4,338 research outputs found
A combinatorial approach to the set-theoretic solutions of the Yang-Baxter equation
A bijective map , where
is a finite set, is called a \emph{set-theoretic solution of the Yang-Baxter
equation} (YBE) if the braid relation
holds in A non-degenerate involutive solution satisfying
, for all , is called \emph{square-free solution}. There
exist close relations between the square-free set-theoretic solutions of YBE,
the semigroups of I-type, the semigroups of skew polynomial type, and the
Bieberbach groups, as it was first shown in a joint paper with Michel Van den
Bergh.
In this paper we continue the study of square-free solutions and the
associated Yang-Baxter algebraic structures -- the semigroup , the
group and the - algebra over a field , generated by
and with quadratic defining relations naturally arising and uniquely
determined by . We study the properties of the associated Yang-Baxter
structures and prove a conjecture of the present author that the three notions:
a square-free solution of (set-theoretic) YBE, a semigroup of I type, and a
semigroup of skew-polynomial type, are equivalent. This implies that the
Yang-Baxter algebra is Poincar\'{e}-Birkhoff-Witt type algebra,
with respect to some appropriate ordering of . We conjecture that every
square-free solution of YBE is retractable, in the sense of Etingof-Schedler.Comment: 34 page
Time dependent correlations in marine stratocumulus cloud base height records
The scaling ranges of time correlations in the cloud base height records of
marine boundary layer stratocumulus are studied applying the Detrended
Fluctuation Analysis statistical method. We have found that time dependent
variations in the evolution of the exponent reflect the diurnal
dynamics of cloud base height fluctuations in the marine boundary layer. In
general, a more stable structure of the boundary layer corresponds to a lower
value of the - indicator, i.e. larger anti-persistence, thus a set of
fluctuations tending to induce a greater stability of the stratocumulus. In
contrast, during periods of higher instability in the marine boundary, less
anti-persistent (more persistent like) behavior of the system drags it out of
equilibrium, corresponding to larger values. From an analysis of the
frequency spectrum, the stratocumulus base height evolution is found to be a
non-stationary process with stationary increments. The occurrence of these
statistics in cloud base height fluctuations suggests the usefulness of similar
studies for the radiation transfer dynamics modeling.Comment: 12 pages, 6 figures; to appear in Int. J. Mod. Phys. C, Vol. 13, No.
2 (2002
Cold air outbreaks and their signature in the ozonometric data at the mountain station near Kislovodsk, Russia
International audienceTwo cases of cold air outbreak in November 2001 are analyzed on the basis of ozone concentration measurements and weather data at the mountain station (43°44' N, 42°43 E, 2070 m a.s.l.) near Kislovodsk, North Caucasus, Russia. Two cold fronts, with fast clearing up in the rear of the cloud zone, passed the station in the morning of 12 and 16 November. In both cases, the ozone concentration drops down to 7?8 ppb within the frontal cloud zones, under 100% relative humidity, RH, and then sharply increases to 43?45 ppb in dry (RH about 50%), cold air during several hours. After a warm front passage (12 November) or RH growth in the cold post-frontal airmass (16 November), the ozone concentration decreases again to its average values near 30 ppb. Neither diurnal cycle nor photochemical generation could produce in the ozonometric data this specific "drop-and-peak'' signature which is considered to be caused by cold outbreak with post-frontal flow of "dry air stream'' type. Synoptic situation and tropopause topography are analyzed using the objective analysis data and METEOSAT images in the water vapor radiation band. It is concluded that the ozone growth in the rear of the cold fronts is associated with sinking of substratospheric or stratospherically influenced air from the areas of tropopause folding. Motion of cold surges with their frontal zones, jet streams, and tropopause folds is analyzed, including development of wave disturbances at the streamers and formation of substratospheric air "tongues'' underflowing the high tropopause and representing "aged'' intrusions
Uncertainties in Arctic sea ice thickness and volume: new estimates and implications for trends
Sea ice volume has decreased
in the last decades, evoked by changes in sea ice area and thickness.
Estimates of sea ice area and thickness rely on a number of geophysical
parameters which introduce large uncertainties. To quantify these
uncertainties we use freeboard retrievals from ICESat and investigate
different assumptions about snow depth, sea ice density and area. We find
that uncertainties in ice area are of minor importance for the estimates of
sea ice volume during the cold season in the Arctic basin. The choice of mean
ice density used when converting sea ice freeboard into thickness mainly
influences the resulting mean sea ice thickness, while snow depth on top of
the ice is the main driver for the year-to-year variability, particularly in
late winter. The absolute uncertainty in the mean sea ice thickness is
0.28 m in February/March and 0.21 m in October/November. The
uncertainty in snow depth contributes up to 70% of the total uncertainty
and the ice density 30–35%, with higher values in October/November. We
find large uncertainties in the total sea ice volume and trend. The mean
total sea ice volume is 10 120 ± 1280 km<sup>3</sup> in October/November
and 13 250 ± 1860 km<sup>3</sup> in February/March for the time period
2005–2007. Based on these uncertainties we obtain trends in sea ice volume
of −1450 ± 530 km<sup>3</sup> a<sup>−1</sup> in October/November and −880 ±
260 km<sup>3</sup> a<sup>−1</sup> in February/March over the ICESat period
(2003–2008). Our results indicate that, taking into account the
uncertainties, the decline in sea ice volume in the Arctic between the ICESat
(2003–2008) and CryoSat-2 (2010–2012) periods may have been less dramatic
than reported in previous studies. However, more work and validation is
required to quantify these changes and analyse possible unresolved biases in
the freeboard retrievals
Supernova Ia: a Converging Delayed Detonation Wave
A model of a carbon-oxygen (C--O) presupernova core with an initial mass 1.33
M_\odot, an initial carbon mass fraction 0.27, and with an average mass
growth-rate 5 x 10^{-7} M_\odot/yr due to accretion in a binary system was
evolved from initial central density 10^9 g/cm^3, and temperature 2.05 x 10^8 K
through convective core formation and its subsequent expansion to the carbon
runaway at the center. The only thermonuclear reaction contained in the
equations of evolution and runaway was the carbon burning reaction 12C + 12C
with an energy release corresponding to the full transition of carbon and
oxygen (with the same rate as carbon) into 56Ni. As a parameter we take
\alpha_c - a ratio of a mixing length to the size of the convective zone. In
spite of the crude assumptions, we obtained a pattern of the runaway acceptable
for the supernova theory with the strong dependence of its duration on
\alpha_c. In the variants with large enough values of \alpha_c=4.0 x 10^{-3}
and 3.0 x 10^{-3} the fuel combustion occurred from the very beginning as a
prompt detonation. In the range of 2.0 x 10^{-3} >= \alpha_c >= 3.0 x 10^{-4}
the burning started as a deflagration with excitation of stellar pulsations
with growing amplitude. Eventually, the detonation set in, which was activated
near the surface layers of the presupernova (with m about 1.33 M_\odot) and
penetrated into the star down to the deflagration front. Excitation of model
pulsations and formation of a detonation front are described in detail for the
variant with \alpha_c=1.0 x 10^{-3}.Comment: 13 pages, 11 figures, to appear in Astronomy Letter
The molecular aspects of personalized anticancer treatment
Only 25% of cancer patients, on average, benefit from therapy. Even in the cases of complete clinical response the tumor progression is an event of high level expectation. The main reasons for tumor progression are: intratumor heterogeneity resulted from clonal evolution, drug resistance, and tumor-promoting microenvironment. The reprogramming of microenvironmental stromal-inflammatory components is expected to allow tumor phenotype reversion. So, to find the new effective markers of tumor progression, drug response and targets for therapy, it could be promising to take into account the tumor-microenvironment heterogeneity and tumor clonal evolution
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