1,438 research outputs found
Velocity control of a 2D dynamic walking robot
In this abstract we introduce velocity control for our 2D dynamic walking robot Dribbel [1] (figure 1) and show that, by âclosing the loopâ, this automatically leads to increased robustness
Geometric dynamics analysis of humanoids - locked inertia
The advantage of geometric dynamics analysis over most classical 3D analysis is that the equations are coordinate- neutral: as long as all quantities are expressed in the same coordinate frame, the equations are correct
Agglomeration economies and entrepreneurship: testing for spatial externalities in the Dutch ICT industry
Although there is growing evidence on the role of agglomeration economies in the formation and growth of firms, both the concepts of agglomeration economies and entrepreneurship tend to be ambiguously defined and measured in the literature. In this study, we aim to improve the conceptualisations and measures of agglomeration economies and entrepreneurship. Indicators of agglomeration economies are analysed in clearly defined urban regimes on three spatial scales in the Netherlands â national zoning, labour market connectedness, and urban size. This is done in order to uncover their effect on two entrepreneurial phases in the firm life cycle - new firm formation and the growth of incumbent firms in the relatively new ICT industry in the Netherlands. In comparison with new firm formation, the growth of incumbent firms is not so much related to spatial clustering of the ICT industry and other localized sources of knowledge economies associated with urban density. Instead, knowledge as an input for growth of incumbent firms is associated with more endogenous (firm internal) learning aspects, reflected by a significant correlate with R&D-investments. Also the effect of local ICT firm competition differs between the two types of firms: a positive effect on new firm formation, but a negative effect on incumbent firm growth. In general, agglomeration economies have stronger effects on the formation of ICT firms than on the growth of ICT firms.agglomeration economics, spatial externalities, entrepreneurship, location, urban regimes, ICT industry
Variety and regional economic growth in the Netherlands
In economic theory, one can distinguish between variety as a source of regional knowledge spillovers, called Jacobs externalities, and variety as a portfolio protecting a region from external shocks. We argue that Jacobs externalities are best measured by related variety (within sectors), while the portfolio argument is better captured by unrelated variety (between sectors). We introduce a methodology based on entropy measures to compute related variety and unrelated variety. Using data at the COROP level for the period 1996-2002, we find that Jacobs externalities enhance employment growth, while unrelated variety dampens unemployment growth. Productivity growth, by contrast, can be explained by traditional determinants including investments and R&D expenditures. Implications for regional policy in The Netherlands follow.evolutionary economic geography, new economic geography, economic variety
The Geography of Internet Adoption by Retailers
Up till now, the literature on Internet adoption by retailers paid little attention to spatial variables. Using data on 27,000 retail outlets in the Netherlands, we investigate the geographical diffusion of Internet adoption by Dutch retailers. More precise, we examine to what extent retail Internet adoption differs between shopping centers, cities, and regions, while controlling for product and organizational variables. Results of the linear and multinomial logistic regressions suggest that shops at city centers are more likely to adopt the Internet than shops located at shopping centers at the bottom of the retail hierarchy. Furthermore, shops in large cities have a higher probability to adopt the Internet than shops in small cities. On the regional level, the likelihood of Internet adoption is higher for shops in core regions than for retail outlets in the periphery. In conclusion, geography seems to matter for retail Internet adoption.evolutionary economics, internet adoption, retailing
Anisotropic interactions of a single spin and dark-spin spectroscopy in diamond
The nitrogen-vacancy (N-V) center in diamond is a promising atomic-scale
system for solid-state quantum information processing. Its spin-dependent
photoluminescence has enabled sensitive measurements on single N-V centers,
such as: electron spin resonance, Rabi oscillations, single-shot spin readout
and two-qubit operations with a nearby 13C nuclear spin. Furthermore, room
temperature spin coherence times as long as 58 microseconds have been reported
for N-V center ensembles. Here, we have developed an angle-resolved
magneto-photoluminescence microscopy apparatus to investigate the anisotropic
electron spin interactions of single N-V centers at room temperature. We
observe negative peaks in the photoluminescence as a function of both magnetic
field magnitude and angle that are explained by coherent spin precession and
anisotropic relaxation at spin level anti-crossings. In addition, precise field
alignment unmasks the resonant coupling to neighboring dark nitrogen spins that
are not otherwise detected by photoluminescence. The latter results demonstrate
a means of investigating small numbers of dark spins via a single bright spin
under ambient conditions.Comment: 13 pages, 4 figure
3D Distribution of Molecular Gas in the Barred Milky Way
We present a new model of the three-dimensional distribution of molecular gas
in the Milky Way Galaxy, based on CO line data. Our analysis is based on a
gas-flow simulation of the inner Galaxy using smoothed-particle hydrodynamics
(SPH) using a realistic barred gravitional potential derived from the observed
COBE/DIRBE near-IR light distribution. The gas model prescribes the gas orbits
much better than a simple circular rotation model and is highly constrained by
observations, but it cannot predict local details. In this study, we provide a
3D map of the observed molecular gas distribution using the velocity field from
the SPH model. A comparison with studies of the Galactic Center region suggests
that the main structures are reproduced but somewhat stretched along the
line-of-sight, probably on account of limited resolution of the underlying SPH
simulation. The gas model will be publicly available and may prove useful in a
number of applications, among them the analysis of diffuse gamma-ray emission
as measured with GLAST.Comment: ApJ in pres
The Radial Extent and Warp of the Ionized Galactic Disk. II. A Likelihood Analysis of Radio-Wave Scattering Toward the Anticenter
We use radio-wave scattering data to constrain the distribution of ionized
gas in the outer Galaxy. Like previous models, our model for the H II disk
includes parameters for the radial scale length and scale height of the H II,
but we allow the H II disk to warp and flare. Our model also includes the
Perseus arm. We use a likelihood analysis on 11 extragalactic sources and 7
pulsars. Scattering in the Perseus arm is no more than 60% of the level
contributed by spiral arms in the inner Galaxy, equivalent to a 1 GHz
scattering diameter of 1.5 mas. Our analysis favors an unwarped, nonflaring
disk with a 1 kpc scale height, though this may reflect the non-uniform and
coarse coverage provided by the available data. The lack of a warp indicates
that VLBI observations near 1 GHz with an orbiting station having baseline
lengths of a few Earth diameters will not be affected by interstellar
scattering at Galactic latitudes |b| ~ 15 degrees. The radial scale length is
15--20 kpc, but the data cannot distinguish between a gradual decrease in the
electron density and a truncated distribution. We favor a truncated one,
because we associate the scattering with massive star formation, which is also
truncated near 20 kpc. The distribution of electron density turbulence
decreases more rapidly with Galactocentric distance than does the hydrogen
distribution. Alternate ionizing and turbulent agents---the intergalactic
ionizing flux and satellite galaxies passing through the disk---do not
contribute significantly to scattering. We cannot exclude the possibility that
a largely ionized, but quiescent disk extends to >~ 100 kpc, similar to that
for some Ly-alpha absorbers.Comment: 34 pages, LaTeX2e with AASTeX aaspp4 macro, 9 figures in 9 PostScript
files, accepted for publication in Ap
High-velocity white dwarfs: thick disk, not dark matter
We present an alternative interpretation of the nature of the extremely cool,
high-velocity white dwarfs identified by Oppenheimer et al (2001) in a
high-latitude astrometric survey. We argue that the velocity distribution of
the majority of the sample is more consistent with the high-velocity tail of a
rotating population, probably the thick disk, rather than a pressure-supported
halo system. Indeed, the observed numbers are well matched by predictions based
on the kinematics of a complete sample of nearby M dwarfs. Analysing only stars
showing retrograde motion gives a local density close to that expected for
white dwarfs in the stellar (R^-3.5) halo. Under our interpretation, none of
the white dwarfs need be assigned to the dark-matter, heavy halo. However,
luminosity functions derived from observations of these stars can set important
constraints on the age of the oldest stars in the Galactic Disk.Comment: 11 pages, 5 figures; accepted for ApJ, 29 May 200
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