21,619 research outputs found
An evolutionary advantage of cooperation
Cooperation is a persistent behavioral pattern of entities pooling and
sharing resources. Its ubiquity in nature poses a conundrum. Whenever two
entities cooperate, one must willingly relinquish something of value to the
other. Why is this apparent altruism favored in evolution? Classical solutions
assume a net fitness gain in a cooperative transaction which, through
reciprocity or relatedness, finds its way back from recipient to donor. We seek
the source of this fitness gain. Our analysis rests on the insight that
evolutionary processes are typically multiplicative and noisy. Fluctuations
have a net negative effect on the long-time growth rate of resources but no
effect on the growth rate of their expectation value. This is an example of
non-ergodicity. By reducing the amplitude of fluctuations, pooling and sharing
increases the long-time growth rate for cooperating entities, meaning that
cooperators outgrow similar non-cooperators. We identify this increase in
growth rate as the net fitness gain, consistent with the concept of geometric
mean fitness in the biological literature. This constitutes a fundamental
mechanism for the evolution of cooperation. Its minimal assumptions make it a
candidate explanation of cooperation in settings too simple for other fitness
gains, such as emergent function and specialization, to be probable. One such
example is the transition from single cells to early multicellular life.Comment: 16 pages, 2 figure
Reduced nonlinear description of Farley-Buneman instability
In the study on nonlinear wave-wave processes in an ionosphere and a
magnetosphere usually the main attention is paid to investigation of plasma
turbulence at well developed stage, when the wide spectrum of plasma wave is
present. On the other side, it is well known that even if the number of
cooperating waves remains small due to a competition of processes of their
instability and attenuation, the turbulence appears in the result of their
stochastic behavior. The regimes of nonlinear dynamics of low frequency waves
excited due to Farley-Buneman instability in weakly ionized and inhomogeneous
ionospheric plasma in the presence of electric current perpendicular to ambient
magnetic field are considered. The problem is essentially three dimensional and
difficult for full numerical simulation, but the strong collisional damping of
waves allow to assume that in this case a perturbed state of plasma can be
described as finite set of interacting waves, some of which are unstable and
other strongly damping. The proposed nonlinear model allow to make full study
of nonlinear stabilization, conditions of stochasticity and to consider the
different regimes and properties of few mode plasma turbulence.Comment: The extended version of work, published in AIP Conf. Proc. 993, 113
(2008
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The effect of multiple knowledge sources on learning and teaching
Current paradigms for machine-based learning and teaching tend to perform their task in isolation from a rich context of existing knowledge. In contrast, the research project presented here takes the view that bringing multiple sources of knowledge to bear is of central importance to learning in complex domains. As a consequence teaching must both take advantage of and beware of interactions between new and existing knowledge. The central process which connects learning to its context is reasoning by analogy, a primary concern of this research. In teaching, the connection is provided by the explicit use of a learning model to reason about the choice of teaching actions. In this learning paradigm, new concepts are incrementally refined and integrated into a body of expertise, rather than being evaluated against a static notion of correctness. The domain chosen for this experimentation is that of learning to solve "algebra story problems." A model of acquiring problem solving skills in this domain is described, including: representational structures for background knowledge, a problem solving architecture, learning mechanisms, and the role of analogies in applying existing problem solving abilities to novel problems. Examples of learning are given for representative instances of algebra story problems. After relating our views to the psychological literature, we outline the design of a teaching system. Finally, we insist on the interdependence of learning and teaching and on the synergistic effects of conducting both research efforts in parallel
First-order phase transitions in outbreaks of co-infectious diseases and the extended general epidemic process
In co-infections, positive feedback between multiple diseases can accelerate
outbreaks. In a recent letter Chen, Ghanbarnejad, Cai, and Grassberger (CGCG)
introduced a spatially homogeneous mean-field model system for such
co-infections, and studied this system numerically with focus on the possible
existence of discontinuous phase transitions. We show that their model
coincides in mean-field theory with the homogenous limit of the extended
general epidemic process (EGEP). Studying the latter analytically, we argue
that the discontinuous transition observed by CGCG is basically a spinodal
phase transition and not a first-order transition with phase-coexistence. We
derive the conditions for this spinodal transition along with predictions for
important quantities such as the magnitude of the discontinuity. We also shed
light on a true first-order transition with phase-coexistence by discussing the
EGEP with spatial inhomogeneities.Comment: 6 pages, 3 figure
Control strategy for cooperating disparate manipulators
To manipulate large payloads typical of space construction, the concept of a small arm mounted on the end of a large arm is introduced. The main purposes of such a configuration are to increase the structural stiffness of the robot by bracing against or locking to a stationary frame, and to maintain a firm position constraint between the robot's base and workpieces by grasping them. Possible topologies for a combination of disparate large and small arms are discussed, and kinematics, dynamics, controls, and coordination of the two arms, especially when they brace at the tip of the small arm, are developed. The feasibility and improvement in performance are verified, not only with analytical work and simulation results but also with experiments on the existing arrangement Robotic Arm Large and Flexible and Small Articulated Manipulator
Project Tech Top study of lunar, planetary and solar topography Final report
Data acquisition techniques for information on lunar, planetary, and solar topograph
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