26,062 research outputs found
A note on log-convexity of q-Catalan numbers
The q-Catalan numbers studied by Carlitz and Riordan are polynomials in q
with nonnegative coefficients. They evaluate, at q=1, to the Catalan numbers:
1, 1, 2, 5, 14,..., a log-convex sequence. We use a combinatorial
interpretation of these polynomials to prove a q-log-convexity result. The
sequence of q-Catalan numbers is not q-log-convex in the narrow sense used by
other authors, so our work suggests a more flexible definition of q-log convex
be adopted
Domain wall switching: optimizing the energy landscape
It has recently been suggested that exchange spring media offer a way to
increase media density without causing thermal instability
(superparamagnetism), by using a hard and a soft layer coupled by exchange.
Victora has suggested a figure of merit xi = 2 E_b/mu_0 m_s H_sw, the ratio of
the energy barrier to that of a Stoner-Wohlfarth system with the same switching
field, which is 1 for a Stoner-Wohlfarth (coherently switching) particle and 2
for an optimal two-layer composite medium. A number of theoretical approaches
have been used for this problem (e.g., various numbers of coupled
Stoner-Wohlfarth layers and continuum micromagnetics). In this paper we show
that many of these approaches can be regarded as special cases or
approximations to a variational formulation of the problem, in which the energy
is minimized for fixed magnetization. The results can be easily visualized in
terms of a plot of the energy as a function of magnetic moment m_z, in which
both the switching field [the maximum slope of E(m_z)] and the stability
(determined by the energy barrier E_b) are geometrically visible. In this
formulation we can prove a rigorous limit on the figure of merit xi, which can
be no higher than 4. We also show that a quadratic anistropy suggested by Suess
et al comes very close to this limit.Comment: Acccepted for proceedings of Jan. 2007 MMM Meeting, paper BE-0
Collaboration : a key competence for competing in the 21st century
It is now an accepted fact that in the 21st century competition will be between networks of organisations and individuals, which efficiently and effectively integrate their competencies and resources in order to compete in a global economy (Bititci et al, 2004). Similarly the SME'2000 conference, which was held in Bologna, concluded that 'SMEs belonging to networks are often more competitive and innovative than those operating in isolation. When working together, SMEs can increase their focus through specialisation in functions that are complementary within their networks'
Buffet test in the National Transonic Facility
A buffet test of a commercial transport model was accomplished in the National Transonic Facility at the NASA Langley Research Center. This aeroelastic test was unprecedented for this wind tunnel and posed a high risk for the facility. Presented here are the test results from a structural dynamics and aeroelastic response point of view. The activities required for the safety analysis and risk assessment are described. The test was conducted in the same manner as a flutter test and employed on-board dynamic instrumentation, real time dynamic data monitoring, and automatic and manual tunnel interlock systems for protecting the model
Effects of 3-d and 4-d-transition metal substitutional impurities on the electronic properties of CrO2
We present first-principles based density functional theory calculations of
the electronic and magnetic structure of CrO2 with 3d (Ti through Cu) and 4d
(Zr through Ag) substitutional impurities. We find that the half-metallicity of
CrO2 remains intact for all of the calculated substitutions. We also observe
two periodic trends as a function of the number of valence electrons: if the
substituted atom has six or fewer valence electrons (Ti-Cr or Zr-Mo), the
number of down spin electrons associated with the impurity ion is zero,
resulting in ferromagnetic (FM) alignment of the impurity magnetic moment with
the magnetization of the CrO2 host. For substituent atoms with eight to ten
(Fe-Ni or Ru-Pd with the exception of Ni), the number of down spin electrons
contributed by the impurity ion remains fixed at three as the number
contributed to the majority increases from one to three resulting in
antiferromagnetic (AFM) alignment between impurity moment and host
magnetization. The origin of this variation is the grouping of the impurity
states into 3 states with approximate "t2g" symmetry and 2 states with
approximate "eg" symmetry. Ni is an exception to the rule because a
Jahn-Teller-like distortion causes a splitting of the Ni eg states. For Mn and
Tc, which have 8 valence electrons, the zero down spin and 3 down spin
configurations are very close in energy. For Cu and Ag atoms, which have 11
valence electrons, the energy is minimized when the substituent ion contributes
5 Abstract down-spin electrons. We find that the interatomic exchange
interactions are reduced for all substitutions except for the case of Fe for
which a modest enhancement is calculated for interactions along certain
crystallographic directions.Comment: 26 pages, 10 figures, 2 table
Splitting The Gluon?
In the strongly correlated environment of high-temperature cuprate
superconductors, the spin and charge degrees of freedom of an electron seem to
separate from each other. A similar phenomenon may be present in the strong
coupling phase of Yang-Mills theories, where a separation between the color
charge and the spin of a gluon could play a role in a mass gap formation. Here
we study the phase structure of a decomposed SU(2) Yang-Mills theory in a mean
field approximation, by inspecting quantum fluctuations in the condensate which
is formed by the color charge component of the gluon field. Our results suggest
that the decomposed theory has an involved phase structure. In particular,
there appears to be a phase which is quite reminiscent of the superconducting
phase in cuprates. We also find evidence that this phase is separated from the
asymptotically free theory by an intermediate pseudogap phase.Comment: Improved discussion of magnetic nature of phases; removed
unsubstantiated speculation about color confinemen
Down-regulation of Fas gene expression in colon cancer is not a result of allelic loss or gene rearrangement.
Expression of Fas, an apoptosis-inducing receptor, in colonic epithelium is progressively reduced during malignant transformation. We have examined the human Fas gene for loss of heterozygosity (LOH) and gross rearrangements in colon tumours and matched normal mucosa. Polymerase chain reaction (PCR) primers were designed to span a DraI restriction fragment length polymorphic site in the gene. Heterozygosity was detected in normal DNA samples by PCR amplification of the polymorphic site and restriction enzyme digestion. Thirty-eight of 88 patients (43%) with colon carcinomas were informative for the assay, and LOH was detected in 6 of the 38 (16%) corresponding tumours. Tumours from three patients with LOH did not express detectable Fas mRNA, and Fas expression was reduced or absent in 7 of 11 tumours from informative patients without LOH. Southern blotting of tumour DNA samples was used to detect rearrangement of the Fas gene, but no altered hybridization patterns were observed in 64 tumours analysed. These findings indicate that disruption of the Fas gene is not primarily responsible for the loss of Fas protein expression reported in colon cancer. We have also shown that loss of Fas gene transcription is common in these tumours, which may be due to epigenetic gene silencing
Planetary Bistatic Radar
Planetary radar observations offer the potential for probing the properties
of characteristics of solid bodies throughout the inner solar system and at
least as far as the orbit of Saturn. In addition to the direct scientific
value, precise orbital determinations can be obtained from planetary radar
observations, which are in turn valuable for mission planning or spacecraft
navigation and planetary defense. The next-generation Very Large Array would
not have to be equipped with a transmitter to be an important asset in the
world's planetary radar infrastructure. Bistatic radar, in which one antenna
transmits (e.g., Arecibo or Goldstone) and another receives, are used commonly
today, with the Green Bank Telescope (GBT) serving as a receiver. The improved
sensitivity of the ngVLA relative to the GBT would improve the signal-to-noise
ratios on many targets and increase the accessible volume specifically for
asteroids. Goldstone-ngVLA bistatic observations would have the potential of
rivaling the sensitivity of Arecibo, but with much wider sky access.Comment: 11 pages, 2 figures, To be published in the ASP Monograph Series,
"Science with a Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco,
CA
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