137 research outputs found
Divergent evolution paths of different genetic families in the Penna model
We present some results of simulations of population growth and evolution,
using the standard asexual Penna model, with individuals characterized by a
string of bits representing a genome containing some possible mutations. After
about 20000 simulation steps, when only a few genetic families are still
present from among rich variety of families at the beginning of the simulation
game, strong peaks in mutation distribution functions are observed. This known
effect is due to evolution rules with hereditary mechanism. The birth and death
balance in the simulation game also leads to elimination of families specified
by different genomes. Number of families versus time follow the
power law, . Our results show the power coefficient exponent
is changing as the time goes. Starting from about --1, smoothly achieves about
--2 after hundreds of steps, and finally has semi-smooth transition to 0, when
only one family exists in the environment. This is in contrast with constant
about --1 as found, for example, in \cite{bib:evolution}. We suspect that
this discrepancy may be due to two different time scales in simulations -
initial stages follow the law, yet for large number of simulation
steps we get , providing random initial population was sufficiently
big to allow for still reliable statistical analysis. The
evolution stage seems to be associated with the Verhulst mechanism of
population elimination due to the limited environmental capacity - when the
standard evolution rules were modified, we observed a plateau () in the
power law in short time scale, again followed by law for longer
times. The modified model uses birth rate controlled by the current population
instead of the standard Verhulst death factor
Strong practical stability based robust stabilization of uncertain discrete linear repetitive processes
Repetitive processes are a distinct class of 2D systems of both theoretical and practical interest whose dynamics evolve over a subset of the positive quadrant in the 2D plane. The stability theory for these processes originally consisted of two distinct concepts termed asymptotic stability and stability along the pass respectively where the former is a necessary condition for the latter. Stability along the pass demands a bounded-input bounded-output property over the complete positive quadrant of the 2D plane and this is a very strong requirement, especially in terms of control law design. A more feasible alternative for some cases is strong practical stability, where previous work has formulated this property and obtained necessary and sufficient conditions for its existence together with Linear Matrix Inequality (LMI) based tests, which then extend to allow control law design. This paper develops considerably simpler, and hence computationally more efficient, stability tests that extend to allow control law design in the presence of uncertainty in process model
Reaction of Stannyl Esters of Phosphorus Acids with Sulfonic Acids Chlorides and Carboxylic Acids Anhydrides. A Novel Synthesis of Phosphoric-Sulfonic and Phosphoric-Carboxylic Anhydrides
New methods employing stannyl phosphates for the synthesis
of phosphorus-sulfonic anhydrides and acylphosphates are presented
The Anderson-Mott transition induced by hole-doping in Nd1-xTiO3
The insulator/metal transition induced by hole-doping due to neodymium
vacancies of the Mott- Hubbard antiferromagnetic insulator, Nd1-xTiO3, is
studied over the composition range 0.010(6) < x < 0.243(10). Insulating p-types
conduction is found for x < 0.071(10). Anderson localization in the presence of
a Mott-Hubbard gap, is the dominant localization mechanism for the range of
0.074(10) < x < 0.089(1) samples. For x < 0.089(1), n-type conduction is
observed and the activation energy extrapolates to zero by x < 0.1. The
0.095(8) < x < 0.203(10) samples are Fermi-liquid metals and the effects of
strong electronic correlations are evident near the metal-to-insulator
boundaries in features such as large Fermi liquid T2 coefficients. For 0.074(9)
< x < 0.112(4), a weak negative magnetoresistance is found below ~ 15 K and it
is attributed to the interaction of conduction electrons with Nd3+ magnetic
moments. Combining information from our companion study of the magnetic
properties of Nd1-xTiO3 solid solution, a phase diagram is proposed. The main
conclusions are that long range antiferromagnetic order disappears before the
onset of metallic behavior and that the Anderson-Mott transition occurs over a
finite range of doping levels. Our results differ from conclusions drawn from a
similar study on the hole doped Nd1-xCaxTiO3 system which found the
co-existence of antiferromagnetic order and metallic behavior and that the Mott
transition occurs at a discrete doping level
Magnetoelastics of a spin liquid: X-ray diffraction studies of Tb2Ti2O7 in pulsed magnetic fields
We report high resolution single crystal x-ray diffraction measurements of
the frustrated pyrochlore magnet Tb2Ti2O7, collected using a novel low
temperature pulsed magnet system. This instrument allows characterization of
structural degrees of freedom to temperatures as low as 4.4 K, and in applied
magnetic fields as large as 30 Tesla. We show that Tb2Ti2O7 manifests
intriguing structural effects under the application of magnetic fields,
including strongly anisotropic giant magnetostriction, a restoration of perfect
pyrochlore symmetry in low magnetic fields, and ultimately a structural phase
transition in high magnetic fields. It is suggested that the magnetoelastic
coupling thus revealed plays a significant role in the spin liquid physics of
Tb2Ti2O7 at low temperatures.Comment: 4 pages, 4 figures, submitted for publicatio
High-field spectroscopy of singlet-triplet transitions in the spin-dimer systems Sr3Cr2O8 and Ba3Cr2O8
Magnetic excitations in the isostructural spin-dimer systems Sr3Cr2O8 and
Ba3Cr2O8 are probed by means of high-field electron spin resonance at
sub-terahertz frequencies. Three types of magnetic modes were observed. One
mode is gapless and corresponds to transitions within excited states, while two
other sets of modes are gapped and correspond to transitions from the ground to
the first excited states. The selection rules of the gapped modes are analyzed
in terms of a dynamical Dzyaloshinskii-Moriya interaction, suggesting the
presence of phonon-assisted effects in the low-temperature spin dynamics of
Sr3Cr2O8 and Ba3Cr2O8Comment: 6 pages, 4 figures, all comments are welcome and appreciate
Catastrophic senescence and semelparity in the Penna aging model
The catastrophic senescence of the Pacific salmon is among the initial tests
used to validate the Penna aging model. Based on the mutation accumulation
theory, the sudden decrease in fitness following reproduction may be solely
attributed to the semelparity of the species. In this work, we report other
consequences of mutation accumulation. Contrary to earlier findings, such
dramatic manifestation of aging depends not only on the choice of breeding
strategy but also on the value of the reproduction age, R, and the mutation
threshold, T. Senescence is catastrophic when . As the organism's
tolerance for harmful genetic mutations increases, the aging process becomes
more gradual. We observe senescence that is threshold dependent whenever T>R.
That is, the sudden drop in survival rate occurs at age equal to the mutation
threshold value
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