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2010


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Policy learning algorithmis for motor learning (Algorithmen zum automatischen Erlernen von Motorfähigkigkeiten)

Peters, J., Kober, J., Schaal, S.

Automatisierungstechnik, 58(12):688-694, 2010, clmc (article)

Abstract
Robot learning methods which allow au- tonomous robots to adapt to novel situations have been a long standing vision of robotics, artificial intelligence, and cognitive sciences. However, to date, learning techniques have yet to ful- fill this promise as only few methods manage to scale into the high-dimensional domains of manipulator robotics, or even the new upcoming trend of humanoid robotics. If possible, scaling was usually only achieved in precisely pre-structured domains. In this paper, we investigate the ingredients for a general ap- proach policy learning with the goal of an application to motor skill refinement in order to get one step closer towards human- like performance. For doing so, we study two major components for such an approach, i. e., firstly, we study policy learning algo- rithms which can be applied in the general setting of motor skill learning, and, secondly, we study a theoretically well-founded general approach to representing the required control structu- res for task representation and execution.

am

link (url) [BibTex]


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Comment on biomimetic ultrathin whitening by capillary-force-induced random clustering of hydrogel micropillar arrays

Bernardino, N. R., Dietrich, S.

ACS Applied Materials \& Interfaces, 2(3):603-604, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Wetting of surfaces covered by elastic hairs

Bernardino, N. R., Blickle, V., Dietrich, S.

Langmuir, 26(10):7233-7241, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Phase diagrams of binary mixtures of oppositely charged colloids

Bier, M., van Roij, R., Dijkstra, M.

Journal of Chemical Physics, 133(12), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Narrow-escape times for diffusion in microdomains with a particle-surface affinity: mean-field results

Oshanin, G., Tamm, M., Vasilyev, O.

Journal of Chemical Physics, 132(23), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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The localization transition of the two-dimensional Lorentz model

Bauer, T., Höfling, Felix, Munk, T., Frey, E., Franosch, T.

European Physical Journal - Special Topics, 189(1):103-118, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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DDFT for Brownian particles and hydrodynamics

Rauscher, M.

Journal of Physics: Condensed Matter, 22(36), 2010 (article)

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link (url) DOI [BibTex]

link (url) DOI [BibTex]


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Colloidal particles in liquid crystal films and at interfaces

Tasinkevych, M., Andrienko, D.

Condensed Matter Physics, 13(3), 2010 (article)

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[BibTex]

[BibTex]


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Phase behavior of ionic liquid crystals

Kondrat, S., Bier, M., Harnau, L.

The Journal of Chemical Physics, 132, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Nanoindentation studies on crosslinking and curing effects of PDMS

Deuschle, J. K., de Souza, E. J., Arzt, E., Enders, S.

International Journal of Materials Research, 101(8):1014-1023, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Fundamental measure theory for hard-sphere mixtures: a review

Roth, R.

Journal of Physics: Condensed Matter, 22(6), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Universality of the glassy transitions in the two-dimensional \pmJ Ising model

Parisen Toldin, F., Pelissetto, A, Vicari, E.

Physical Review E, 82(2), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Phoretic motion of spheroidal particles due to self-generated solute gradients

Popescu, M. N., Dietrich, S., Tasinkevych, M., Ralston, J.

European Physical Journal E, 31(04):351-367, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Vapor pressure of ionic liquids

Bier, M., Dietrich, S.

Molecular Physics, 108(2):211-214, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Colloidal aggregation and critical Casimir forces

Gambassi, A., Dietrich, S.

Physical Review Letters, 105(5), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Localization phenomena in models of ion-conduction glass formers

Horbach, J., Voigtmann, T., Höfling, F., Franosch, T.

European Physical Journal - Special Topics, 189(1):141-145, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Tensorial density functional theory for non-spherical hard-body fluids

Hansen-Goos, H., Mecke, K.

Journal of Physics: Condensed Matter, 22(36), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Thermoresponsive colloidal molecules

Hoffmann, M., Siebenbürger, M., Harnau, L., Hund, M., Hanske, C., Wagner, C. S., Drechsler, M., Ballauff, M.

Soft Matter, 6, pages: 1125-1128, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Critical Casimir forces and adsorption profiles in the presence of a chemically structured substrate

Parisen Toldin, F., Dietrich, S.

Journal of Statistical Mechanics: Theory and Experiment, (Nov.), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Free energy of colloidal particles at the surface of sessile drops

Guzowski, J., Tasinkevych, M., Dietrich, S.

European Physical Journal, 33(3):219-242, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Casimir interactions in Ising strips with boundary fields: exact results

Abraham, D. B., Maciolek, A.

Physical Review Letters, 105(5), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Ballistic deposition patterns beneath a growing Kardar-Parisi-Zhang interface

Khanin, K., Nechaev, S., Oshanin, G., Sobolevski, A., Vasilyev, O.

Physical Review E, 82(6), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


Molecular QED of coherent and incoherent sum-frequency and second-harmonic generation in chiral liquids in the presence of a static electric field
Molecular QED of coherent and incoherent sum-frequency and second-harmonic generation in chiral liquids in the presence of a static electric field

Fischer, P., Salam, A.

MOLECULAR PHYSICS, 108(14):1857-1868, 2010 (article)

Abstract
Coherent second-order nonlinear optical processes are symmetry forbidden in centrosymmetric environments in the electric-dipole approximation. In liquids that contain chiral molecules, however, and which therefore lack mirror image symmetry, coherent sum-frequency generation is possible, whereas second-harmonic generation remains forbidden. Here we apply the theory of molecular quantum electrodynamics to the calculation of the matrix element, transition rate, and integrated signal intensity for sum-frequency and second-harmonic generation taking place in a chiral liquid in the presence and absence of a static electric field, to examine which coherent and incoherent processes exist in the electric-dipole approximation in liquids. Third- and fourth-order time-dependent perturbation theory is employed in combination with single-sided Feynman diagrams to evaluate two contributions arising from static field-free and field-induced processes. It is found that, in addition to the coherent term, an incoherent process exists for sum-frequency generation in liquids. Surprisingly, in the case of dc-field-induced second-harmonic generation, the incoherent contribution is found to always vanish for isotropic chiral liquids even though hyper-Rayleigh second-harmonic generation and electric-field-induced second-harmonic generation are both independently symmetry allowed in any liquid.

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DOI [BibTex]


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Vapour pressure of ionic liquids. Corrigendum

Bier, M., Dietrich, S.

Molecular Physics, 108(10):1413-1413, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Wetting of nanopatterned grooved surfaces

Hofmann, T., Tasinkevych, M., Checco, A., Dobisz, E., Dietrich, S., Ocko, B. M.

Physical Review Letters, 104(10), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Critical Casimir effect for colloids close to chemically patterned substrates

Tröndle, M., Kondrat, S., Gambassi, A., Harnau, L., Dietrich, S.

Journal of Chemical Physics, 133(7), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Dynamics of colloidal particles with capillary interactions

Dominguez, A., Oettel, M., Dietrich, S.

Physical Review E, 82(1), 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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A fluorescence correlation spectroscopy study of macromolecular tracer diffusion in polymer solutions

Zettl, U., Ballauff, M., Harnau, L.

Journal of Physics: Condensed Matter, 22, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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Anomalous transport resolved in space and time by fluorescence correlation spectroscopy

Höfling, F., Bamberg, K. U., Franosch, T.

Soft Matter, 7(4):1358-1363, 2010 (article)

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DOI [BibTex]

DOI [BibTex]


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A Bayesian approach to nonlinear parameter identification for rigid-body dynamics

Ting, J., DSouza, A., Schaal, S.

Neural Networks, 2010, clmc (article)

Abstract
For complex robots such as humanoids, model-based control is highly beneficial for accurate tracking while keeping negative feedback gains low for compliance. However, in such multi degree-of-freedom lightweight systems, conventional identification of rigid body dynamics models using CAD data and actuator models is inaccurate due to unknown nonlinear robot dynamic effects. An alternative method is data-driven parameter estimation, but significant noise in measured and inferred variables affects it adversely. Moreover, standard estimation procedures may give physically inconsistent results due to unmodeled nonlinearities or insufficiently rich data. This paper addresses these problems, proposing a Bayesian system identification technique for linear or piecewise linear systems. Inspired by Factor Analysis regression, we develop a computationally efficient variational Bayesian regression algorithm that is robust to ill-conditioned data, automatically detects relevant features, and identifies input and output noise. We evaluate our approach on rigid body parameter estimation for various robotic systems, achieving an error of up to three times lower than other state-of-the-art machine learning methods.

am

link (url) [BibTex]


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A first optimal control solution for a complex, nonlinear, tendon driven neuromuscular finger model

Theodorou, E. A., Todorov, E., Valero-Cuevas, F.

Proceedings of the ASME 2010 Summer Bioengineering Conference August 30-September 2, 2010, Naples, Florida, USA, 2010, clmc (article)

Abstract
In this work we present the first constrained stochastic op- timal feedback controller applied to a fully nonlinear, tendon driven index finger model. Our model also takes into account an extensor mechanism, and muscle force-length and force-velocity properties. We show this feedback controller is robust to noise and perturbations to the dynamics, while successfully handling the nonlinearities and high dimensionality of the system. By ex- tending prior methods, we are able to approximate physiological realism by ensuring positivity of neural commands and tendon tensions at all timesthus can, for the first time, use the optimal control framework to predict biologically plausible tendon tensions for a nonlinear neuromuscular finger model. METHODS 1 Muscle Model The rigid-body triple pendulum finger model with slightly viscous joints is actuated by Hill-type muscle models. Joint torques are generated by the seven muscles of the index fin-

am

PDF [BibTex]

PDF [BibTex]


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Efficient learning and feature detection in high dimensional regression

Ting, J., D’Souza, A., Vijayakumar, S., Schaal, S.

Neural Computation, 22, pages: 831-886, 2010, clmc (article)

Abstract
We present a novel algorithm for efficient learning and feature selection in high- dimensional regression problems. We arrive at this model through a modification of the standard regression model, enabling us to derive a probabilistic version of the well-known statistical regression technique of backfitting. Using the Expectation- Maximization algorithm, along with variational approximation methods to overcome intractability, we extend our algorithm to include automatic relevance detection of the input features. This Variational Bayesian Least Squares (VBLS) approach retains its simplicity as a linear model, but offers a novel statistically robust â??black- boxâ? approach to generalized linear regression with high-dimensional inputs. It can be easily extended to nonlinear regression and classification problems. In particular, we derive the framework of sparse Bayesian learning, e.g., the Relevance Vector Machine, with VBLS at its core, offering significant computational and robustness advantages for this class of methods. We evaluate our algorithm on synthetic and neurophysiological data sets, as well as on standard regression and classification benchmark data sets, comparing it with other competitive statistical approaches and demonstrating its suitability as a drop-in replacement for other generalized linear regression techniques.

am

link (url) [BibTex]

link (url) [BibTex]


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Stochastic Differential Dynamic Programming

Theodorou, E., Tassa, Y., Todorov, E.

In the proceedings of American Control Conference (ACC 2010) , 2010, clmc (article)

Abstract
We present a generalization of the classic Differential Dynamic Programming algorithm. We assume the existence of state- and control-dependent process noise, and proceed to derive the second-order expansion of the cost-to-go. Despite having quartic and cubic terms in the initial expression, we show that these vanish, leaving us with the same quadratic structure as standard DDP.

am

PDF [BibTex]

PDF [BibTex]


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Learning control in robotics – trajectory-based opitimal control techniques

Schaal, S., Atkeson, C. G.

Robotics and Automation Magazine, 17(2):20-29, 2010, clmc (article)

Abstract
In a not too distant future, robots will be a natural part of daily life in human society, providing assistance in many areas ranging from clinical applications, education and care giving, to normal household environments [1]. It is hard to imagine that all possible tasks can be preprogrammed in such robots. Robots need to be able to learn, either by themselves or with the help of human supervision. Additionally, wear and tear on robots in daily use needs to be automatically compensated for, which requires a form of continuous self-calibration, another form of learning. Finally, robots need to react to stochastic and dynamic environments, i.e., they need to learn how to optimally adapt to uncertainty and unforeseen changes. Robot learning is going to be a key ingredient for the future of autonomous robots. While robot learning covers a rather large field, from learning to perceive, to plan, to make decisions, etc., we will focus this review on topics of learning control, in particular, as it is concerned with learning control in simulated or actual physical robots. In general, learning control refers to the process of acquiring a control strategy for a particular control system and a particular task by trial and error. Learning control is usually distinguished from adaptive control [2] in that the learning system can have rather general optimization objectivesâ??not just, e.g., minimal tracking errorâ??and is permitted to fail during the process of learning, while adaptive control emphasizes fast convergence without failure. Thus, learning control resembles the way that humans and animals acquire new movement strategies, while adaptive control is a special case of learning control that fulfills stringent performance constraints, e.g., as needed in life-critical systems like airplanes. Learning control has been an active topic of research for at least three decades. However, given the lack of working robots that actually use learning components, more work needs to be done before robot learning will make it beyond the laboratory environment. This article will survey some ongoing and past activities in robot learning to assess where the field stands and where it is going. We will largely focus on nonwheeled robots and less on topics of state estimation, as typically explored in wheeled robots [3]â??6], and we emphasize learning in continuous state-action spaces rather than discrete state-action spaces [7], [8]. We will illustrate the different topics of robot learning with examples from our own research with anthropomorphic and humanoid robots.

am

link (url) [BibTex]

link (url) [BibTex]


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Learning, planning, and control for quadruped locomotion over challenging terrain

Kalakrishnan, M., Buchli, J., Pastor, P., Mistry, M., Schaal, S.

International Journal of Robotics Research, 30(2):236-258, 2010, clmc (article)

Abstract
We present a control architecture for fast quadruped locomotion over rough terrain. We approach the problem by decomposing it into many sub-systems, in which we apply state-of-the-art learning, planning, optimization, and control techniques to achieve robust, fast locomotion. Unique features of our control strategy include: (1) a system that learns optimal foothold choices from expert demonstration using terrain templates, (2) a body trajectory optimizer based on the Zero- Moment Point (ZMP) stability criterion, and (3) a floating-base inverse dynamics controller that, in conjunction with force control, allows for robust, compliant locomotion over unperceived obstacles. We evaluate the performance of our controller by testing it on the LittleDog quadruped robot, over a wide variety of rough terrains of varying difficulty levels. The terrain that the robot was tested on includes rocks, logs, steps, barriers, and gaps, with obstacle sizes up to the leg length of the robot. We demonstrate the generalization ability of this controller by presenting results from testing performed by an independent external test team on terrain that has never been shown to us.

am

link (url) Project Page [BibTex]

link (url) Project Page [BibTex]

2001


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Polymer depletion interaction between two parallel repulsive walls

Schlesener, F., Hanke, A., Klimpel, R., Dietrich, S.

Physical Review E, 63, 2001 (article)

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[BibTex]

2001


[BibTex]


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Positron annihilation studies on the migration of deformation induced vacancies in stainless steel AISI 316 L

Holzwarth, U., Barbieri, A., Hansen-Ilzhofer, S., Schaaff, P., Haaks, M.

Applied Physics A-Materials Science \& Processing, 73(4):467-475, 2001 (article)

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[BibTex]

[BibTex]


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Self-diffusion of 71Ge in Si-Ge

Strohm, A., Voss, T., Frank, W., Raisanen, J., Dietrich, M.

Physica B, 308, pages: 542-545, 2001 (article)

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[BibTex]

[BibTex]


Isotropic second-order nonlinear optical susceptibilities
Isotropic second-order nonlinear optical susceptibilities

Fischer, P., Buckingham, A., Albrecht, A.

PHYSICAL REVIEW A, 64(5), 2001 (article)

Abstract
The second-order nonlinear optical susceptibility, in the electric dipole approximation, is only nonvanishing for materials that are noncentrosymmetric. Should the medium be isotropic, then only a chiral system. such as an optically active liquid, satisfies this symmetry requirement. We derive the quantum-mechanical form of the isotropic component of the sum- and difference-frequency susceptibility and discuss its unusual spectral properties. We show that any coherent second-order nonlinear optical process in a system of randomly oriented molecules requires the medium to be chiral. and the incident frequencies to be different and nonzero. Furthermore, a minimum of two nondegenerate excited molecular states are needed for the isotropic part of the susceptibility to be nonvanishing. The rotationally invariant susceptibility is zero in the static field limit and shows exceptionally sensitive resonance and dephasing effects that are particular to chiral centers.

pf

DOI [BibTex]

DOI [BibTex]


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Crystal structures and freezing of dipolar fluids

Groh, B., Dietrich, S.

Physical Review E, 63(02), 2001 (article)

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[BibTex]

[BibTex]


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Non-Gaussian morphology on large scales: Minkowski functionals of the REFLEX cluster catalogue

Kerscher, M., Mecke, K., Schuecker, P., Böhringer, H., Guzzo, L., Collins, C. A., Schindler, S., De Grandi, S., Cruddace, R.

Astronomy & Astrophysics, 377(1):1-16, 2001 (article)

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[BibTex]

[BibTex]


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Adsorption isotherms of hydrogen: The role of thermal fluctuations

Vorberg, J., Herminghaus, S., Mecke, K.

Physical Review Letters, 87(19), 2001 (article)

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[BibTex]

[BibTex]


Reply to ``Comment on `Phenomenological damping in optical response tensors'{''}
Reply to “Comment on ‘Phenomenological damping in optical response tensors’”

Buckingham, A., Fischer, P.

PHYSICAL REVIEW A, 63(4), 2001 (article)

Abstract
We show that damping factors must not be incorporated in the perturbation of the ground state by a static electric field. If they are included, as in the theory of Stedman et al. {[}preceding Comment. Phys. Rev. A 63, 047801 (2001)], then there would be an electric dipole in the y direction induced in a hydrogen atom in the M-s = + 1/2 state by a static electric field in the x direction. Such a dipole is excluded by symmetry.

pf

DOI [BibTex]


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Morphological fluctuations of large-scale structure: The PSCz survey

Kerscher, M., Mecke, K., Schmalzing, J., Beisbart, C., Buchert, T., Wagner, H.

Astronomy \& Astrophysics, 373(1):1-11, 2001 (article)

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[BibTex]

[BibTex]


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Polydisperse fluid in contact with a semipermeable membrane

Bryk, P., Patrykiejew, A., Sokolowski, S.

Molecular Physics, 99(20):1709-1717, 2001 (article)

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[BibTex]

[BibTex]


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Diffusion of 195Au in amorphous Si3N4 and Si4N3

Voss, T., Matics, S., Strohm, A., Frank, W., Bilger, G.

Physica B, 308, pages: 431-433, 2001 (article)

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[BibTex]

[BibTex]


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Synchronized robot drumming by neural oscillator

Kotosaka, S., Schaal, S.

Journal of the Robotics Society of Japan, 19(1):116-123, 2001, clmc (article)

Abstract
Sensory-motor integration is one of the key issues in robotics. In this paper, we propose an approach to rhythmic arm movement control that is synchronized with an external signal based on exploiting a simple neural oscillator network. Trajectory generation by the neural oscillator is a biologically inspired method that can allow us to generate a smooth and continuous trajectory. The parameter tuning of the oscillators is used to generate a synchronized movement with wide intervals. We adopted the method for the drumming task as an example task. By using this method, the robot can realize synchronized drumming with wide drumming intervals in real time. The paper also shows the experimental results of drumming by a humanoid robot.

am

[BibTex]

[BibTex]