Contact process on generalized Fibonacci chains: infinite-modulation criticality and double-log periodic oscillations

We study the nonequilibrium phase transition of the contact process with aperiodic transition rates using a real-space renormalization group as well as Monte-Carlo simulations. The transition rates are modulated according to the generalized Fibonacci sequences defined by the inflation rules A $\to$ AB$^k$ and B $\to$ A. For $k=1$ and 2, the aperiodic fluctuations are irrelevant, and the nonequilibrium transition is in the clean directed percolation universality class. For $k\ge 3$, the aperiodic fluctuations are relevant. We develop a complete theory of the resulting unconventional "infinite-modulation" critical point which is characterized by activated dynamical scaling. Moreover, observables such as the survival probability and the size of the active cloud display pronounced double-log periodic oscillations in time which reflect the discrete scale invariance of the aperiodic chains. We illustrate our theory by extensive numerical results, and we discuss relations to phase transitions in other quasiperiodic systems.Comment: 12 pages, 9 eps figures included, final version as publishe

African portfolio entrepreneurship and the creation of jobs

Purpose The purpose of this paper is to focus on large-scale portfolio entrepreneurship and its impact on the creation of stable wage employment in African economies. Design/methodology/approach The three studies focussed on Egypt, Uganda, and Malawi were all exploratory, inductive, and qualitative studies, which involved semi-structured interviews with 65 entrepreneurial founders of some of these countries’ most prominent business portfolios between 2009 and 2012. The data were collected through face-to-face interviews, which lasted between one and four hours, with the founders of each of these portfolios. Findings This inductive and qualitative study finds a connection between the creation of stable wage-paying jobs and portfolio entrepreneurship in three countries, representing three of the four different archetypal African economies. It also finds a strong connection between the development of new industries and portfolio entrepreneurship. Practical implications The practical and societal implications of these findings are incredibly important. The current and looming shortage of stable wage employment in Africa is reaching calamitous proportions. The growth in religion-affiliated terrorism and high-risk economic migration to Europe can be directly related to the lack of employment opportunities in African nations. The findings indicate that portfolio entrepreneurs are major players in the creation of such employment opportunities and government policies focussing on this area, as compared to focussing solely on SMEs, may be more effective in mitigating some of the drivers for emigration and terrorism. Originality/value This is the only study of its kind that investigates the role of large-scale portfolio entrepreneurship in the growth of employment opportunities in Africa

Multicore-optimized wavefront diamond blocking for optimizing stencil updates

The importance of stencil-based algorithms in computational science has focused attention on optimized parallel implementations for multilevel cache-based processors. Temporal blocking schemes leverage the large bandwidth and low latency of caches to accelerate stencil updates and approach theoretical peak performance. A key ingredient is the reduction of data traffic across slow data paths, especially the main memory interface. In this work we combine the ideas of multi-core wavefront temporal blocking and diamond tiling to arrive at stencil update schemes that show large reductions in memory pressure compared to existing approaches. The resulting schemes show performance advantages in bandwidth-starved situations, which are exacerbated by the high bytes per lattice update case of variable coefficients. Our thread groups concept provides a controllable trade-off between concurrency and memory usage, shifting the pressure between the memory interface and the CPU. We present performance results on a contemporary Intel processor