investigate a very simple model describing the evolution of protein-protein interaction networks via duplication and divergence. The model exhibits a remarkably rich behavior depending on a single parameter, the probability to retain a duplicated link during divergence. When this parameter is large, the network growth is not self-averaging and an average node degree increases algebraically. The lack of self-averaging results in a great diversity of networks grown out of the same initial condition. When less than a half of links are (on average) preserved after divergence, the growth is self-averaging, the average degree increases very slowly or tends to a constant, and a degree distribution has a power-law tail. The predicted degree distributions are in a very good agreement with the distributions observed in real protein networks.

Computer Simulation↗
PubMed2004

Anomalously slow phase transitions in self-gravitating systems.

The kinetics of collapse and explosion transitions in microcanonical self-gravitating ensembles is analyzed. A system of point particles interacting via an attractive soft Coulomb potential and confined to a spherical container is considered. We observed that for 100-200 particles collapse takes 10(3) - 10(4) particle crossing times to complete; i.e., it is by two to three orders of magnitude slower than the velocity relaxation. In addition, it is found that the collapse time decreases rapidly with an increase of the soft-core radius. We found that such an anomalously long collapse time is caused by the slow energy exchange between a higher-temperature compact core and relatively cold diluted halo. The rate of energy exchange between the faster modes of the core particles and slower-moving particles of the halo is exponentially small in the ratio of the frequencies of these modes. As the soft-core radius increases and the typical core modes become slower, the ratio of core and halo frequencies decreases and the collapse accelerates. Implications for astrophysical systems and phase transition kinetics are discussed.

Journal Article↗
PubMed2003

Collapses and explosions in self-gravitating systems.

Collapse and explosion (reverse to collapse) transitions in self-gravitating systems are studied by molecular dynamics simulations. A microcanonical ensemble of point particles confined to a spherical box is considered. The particles interact via an attractive soft Coulomb potential. It is observed that a collapse indeed takes place when the energy of the uniform state is set near or below the metastability-instability threshold (collapse energy) as predicted by the mean-field theory. Similarly, an explosion occurs when the energy of the core-halo state is increased above the explosion energy, where according to the mean-field predictions the core-halo state becomes unstable. For systems consisting of 125-500 particles, the collapse takes about 10(5) single-particle crossing times to complete, while a typical explosion is by an order of magnitude faster. A finite lifetime of metastable states is observed. It is also found that the mean-field description of the uniform and core-halo states is exact within the statistical uncertainty of the molecular dynamics data.

Journal Article↗
PubMed2002

Phase diagram of self-attracting systems.

A phase diagram of microcanonical ensembles of self-attracting particles is studied for two types of short-range potential regularizations: self-gravitating fermions and classical particles interacting via an attractive soft -(r(2)+r(2)(0))(-1/2) Coulomb potential. When the range of regularization is sufficiently short, the self-attracting systems exhibit gravitational or collapselike transition. As the fermionic degeneracy or the softness radius increases, the gravitational phase transition crosses over to a normal first-order phase transition, becomes second-order at a critical point, and finally disappears. Applicability of a commonly used saddle-point or mean-field approximation and importance of metastable states is discussed.

Journal Article↗
PubMed2001

Collapse in systems with attractive nonintegrable potentials.

Collapse, or a gravitational-like phase transition, is found in microcanonical ensembles of particles with an attractive 1/r(alpha) potential not only for alpha = 1 but for all 0<alpha<3. The phase behavior of the system is complex: If an effective sufficiently short-range cutoff is applied, the density of the collapsed phase is finite everywhere; if not, the collapse results in a density singularity. Also, with increasing effective cutoff range, the gravitational phase transition will cross over to a normal first-order phase transition.

Journal Article↗
PubMed2001

Phase transitions in systems with 1/r(alpha) attractive interactions.

Collapse, or a gravitational-like phase transition is studied in a microcanonical ensemble of particles with an attractive 1/r(alpha) potential. A mean-field continuous integral equation is used to determine a saddle-point density profile that extremizes the entropy functional. For all 0 < alpha < 3, a critical energy is determined below which the entropy of the system exhibits a discontinuous jump. If an effective short-range cutoff is applied, the entropy jump is finite; if not, the entropy diverges to +infinity. A stable integral equation solution represents a state with maximal entropy; the reverse is always true only for a modified integral equation introduced here.

Journal Article↗
PubMed2000

Phase transition in a traffic model with passing

We investigate a traffic model in which cars either move freely with quenched intrinsic velocities or belong to clusters formed behind slower cars. In each cluster, the next-to-leading car is allowed to pass and resume free motion. The model undergoes a phase transition from a disordered phase for the high passing rate to a jammed phase for the low rate. In the disordered phase, the cluster size distribution decays exponentially in the large size limit. In the jammed phase, the distribution of finite clusters is independent on the passing rate, but it accounts only for a fraction of all cars; the "excessive" cars form an infinite cluster moving with the smallest velocity. Mean-field equations, describing the model in the framework of Maxwell approximation, correctly predict the existence of phase transition and adequately describe the disordered phase; properties of the jammed phase are studied numerically.

Journal Article↗
PubMed1999

Persistence in systems with algebraic interaction.

Persistence in coarsening one-dimensional spin systems with a power-law interaction r(-1-sigma) is considered. Numerical studies indicate that for sufficiently large values of the interaction exponent sigma (sigma > or =1/2 in our simulations), persistence decays as an algebraic function of the length scale L, P(L) approximately L(-theta). The persistence exponent theta is found to be independent on the force exponent sigma and close to its value for the extremal (sigma-->infinity) model, theta =0.175 075 88. For smaller values of the force exponent (sigma < 1/2), finite size effects prevent the system from reaching the asymptotic regime. Scaling arguments suggest that in order to avoid significant boundary effects for small sigma, the system size should grow as [O(1/sigma)](1/sigma).

Journal Article↗