200 research outputs found
On periodic solutions of 2-periodic Lyness difference equations
We study the existence of periodic solutions of the non--autonomous periodic
Lyness' recurrence u_{n+2}=(a_n+u_{n+1})/u_n, where {a_n} is a cycle with
positive values a,b and with positive initial conditions. It is known that for
a=b=1 all the sequences generated by this recurrence are 5-periodic. We prove
that for each pair (a,b) different from (1,1) there are infinitely many initial
conditions giving rise to periodic sequences, and that the family of
recurrences have almost all the even periods. If a is not equal to b, then any
odd period, except 1, appears.Comment: 27 pages; 1 figur
Self-expandable metal stents in the treatment of acute esophageal variceal bleeding
Acute variceal bleeding (AVB) is a life-threatening complication in patients with cirrhosis. Hemostatic therapy of AVB includes early administration of vasoactive drugs that should be combined with endoscopic therapy, preferably banding ligation. However, failure to control bleeding or early rebleed within 5 days still occurs in 15-20% of patients with AVB. In these cases, a second endoscopic therapy may be attempted (mild bleeding in a hemodynamically stable patient) or we can use a balloon tamponade as a bridge to definitive derivative treatment (i.e., a transjugular intrahepatic portosystemic shunt). Esophageal balloon tamponade provides initial control in up to 80% of AVB, but it carries a high risk of major complications, especially in cases of long duration of tamponade (>24 h) and when tubes are inserted by inexperienced staff. Preliminary reports suggest that self-expandable covered esophageal metallic stents effectively control refractory AVB (i.e., ongoing bleeding despite pharmacological and endoscopic therapy or massive bleeding precluding endoscopic therapy) with a low incidence of complications. Thus, covered self-expanding metal stents may represent an alternative to the Sengstaken-Blakemore balloon for the temporary control of bleeding in treatment failures. Further studies are required to determine the role of this new device in AVB
Evaluation of mechanically activated kaolin as alkali-activated material precursor
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On the athermal character of structural phase transitions
The significance of thermal fluctuations on nucleation in structural
first-order phase transitions has been examined. The prototype case of
martensitic transitions has been experimentally investigated by means of
acoustic emission techniques. We propose a model based on the mean
first-passage time to account for the experimental observations. Our study
provides a unified framework to establish the conditions for isothermal and
athermal transitions to be observed.Comment: 5 pages, 4 figures, accepted in Phys. Rev. Let
Phase diagram of Fe-doped Ni-Mn-Ga ferromagnetic shape-memory alloys
We have studied the effect of Fe addition on the structural and magnetic
transitions in the magnetic shape memory alloy Ni-Mn-Ga by substituting
systematically each atomic species by Fe. Calorimetric and AC susceptibility
measurements have been carried out in order to study the magnetic and
structural transformation properties. We find that the addition of Fe modifies
the structural and magnetic transformation temperatures. Magnetic transition
temperatures are displaced to higher values when Fe is substituted into
Ni-Mn-Ga, while martensitic and premartensitic transformation temperatures
shift to lower values. Moreover, it has been found that the electron per atom
concentration essentially governs the phase stability in the quaternary system.
However, the observed scaling of transition temperatures with differs
from that reported in the related ternary system Ni-Mn-Ga.Comment: 8 pages, 8 figures. Accepted for publication in the Physical Review
Martensitic transition and magnetoresistance in a Cu-Al-Mn shape memory alloy. Influence of aging
We have studied the effect of ageing within the miscibility gap on the
electric, magnetic and thermodynamic properties of a non-stoichiometric Heusler
Cu-Al-Mn shape-memory alloy, which undergoes a martensitic transition from a
-based (-phase) towards a close-packed structure (-phase).
Negative magnetoresistance which shows an almost linear dependence on the
square of magnetization with different slopes in the - and -phases,
was observed. This magnetoresistive effect has been associated with the
existence of Mn-rich clusters with the CuAlMn-structure. The effect of an
applied magnetic field on the martensitic transition has also been studied. The
entropy change between the - and -phases shows negligible dependence
on the magnetic field but it decreases significantly with annealing time within
the miscibility gap. Such a decrease is due to the increasing amount of
CuMnAl-rich domains that do not transform martensitically.Comment: 9 pages, 9 figures, accepted for publication in PR
Global periodicity conditions for maps and recurrences via Normal Forms
We face the problem of characterizing the periodic cases in parametric
families of (real or complex) rational diffeomorphisms having a fixed point.
Our approach relies on the Normal Form Theory, to obtain necessary conditions
for the existence of a formal linearization of the map, and on the introduction
of a suitable rational parametrization of the parameters of the family. Using
these tools we can find a finite set of values p for which the map can be
p-periodic, reducing the problem of finding the parameters for which the
periodic cases appear to simple computations. We apply our results to several
two and three dimensional classes of polynomial or rational maps. In particular
we find the global periodic cases for several Lyness type recurrences.Comment: 25 page
Alkali activated binders based on municipal solid waste Incineration bottom ash
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Cooling and heating by adiabatic magnetization in the NiMnIn magnetic shape memory alloy
We report on measurements of the adiabatic temperature change in the inverse
magnetocaloric NiMnIn alloy. It is shown that this alloy
heats up with the application of a magnetic field around the Curie point due to
the conventional magnetocaloric effect. In contrast, the inverse magnetocaloric
effect associated with the martensitic transition results in the unusual
decrease of temperature by adiabatic magnetization. We also provide
magnetization and specific heat data which enable to compare the measured
temperature changes to the values indirectly computed from thermodynamic
relationships. Good agreement is obtained for the conventional effect at the
second-order paramagnetic-ferromagnetic phase transition. However, at the first
order structural transition the measured values at high fields are lower than
the computed ones. Irreversible thermodynamics arguments are given to show that
such a discrepancy is due to the irreversibility of the first-order martensitic
transition.Comment: 5 pages, 4 figures. Accepted for publication in the Physical Review
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