2,603 research outputs found
On NP-Hardness of the Paired de Bruijn Sound Cycle Problem
The paired de Bruijn graph is an extension of de Bruijn graph incorporating
mate pair information for genome assembly proposed by Mevdedev et al. However,
unlike in an ordinary de Bruijn graph, not every path or cycle in a paired de
Bruijn graph will spell a string, because there is an additional soundness
constraint on the path. In this paper we show that the problem of checking if
there is a sound cycle in a paired de Bruijn graph is NP-hard in general case.
We also explore some of its special cases, as well as a modified version where
the cycle must also pass through every edge.Comment: Peer-reviewed and presented as part of the 13th Workshop on
Algorithms in Bioinformatics (WABI2013
Diffusion of Elements in the Interstellar Medium in Early-Type Galaxies
We consider the role of diffusion in the redistribution of elements in the
hot interstellar medium (ISM) of early-type galaxies. It is well known that
gravitational sedimentation can affect significantly the abundances of helium
and heavy elements in the intracluster gas of massive galaxy clusters. The
self-similarity of the temperature profiles and tight mass--temperature
relation of relaxed cool-core clusters suggest that the maximum effect of
sedimentation take place in the most massive virialized objects in the
Universe. However, Chandra and XMM-Newton observations demonstrate more complex
scaling relations between the masses of early-type galaxies and other
parameters, such as the ISM temperature and gas mass fraction. An important
fact is that early-type galaxies can show both decreasing and increasing radial
temperature profiles. We have calculated the diffusion based on the observed
gas density and temperature distributions for 13 early-type galaxies that
belonging to the different environments and cover a wide range of X-ray
luminosities. To estimate the maximum effect of sedimentation and thermal
diffusion, we have solved the full set of Burgers' equations for a
non-magnetized ISM plasma. The results obtained demonstrate a considerable
increase of the He/H ratio within one effective radius for all galaxies of our
sample. For galaxies with a flat or declining radial temperature profile the
average increase of the helium abundance is 60\% in one billion years of
diffusion. The revealed effect can introduce a significant bias in the metal
abundance measurements based on X-ray spectroscopy and can affect the evolution
of stars that could be formed from a gas with a high helium abundance.Comment: 15 pages, 9 figures, 3 tables, published in Astronomy Letters, 2017,
Volume 43, Issue 5, pp.285-30
Impact of thermal diffusion and other abundance anomalies on cosmological uses of galaxy clusters
Depending on the topology of the magnetic field and characteristics of
turbulent motions, diffusion can significantly affect the distribution of
elements, in particular helium, in the intracluster medium (ICM). As has been
noted previously, an incorrect assumption about the helium abundance will lead
to an error in the iron abundance determined from X-ray spectroscopy. The
corresponding effect on the temperature measurement is negligibly small. An
incorrectly assumed helium abundance will also lead to a systematic error in
angular distance measurements based on X-ray and Sunyaev-Zeldovich (SZ)
observations of clusters of galaxies. Its magnitude is further amplified by the
associated error in the metal abundance determination, the impact being larger
at lower ICM temperatures. Overall, a factor of 2-5 error in the helium
abundance will lead to an ~ 10-25 % error in the angular distance.
We solve the full set of Burgers equations for a multi-component intracluster
plasma to determine the maximal effect of diffusion on the interpretation of
X-ray and microwave observations of clusters of galaxies. For an isothermal
cluster, gravitational sedimentation can lead to up to a factor of ~ 5-10
enhancements of helium and metal abundances in the cluster center on a ~ 3-7
Gyr timescale. In cool-core clusters on the contrary, thermal diffusion can
counteract gravitational sedimentation and effectively remove helium and metals
from the cluster inner core. In either case, a significant, up to ~ 40 %, error
in the metal abundances determined by means of X-ray spectroscopy is possible.
The angular distance determined from X-ray and SZ data can be underestimated by
up to ~ 10-25 %.Comment: 11 pages, 10 figure
-point amplitudes for d=2 c=1 Discrete States from String Field Theory
Starting from string field theory for 2d gravity coupled to c=1 matter we
analyze N-point off-shell tree amplitudes of discrete states. The amplitudes
exhibit the pole structure and we use the oscillator representation to extract
the residues. The residues are generated by a simple effective action. We show
that the effective action can be directly deduced from a string field action in
a special transversal-like gauge.Comment: 12 pages, latex, 1 figur
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