10 research outputs found
Random sampling vs. exact enumeration of attractors in random Boolean networks
We clarify the effect different sampling methods and weighting schemes have
on the statistics of attractors in ensembles of random Boolean networks (RBNs).
We directly measure cycle lengths of attractors and sizes of basins of
attraction in RBNs using exact enumeration of the state space. In general, the
distribution of attractor lengths differs markedly from that obtained by
randomly choosing an initial state and following the dynamics to reach an
attractor. Our results indicate that the former distribution decays as a
power-law with exponent 1 for all connectivities in the infinite system
size limit. In contrast, the latter distribution decays as a power law only for
K=2. This is because the mean basin size grows linearly with the attractor
cycle length for , and is statistically independent of the cycle length
for K=2. We also find that the histograms of basin sizes are strongly peaked at
integer multiples of powers of two for
Activity Dependent Branching Ratios in Stocks, Solar X-ray Flux, and the Bak-Tang-Wiesenfeld Sandpile Model
We define an activity dependent branching ratio that allows comparison of
different time series . The branching ratio is defined as . The random variable is the value of the next signal given
that the previous one is equal to , so . If
, the process is on average supercritical when the signal is equal to
, while if , it is subcritical. For stock prices we find
within statistical uncertainty, for all , consistent with an ``efficient
market hypothesis''. For stock volumes, solar X-ray flux intensities, and the
Bak-Tang-Wiesenfeld (BTW) sandpile model, is supercritical for small
values of activity and subcritical for the largest ones, indicating a tendency
to return to a typical value. For stock volumes this tendency has an
approximate power law behavior. For solar X-ray flux and the BTW model, there
is a broad regime of activity where , which we interpret as an
indicator of critical behavior. This is true despite different underlying
probability distributions for , and for . For the BTW model the
distribution of is Gaussian, for sufficiently larger than one, and
its variance grows linearly with . Hence, the activity in the BTW model
obeys a central limit theorem when sampling over past histories. The broad
region of activity where is close to one disappears once bulk dissipation
is introduced in the BTW model -- supporting our hypothesis that it is an
indicator of criticality.Comment: 7 pages, 11 figure
Complex Network Analysis of State Spaces for Random Boolean Networks
We apply complex network analysis to the state spaces of random Boolean
networks (RBNs). An RBN contains Boolean elements each with inputs. A
directed state space network (SSN) is constructed by linking each dynamical
state, represented as a node, to its temporal successor. We study the
heterogeneity of an SSN at both local and global scales, as well as
sample-to-sample fluctuations within an ensemble of SSNs. We use in-degrees of
nodes as a local topological measure, and the path diversity [Phys. Rev. Lett.
98, 198701 (2007)] of an SSN as a global topological measure. RBNs with exhibit non-trivial fluctuations at both local and global scales,
while K=2 exhibits the largest sample-to-sample, possibly non-self-averaging,
fluctuations. We interpret the observed ``multi scale'' fluctuations in the
SSNs as indicative of the criticality and complexity of K=2 RBNs. ``Garden of
Eden'' (GoE) states are nodes on an SSN that have in-degree zero. While
in-degrees of non-GoE nodes for SSNs can assume any integer value between
0 and , for K=1 all the non-GoE nodes in an SSN have the same in-degree
which is always a power of two
Concentration Independent Modulation of Local Micromechanics in a Fibrin Gel
Methods for tuning extracellular matrix (ECM) mechanics in 3D cell culture that rely on increasing the concentration of either protein or cross-linking molecules fail to control important parameters such as pore size, ligand density, and molecular diffusivity. Alternatively, ECM stiffness can be modulated independently from protein concentration by mechanically loading the ECM. We have developed a novel device for generating stiffness gradients in naturally derived ECMs, where stiffness is tuned by inducing strain, while local mechanical properties are directly determined by laser tweezers based active microrheology (AMR). Hydrogel substrates polymerized within 35 mm diameter Petri dishes are strained non-uniformly by the precise rotation of an embedded cylindrical post, and exhibit a position-dependent stiffness with little to no modulation of local mesh geometry. Here we present the device in the context of fibrin hydrogels. First AMR is used to directly measure local micromechanics in unstrained hydrogels of increasing fibrin concentration. Changes in stiffness are then mapped within our device, where fibrin concentration is held constant. Fluorescence confocal imaging and orbital particle tracking are used to quantify structural changes in fibrin on the micro and nano levels respectively. The micromechanical strain stiffening measured by microrheology is not accompanied by ECM microstructural changes under our applied loads, as measured by confocal microscopy. However, super-resolution orbital tracking reveals nanostructural straightening, lengthening, and reduced movement of fibrin fibers. Furthermore, we show that aortic smooth muscle cells cultured within our device are morphologically sensitive to the induced mechanical gradient. Our results demonstrate a powerful cell culture tool that can be used in the study of mechanical effects on cellular physiology in naturally derived 3D ECM tissues
Do users in Qatar perceive intellectual capital information to be value relevant for decision-making purposes?
This paper examines the perception of users on the value relevance of intellectual capital (IC) information for decision-making. The data were collected through questionnaires distributed to Qatari companies. The paper found that users perceive that IC information, internal capital information, and external capital information are relevant for decision-making. Thus, it has value relevance. Human capital was not perceived as relevant. The findings are useful for companies to better understand the needs of information users. They assist regulatory bodies in Qatar towards developing guidelines and standards for better corporate disclosure of IC, thereby helping in decision-making. This is the first research that examines the value relevance of IC information in Qatar. 2016 Inderscience Enterprises Ltd.Scopu