21,333 research outputs found
A fast version of the k-means classification algorithm for astronomical applications
Context. K-means is a clustering algorithm that has been used to classify
large datasets in astronomical databases. It is an unsupervised method, able to
cope very different types of problems. Aims. We check whether a variant of the
algorithm called single-pass k-means can be used as a fast alternative to the
traditional k-means. Methods. The execution time of the two algorithms are
compared when classifying subsets drawn from the SDSS-DR7 catalog of galaxy
spectra. Results. Single-pass k-means turn out to be between 20 % and 40 %
faster than k-means and provide statistically equivalent classifications. This
conclusion can be scaled up to other larger databases because the execution
time of both algorithms increases linearly with the number of objects.
Conclusions. Single-pass k-means can be safely used as a fast alternative to
k-means
The symmetries of the Dirac--Pauli equation in two and three dimensions
We calculate all symmetries of the Dirac-Pauli equation in two-dimensional
and three-dimensional Euclidean space. Further, we use our results for an
investigation of the issue of zero mode degeneracy. We construct explicitly a
class of multiple zero modes with their gauge potentials.Comment: 22 pages, Latex. Final version as published in JMP. Contains an
additional subsection (4.2) with the explicit construction of multiple zero
mode
A mechanism for unipolar resistance switching in oxide non-volatile memory devices
Building on a recently introduced model for non-volatile resistive switching,
we propose a mechanism for unipolar resistance switching in
metal-insulator-metal sandwich structures. The commutation from the high to low
resistance state and back can be achieved with successive voltage sweeps of the
same polarity. Electronic correlation effects at the metal-insulator interface
are found to play a key role to produce a resistive commutation effect in
qualitative agreement with recent experimental reports on binary transition
metal oxide based sandwich structures.Comment: 4 pages, 2 figure
Evidence of ongoing radial migration in NGC 6754: Azimuthal variations of the gas properties
Understanding the nature of spiral structure in disk galaxies is one of the
main, and still unsolved questions in galactic astronomy. However, theoretical
works are proposing new testable predictions whose detection is becoming
feasible with recent development in instrumentation. In particular, streaming
motions along spiral arms are expected to induce azimuthal variations in the
chemical composition of a galaxy at a given galactic radius. In this letter we
analyse the gas content in NGC 6754 with VLT/MUSE data to characterise its 2D
chemical composition and H line-of-sight velocity distribution. We find
that the trailing (leading) edge of the NGC 6754 spiral arms show signatures of
tangentially-slower, radially-outward (tangentially-faster, radially-inward)
streaming motions of metal-rich (poor) gas over a large range of radii. These
results show direct evidence of gas radial migration for the first time. We
compare our results with the gas behaviour in a -body disk simulation
showing spiral morphological features rotating with a similar speed as the gas
at every radius, in good agreement with the observed trend. This indicates that
the spiral arm features in NGC 6754 may be transient and rotate similarly as
the gas does at a large range of radii.Comment: 8 pages, 4 figures, accepted for publication in ApJL 2016 September
2
A standardised method for measuring in situ denitrification in shallow aquifers: numerical validation and measurements in riparian wetlands
A tracer test to examine in situ denitrification in shallow groundwater by a piezometer with a packer system used bromide as a tracer of dilution and acetylene (10%) to block the denitrification process at the nitrous oxide stage. During the test, dissolved oxygen, nitrate (NO3-), bromide (Br-), nitrous oxide (N2O) and dissolved organic carbon (DOC) were measured. To calibrate the experimental method, comparison with numerical simulations of the groundwater transfer were carried out, taking into account the environmental characteristics. The method was tested by measurements undertaken in different environmental conditions (geology, land use and hydrology) in two riparian wetlands. Denitrification rates measured by this method ranged from 5.7 10-6 g N-NO3-L-1 h-1 to 1.97 10-3 g N-NO3-L-1 h-1 The method is applicable in shallow aquifers with a permeability from 10-2 to 10-4m s-1
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