Generating upward sweeps in population using the Turchin--Korotayev model

The works of [Cha-DunAlvInoNieCarFieLaw,Cha-Dun] describe upward sweeps in populations of city-states and attempt to characterize such phenomenon. The model proposed in both [TurKor,Tur] describes how the population, state resources and internal conflict influence each other over time. We show that one can obtain an upward sweep in the population by altering particular parameters of the system of differential equations constituting the model given in [TurKor,Tur]. Moreover, we show that such a system has a unstable critical point and propose an approach for determining bifurcation points in the parameter space for the model.Comment: 20 pages, 13 figures. Contains Matlab code for those interested in adapting and/or extending the model. Comments are welcom

Properties of the flow on a polygonal Andreev billiard

A formal definition of a (mathematical) polygonal Andreev billiard and a construction of an equivalence relation that captures the dynamics described in physical toy model of Andreev reflection are given. The continuous flow and discrete flow on the respective phase spaces. It is then shown that the continuous flow preserves the absolute value of the volume element $dx\wedge dy\wedge d\theta$ and the billiard (collision) map preserves the measure $\cos \phi dr d\phi$, respectively. One can then characterize the dynamics of a rational polygonal Andreev billiard table. Finally, a discussions of the effect of a fractal perturbation of the toy model of a rectangular nanowire lying upon a superconducting medium is given.Comment: This is a preliminary report and open for comments and suggestions. 17 pages, 15 figures. Makes reference to arXiv:1503.0849

Sequences of compatible periodic hybrid orbits of prefractal Koch snowflake billiards

The Koch snowflake KS is a nowhere differentiable curve. The billiard table Omega(KS) with boundary KS is, a priori, not well defined. That is, one cannot a priori determine the minimal path traversed by a billiard ball subject to a collision in the boundary of the table. It is this problem which makes Omega(KS) such an interesting, yet difficult, table to analyze. In this paper, we approach this problem by approximating (from the inside) Omega(KS) by well-defined (prefractal) rational polygonal billiard tables Omega(KS_n). We first show that the flat surface S(KS_n) determined from the rational billiard Omega(KS_n) is a branched cover of the singly punctured hexagonal torus. Such a result, when combined with the results of [Gut2], allows us to define a sequence of compatible orbits of prefractal billiards. We define a hybrid orbit of a prefractal billiard Omega(KS_n) and show that every dense orbit of a prefractal billiard is a dense hybrid orbit of Omega(KS_n). This result is key in obtaining a topological dichotomy for a sequence of compatible orbits. Furthermore, we determine a sufficient condition for a sequence of compatible orbits to be a sequence of compatible periodic hybrid orbits. We then examine the limiting behavior of a sequence of compatible periodic hybrid orbits. We show that the trivial limit of particular (eventually) constant sequences of compatible hybrid orbits constitutes an orbit of Omega(KS). In addition, we show that the union of two suitably chosen nontrivial polygonal paths connects two elusive limit points of the Koch snowflake. Finally, we discuss how it may be possible for our results to be generalized to other fractal billiard tables and how understanding the structures of the Veech groups of the prefractal billiards may help in determining `fractal flat surfaces' naturally associated with the billiard flows.Comment: 21 pages, 15 figure

The Wild, Elusive Singularities of the T-fractal Surface

We give a rigorous definition of the T-fractal translation surface, and describe some its basic geometric and dynamical properties. In particular, we study the singularities attached to the surface by its metric completion and show there exists a Cantor set of "elusive singularities." We show these elusive singularities can be thought of as a generalization of the wild singularities introduced by Bowman and Valdez. In particular, we show that every elusive singularities has an infinite discrete set of rotational components.Comment: 30 pages, 18 figures. Comments welcom

Towards the Koch Snowflake Fractal Billiard: Computer Experiments and Mathematical Conjectures

In this paper, we attempt to define and understand the orbits of the Koch snowflake fractal billiard $KS$. This is a priori a very difficult problem because $\partial(KS)$, the snowflake curve boundary of $KS$, is nowhere differentiable, making it impossible to apply the usual law of reflection at any point of the boundary of the billiard table. Consequently, we view the prefractal billiards $KS_n$ (naturally approximating $KS$ from the inside) as rational polygonal billiards and examine the corresponding flat surfaces of $KS_n$, denoted by $\mathcal{S}_{KS_n}$. In order to develop a clearer picture of what may possibly be happening on the billiard $KS$, we simulate billiard trajectories on $KS_n$ (at first, for a fixed $n\geq 0$). Such computer experiments provide us with a wealth of questions and lead us to formulate conjectures about the existence and the geometric properties of periodic orbits of $KS$ and detail a possible plan on how to prove such conjectures.Comment: 26 pages, color figures (For crisper figures, please contact the second author

Families of Periodic Orbits of the Koch Snowflake Fractal Billiard

We describe the periodic orbits of the prefractal Koch snowflake billiard (the nth inner rational polygonal approximation of the Koch snowflake billiard). In the case of the finite (prefractal) billiard table, we focus on the direction given by an initial angle of pi/3, and define 1) a compatible sequence of piecewise Fagnano orbits, 2) an eventually constant compatible sequence of orbits and 3) a compatible sequence of generalized piecewise Fagnano orbits. In the case of the infinite (fractal) billiard table, we will describe what we call stabilizing periodic orbits of the Koch snowflake fractal billiard. In a sense, we show that it is possible to define billiard dynamics on a Cantor set. In addition, we will show that the inverse limit of the footprints of orbits of the prefractal approximations exists in a specific situation and provide a plausibility argument as to why such an inverse limit of footprints should constitute the footprint of a well-defined periodic orbit of the fractal billiard. Using known results for the inverse limit of a sequence of finite spaces, we deduce that the footprint (i.e., the intersection of the orbit with the boundary) of a piecewise Fagnano orbit is a topological Cantor set and a self-similar Cantor set. We allude to a possible characterization of orbits with an initial direction of pi/3. Such a characterization would allow one to describe an orbit with an initial direction of pi/3 of the Koch snowflake billiard as either a piecewise Fagnano orbit, a stabilizing orbit or a generalized piecewise Fagnano orbit. We then close the paper by discussing several outstanding open problems and conjectures about the Koch snowflake fractal billiard, the associated 'fractal flat surface', and possible connections with the associated fractal drum. In the long-term, the present work may help lay the foundations for a general theory of fractal billiards.Comment: This paper contains an index of notation, a table of contents and 31 figures; it is 63 pages in lengt

Nontrivial paths and periodic orbits of the TT-fractal billiard table

We introduce and prove numerous new results about the orbits of the $T$-fractal billiard. Specifically, in Section 3, we give a variety of sufficient conditions for the existence of a sequence of compatible periodic orbits. In Section 4, we examine the limiting behavior of particular sequences of compatible periodic orbits and, more interesting, in Section 5, the limiting behavior of a particular sequence of compatible singular orbits. The latter seems to indicate that the classification of orbits may not be so straightforward. Additionally, sufficient conditions for the existence of particular nontrivial paths is given in Section 4. The proofs of two results stated in [LapNie4] appear here for the first time, as well. A discussion of our results and directions for future research is then given in Section 6.Comment: 20 Figures, 35 pages, two results from arXiv:1210.0282 are generalized and proved in this article. Many new results appear here. Comments welcome. Version 3 contains minor grammatical changes and the presentation of some results has greatly improved. To appear in the journal Nonlinearit

Impact of baryonic streaming velocities on the formation of supermassive black holes via direct collapse

Baryonic streaming motions produced prior to the epoch of recombination became supersonic during the cosmic dark ages. Various studies suggest that such streaming velocities change the halo statistics and also influence the formation of Population III stars. In this study, we aim to explore the impact of streaming velocities on the formation of supermassive black holes at $z>10$ via the direct collapse scenario. To accomplish this goal, we perform cosmological large eddy simulations for two halos of a few times $\rm 10^{7} M_{\odot}$ with initial streaming velocities of 3, 6 and 9 $\rm km/s$. These massive primordial halos illuminated by the strong Lyman Werner flux are the potential cradles for the formation of direct collapse seed black holes. To study the evolution for longer times, we employ sink particles and track the accretion for 10,000 years. Our findings show that higher streaming velocities increase the circular velocities from about 14 $\rm km/s$ to 16 $\rm km/s$. They also delay the collapse of halos for a few million years, but do not have any significant impact on the halo properties such as turbulent energy, radial velocity, density and accretion rates. Sink particles of about $\rm \sim 10^5 M_{\odot}$ are formed at the end of our simulations and no clear distribution of sink masses is observed in the presence of streaming motions. It is further found that the impact of streaming velocities is less severe in massive halos compared to the minihalos as reported in the previous studies.Comment: Matches the accepted vesion, to be appeared MNRA

The formation of massive Pop III stars in the presence of turbulence

Population III stars forming in the infant universe at z=30 heralded the end of the cosmic dark ages. They are presumed to be assembled in so-called minihaloes with virial temperatures of a few thousand K where collapse is triggered by molecular hydrogen cooling. A central question concerns their final masses, and whether fragmentation occurs during their formation. While studies employing Lagrangian codes suggest fragmentation via a self-gravitating disk, recent high resolution simulations indicated that disk formation is suppressed. Here we report the first high-resolution large-eddy simulations performed with the Eulerian grid-based code Enzo following the evolution beyond the formation of the first peak, to investigate the accretion of the central massive clump and potential fragmentation. For a total of 3 halos, we see that a disk forms around the first clump. The central clump reaches $\sim10$ solar masses after 40 years, while subsequent accretion is expected at a rate of $10^{-2}$ solar masses per year. In one of these halos, additional clumps form as a result of fragmentation which proceeds at larger scales. We note that subgrid-scale turbulence yields relevant contributions to the stability of the protostellar disks. We conclude that the first protostar may reach masses up to $\rm 40-100 M_{\odot}$, which are only limited by the effect of radiative feedback.Comment: Accepted for publication in APJL, comments are welcom

The small scale dynamo and the amplification of magnetic fields in massive primordial haloes

While present standard model of cosmology yields no clear prediction for the initial magnetic field strength, efficient dynamo action may compensate for initially weak seed fields via rapid amplification. In particular, the small-scale dynamo is expected to exponentially amplify any weak magnetic field in the presence of turbulence. We explore whether this scenario is viable using cosmological magneto-hydrodynamics simulations modeling the formation of the first galaxies, which are expected to form in so-called atomic cooling halos with virial temperatures $\rm T_{vir} \geq 10^{4}$ K. As previous calculations have shown that a high Jeans resolution is needed to resolve turbulent structures and dynamo effects, our calculations employ resolutions of up to 128 cells per Jeans length. The presence of the dynamo can be clearly confirmed for resolutions of at least 64 cells per Jeans length, while saturation occurs at approximate equipartition with turbulent energy. As a result of the large Reynolds numbers in primordial galaxies, we expect saturation to occur at early stages, implying magnetic field strengths of \sim0.1 $\mu$G at densities of 10^4 cm^{-3}.Comment: Matches the accepted version to be appeared in MNRA