13,729 research outputs found
Gaugino condensation in an improved heterotic M-theory
Gaugino condensation is discussed in the context of a consistent new version
of low energy heterotic M-theory. The four dimensional reduction of the theory
is described, based on simple boson and fermion backgrounds. This is
generalised to include gaugino condenstates and various background fluxes, some
with non-trivial topology. It is found that condensate and quantised flux
contributions to the four-dimensional superpotential contain no corrections due
to the warping of the higher dimensional metric.Comment: 11 pages, 4 figures, LaTe
A new look at anomaly cancellation in heterotic -theory
This paper considers anomaly cancellation for eleven-dimensional supergravity
on a manifold with boundary and theories related to heterotic -theory. The
Green-Schwarz mechanism is implemented without introducing distributions. The
importance of the supersymmetry anomaly in constructing the low energy action
is discussed and it is argued that a recently proposed action for heterotic
-theory gives a supersymmetric theory to all orders in the gravitational
coupling .Comment: 4 pages LaTe
Phase transition in a mean-field model for sympatric speciation
We introduce an analytical model for population dynamics with intra-specific
competition, mutation and assortative mating as basic ingredients. The set of
equations that describes the time evolution of population size in a mean-field
approximation may be decoupled. We find a phase transition leading to sympatric
speciation as a parameter that quantifies competition strength is varied. This
transition, previously found in a computational model, occurs to be of first
order.Comment: accepted for Physica
The Penna model for biological ageing on a lattice: spatial consequences of child-care
We introduce a square lattice into the Penna bit-string model for biological
ageing and study the evolution of the spatial distribution of the population
considering different strategies of child-care. Two of the strategies are
related to the movements of a whole family on the lattice: in one case the
mother cannot move if she has any child younger than a given age, and in the
other case if she moves, she brings these young children with her. A stronger
condition has also been added to the second case, considering that young
children die with a higher probability if their mothers die, this probability
decreasing with age. We show that a highly non uniform occupation can be
obtained when child-care is considered, even for an uniform initial occupation
per site. We also compare the standard survival rate of the model with that
obtained when the spacial lattice is considered (without any kind of
child-care).Comment: 8 pages, 6 Postscript figure
Computer simulations on the sympatric speciation modes for the Midas cichlid species complex
Cichlid fishes are one of the best model system for the study of evolution of the species. Inspired by them, in this paper we simulated the splitting of a single species into two separate ones via random mutations, with both populations living together in sympatry, sharing the same habitat. We study the ecological, mating and genetic conditions needed to reproduce the polychromatism and polymorphism of three species of the Midas Cichlid species complex. Our results show two scenarios for the A. Citrinellus speciation process, one with and the other without disruptive natural selection. 
In the first scenario, the ecological and genetic conditions are sufficient to create two new species, while in the second the mating and genetic conditions must be synchronized in order to control the velocity of genetic drift
Applications and Sexual Version of a Simple Model for Biological Ageing
We use a simple model for biological ageing to study the mortality of the
population, obtaining a good agreement with the Gompertz law. We also simulate
the same model on a square lattice, considering different strategies of
parental care. The results are in agreement with those obtained earlier with
the more complicated Penna model for biological ageing. Finally, we present the
sexual version of this simple model.Comment: For Int.J.Mod.Phys.C Dec. 2001; 11 pages including 6 fig
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