141 research outputs found
Cascades of Failure and Extinction in Evolving Complex Systems
There is empirical evidence from a range of disciplines that as the connectivity of a network increases, we observe an increase in the average fitness of the system. But at the same time, there is an increase in the proportion of failure/extinction events which are extremely large. The probability of observing an extreme event remains very low, but it is markedly higher than in the system with lower degrees of connectivity. We are therefore concerned with systems whose properties are not static but which evolve dynamically over time. The focus in this paper, motivated by the empirical examples, is on networks in which the robustness or fragility of the vertices is not given, but which themselves evolve over time We give examples from complex systems such as outages in the US power grid, the robustness properties of cell biology networks, and trade links and the propagation of both currency crises and disease. We consider systems which are populated by agents which are heterogeneous in terms of their fitness for survival. The agents are connected on a network, which evolves over time. In each period agents take self-interested decisions to increase their fitness for survival to form alliances which increase the connectivity of the network. The network is subjected to external negative shocks both with respect to the size of the shock and the spatial impact of the shock. We examine the size/frequency distribution of extinctions and how this distribution evolves as the connectivity of the network grows. The results are robust with respect to the choice of statistical distribution of the shocks. The model is deliberately kept as parsimonious and simple as possible, and refrains from incorporating features such as increasing returns and externalities arising from preferential attachment which might bias the model in the direction of having the empirically observed features of many real world networks. The model still generates results consistent with the empirical evidence: increasing the number of connections causes an increase in the average fitness of agents, yet at the same time makes the system as whole more vulnerable to catastrophic failure/extinction events on an near-global scale.Agent-Based Model; Connectivity; Complex Systems; Networks
Early Warning Analysis for Social Diffusion Events
There is considerable interest in developing predictive capabilities for
social diffusion processes, for instance to permit early identification of
emerging contentious situations, rapid detection of disease outbreaks, or
accurate forecasting of the ultimate reach of potentially viral ideas or
behaviors. This paper proposes a new approach to this predictive analytics
problem, in which analysis of meso-scale network dynamics is leveraged to
generate useful predictions for complex social phenomena. We begin by deriving
a stochastic hybrid dynamical systems (S-HDS) model for diffusion processes
taking place over social networks with realistic topologies; this modeling
approach is inspired by recent work in biology demonstrating that S-HDS offer a
useful mathematical formalism with which to represent complex, multi-scale
biological network dynamics. We then perform formal stochastic reachability
analysis with this S-HDS model and conclude that the outcomes of social
diffusion processes may depend crucially upon the way the early dynamics of the
process interacts with the underlying network's community structure and
core-periphery structure. This theoretical finding provides the foundations for
developing a machine learning algorithm that enables accurate early warning
analysis for social diffusion events. The utility of the warning algorithm, and
the power of network-based predictive metrics, are demonstrated through an
empirical investigation of the propagation of political memes over social media
networks. Additionally, we illustrate the potential of the approach for
security informatics applications through case studies involving early warning
analysis of large-scale protests events and politically-motivated cyber
attacks
Predictive Analysis for Social Processes II: Predictability and Warning Analysis
This two-part paper presents a new approach to predictive analysis for social
processes. Part I identifies a class of social processes, called positive
externality processes, which are both important and difficult to predict, and
introduces a multi-scale, stochastic hybrid system modeling framework for these
systems. In Part II of the paper we develop a systems theory-based,
computationally tractable approach to predictive analysis for these systems.
Among other capabilities, this analytic methodology enables assessment of
process predictability, identification of measurables which have predictive
power, discovery of reliable early indicators for events of interest, and
robust, scalable prediction. The potential of the proposed approach is
illustrated through case studies involving online markets, social movements,
and protest behavior
Predictive Non-equilibrium Social Science
Non-Equilibrium Social Science (NESS) emphasizes dynamical phenomena, for
instance the way political movements emerge or competing organizations
interact. This paper argues that predictive analysis is an essential element of
NESS, occupying a central role in its scientific inquiry and representing a key
activity of practitioners in domains such as economics, public policy, and
national security. We begin by clarifying the distinction between models which
are useful for prediction and the much more common explanatory models studied
in the social sciences. We then investigate a challenging real-world predictive
analysis case study, and find evidence that the poor performance of standard
prediction methods does not indicate an absence of human predictability but
instead reflects (1.) incorrect assumptions concerning the predictive utility
of explanatory models, (2.) misunderstanding regarding which features of social
dynamics actually possess predictive power, and (3.) practical difficulties
exploiting predictive representations.Comment: arXiv admin note: substantial text overlap with arXiv:1212.680
Cartesian control of redundant robots
A Cartesian-space position/force controller is presented for redundant robots. The proposed control structure partitions the control problem into a nonredundant position/force trajectory tracking problem and a redundant mapping problem between Cartesian control input F is a set member of the set R(sup m) and robot actuator torque T is a set member of the set R(sup n) (for redundant robots, m is less than n). The underdetermined nature of the F yields T map is exploited so that the robot redundancy is utilized to improve the dynamic response of the robot. This dynamically optimal F yields T map is implemented locally (in time) so that it is computationally efficient for on-line control; however, it is shown that the map possesses globally optimal characteristics. Additionally, it is demonstrated that the dynamically optimal F yields T map can be modified so that the robot redundancy is used to simultaneously improve the dynamic response and realize any specified kinematic performance objective (e.g., manipulability maximization or obstacle avoidance). Computer simulation results are given for a four degree of freedom planar redundant robot under Cartesian control, and demonstrate that position/force trajectory tracking and effective redundancy utilization can be achieved simultaneously with the proposed controller
Obstacle avoidance for redundant robots using configuration control
A redundant robot control scheme is provided for avoiding obstacles in a workspace during the motion of an end effector along a preselected trajectory by stopping motion of the critical point on the robot closest to the obstacle when the distance between is reduced to a predetermined sphere of influence surrounding the obstacle. Algorithms are provided for conveniently determining the critical point and critical distance
Cascades of Failure and Extinction in Evolving Complex Systems
There is empirical evidence from a range of disciplines that as the
connectivity of a network increases, we observe an increase in the average
fitness of the system. But at the same time, there is an increase in the
proportion of failure/extinction events which are extremely large. The
probability of observing an extreme event remains very low, but it is markedly
higher than in the system with lower degrees of connectivity. We give examples
from complex systems such as outages in the US power grid, the robustness
properties of cell biology networks, and trade links and the propagation of
both currency crises and disease. We consider networks which are populated by
agents which are heterogeneous in terms of their fitness for survival. The
network evolves over time, and in each period agents take self-interested
decisions to increase their fitness for survival to form alliances which
increase the connectivity of the network. The network is subjected to external
negative shocks both with respect to the size of the shock and the spatial
impact of the shock. We examine the size/frequency distribution of extinctions
and how this distribution evolves as the connectivity of the network grows. The
results are robust with respect to the choice of statistical distribution of
the shocks. We find that increasing the number of connections causes an
increase in the average fitness of agents, yet at the same time makes the
system as whole more vulnerable to catastrophic failure/extinction events on an
near-global scale.Comment: 15 pages, 5 figure
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