Hurst's exponent behaviour, weak-form stock market efficiency and financial liberalization: the Tunisian case

In this paper, we test the weak-form stock market efficiency for the Tunisian stock market (TSE). Our empirical approach is founded on the analysis of the behaviour over time of the Hurst's exponent. Thus, we computed the Hurst's exponent using a “rolling sample†with a time window of 4 years. The sample data covers in daily frequency the period (January, 1997- October 2007). Since the classical R/S analysis is strongly affected by short-range dependencies both in the mean and the conditional variance of TSE daily volatility, daily stock returns were filtered using the traditional AR-GARCH(1,1) model. Our results for Hurst's and filtered Hurst's exponents behaviour analysis show a strong evidence of long-range dependence with persistent behaviour of the TSE. However, during the last two years, the filtered Hurst's exponent seems to exhibit a switching regime behaviour with alternating persistent and antipersistent behaviour but where it was somewhat close to 0.5.The nonparametric statistic approach results reveal that some TSE reforms including the launching of the Electronic quotation system on April, 1998, the fiscal regime for holdings, the security reinforcement laws, the legal protection of minority shareholder may play a role in understanding the Hurst's exponent behaviour over timefinancial reforms, long-range dependence; weak-form efficiency; Hurst's exponent; rolling sample approach.

Generation of planar tensegrity structures through cellular multiplication

Tensegrity structures are frameworks in a stable self-equilibrated prestress state that have been applied in various fields in science and engineering. Research into tensegrity structures has resulted in reliable techniques for their form finding and analysis. However, most techniques address topology and form separately. This paper presents a bio-inspired approach for the combined topology identification and form finding of planar tensegrity structures. Tensegrity structures are generated using tensegrity cells (elementary stable self-stressed units that have been proven to compose any tensegrity structure) according to two multiplication mechanisms: cellular adhesion and fusion. Changes in the dimension of the self-stress space of the structure are found to depend on the number of adhesion and fusion steps conducted as well as on the interaction among the cells composing the system. A methodology for defining a basis of the self-stress space is also provided. Through the definition of the equilibrium shape, the number of nodes and members as well as the number of self-stress states, the cellular multiplication method can integrate design considerations, providing great flexibility and control over the tensegrity structure designed and opening the door to the development of a whole new realm of planar tensegrity systems with controllable characteristics.Comment: 29 pages, 19 figures, to appear at Applied Mathematical Modelin

Investigations in an external-loop airlift photobioreactor with annular light chambers and swirling flow

Photosynthetic microorganisms could serve as valuable compounds, but also for environmental applications. Their production under controlled conditions implies to design specific reactors, named photobioreactors, in which light supply is the main constraint. This paper was devoted to an original external-loop airlift photobioreactor (PBR) with annular light chambers in which a swirling motion was induced. The aim was to characterize this novel geometrical configuration in terms of gas–liquid hydrodynamics, and to test its potentiality for algal cultures. This PBR consisted of two identical columns connected by flanges defining tangential inlets, each column being made of two transparent concentric tubes (6 L in liquid volume, 50 m−1 in specific illuminated area). Firstly, the global flow characteristics (circulation and mixing times) were determined by a tracer method and modelled by an axial dispersed plug flow with complete recirculation (Péclet number). By means of a double optical probe, both local and global time-averaged parameters of the gas phase were measured, namely void fraction, bubble velocity, frequency and size. The gas–liquid mass transfer were also characterized, in tap water and in culture medium, by measuring overall volumetric mass transfer coefficients. In a second time, cultures of the microalga Chlamydomonas reinhardtii were run in batch mode. The variations of biomass concentration and pigment content with time from inoculation were successfully obtained. All these findings highlighted: (i) some significant differences in terms of gas–liquid hydrodynamics between the present PBR and the usual airlift systems, (ii) the interest of this configuration for algal cultures, even if complementary studies and technological improvements are still required for definitively validating its scale-up

Exponential Energy Decay for Damped Klein-Gordon Equation with Nonlinearities of Arbitrary Growth

We derive a uniform exponential decay of the total energy for the nonlinear Klein-Gordon equation with a damping around spatial infinity in the whole space or in the exterior of a star shaped obstacle