83,981 research outputs found
The effect of static incubation on the yolk sac vasculature of the Japanese quail (Coturnix c. japonica)
Static incubation affects early embryonic development with, notably, a reduction area vasculosa expansion and diminished sub-embryonic fluid (SEF) volume, effects produced during a ‘critical’ period (3-7 days in the chick) (Baggott et al., 2002). Also, as noted by Babiker & Baggott (1992), SEF is produced in bulk only after the appearance of the yolk sac vasculature (YSV), which undergoes extensive proliferation before and during the critical period. Quantification of such changes in YSV requires estimates of both the quantity of vessels and the degree of branching. In the chick, total vessel length increased linearly up to 160h of incubation, whereas branching was maximal by about 96 h (Vico et al., 1998); so, by the critical period branching is complete yet vessel growth continues. It would seem likely, therefore, that a lack of turning would reduce both measures of YSV proliferation during the critical period. In quail the effect of static incubation seems not to be simply due to retardation of YSV proliferation, as vascular density index was reduced in unturned eggs in the middle of the critical period, only to increase again by 168 h. Also early in the critical period fractal dimension was 1.70 (as in the chick, Vico et al., 1998), yet then decreased in unturned eggs, although not significantly, and subsequently an increase occurred. Thus during the critical period static incubation specifically affects the structuring of the YSV but whether this is because of, or independent of, retardation of area vasculosa expansion is not known
Finite-Blocklength Channel Coding Rate Under a Long-Term Power Constraint
This paper investigates the maximal channel coding rate achievable at a given blocklength and error probability , when the codewords are subject to a long-term (i.e., averaged-over-all-codeword) power constraint. The second-order term in the large-n expansion of the maximal channel coding rate is characterized both for AWGN channels and for quasi-static fading channels with perfect channel state information at the transmitter and the receiver. It is shown that in both cases the second-order term is proportional to
Bosonization, Pairing, and Superconductivity of the Fermionic Tonks-Girardeau Gas
We determine some exact static and time-dependent properties of the fermionic
Tonks-Girardeau (FTG) gas, a spin-aligned one-dimensional Fermi gas with
infinitely strongly attractive zero-range odd-wave interactions. We show that
the two-particle reduced density matrix exhibits maximal off-diagonal
long-range order, and on a ring an FTG gas with an even number of atoms has a
highly degenerate ground state with quantization of Coriolis rotational flux
and high sensitivity to rotation and to external fields and accelerations. For
a gas initially under harmonic confinement we show that during an expansion the
momentum distribution undergoes a "dynamical bosonization", approaching that of
an ideal Bose gas without violating the Pauli exclusion principle.Comment: v3: 4 pages, 2 figures, revtex4. Section on the fermionic TG gas on a
ring revised, emphasizing degeneracy of ground state for even N and resultant
high sensitivity to external fields. Submitted to PR
Geometry of a Centrosymmetric Electric Charge
The gravitational description given for an electric on the basis of exact
solution of the Einstein-Maxwell equations eliminates Coulomb divergence. The
internal pulsating semiconfined world formed by neutral dust is smoothly joined
with parallel Reissner-Nordstrem vacuum worlds via two static bottlenecks. The
charge, rest mass, and electric field are expressed in terms of the space
curvatures. The internal and external parameters of the maximon, electron, and
the universe form a power series.Comment: 12 pages, 2 figures, 1 tabl
The economic basis of periodic enzyme dynamics
Periodic enzyme activities can improve the metabolic performance of cells. As
an adaptation to periodic environments or by driving metabolic cycles that can
shift fluxes and rearrange metabolic processes in time to increase their
efficiency. To study what benefits can ensue from rhythmic gene expression or
posttranslational modification of enzymes, I propose a theory of optimal enzyme
rhythms in periodic or static environments. The theory is based on kinetic
metabolic models with predefined metabolic objectives, scores the effects of
harmonic enzyme oscillations, and determines amplitudes and phase shifts that
maximise cell fitness. In an expansion around optimal steady states, the
optimal enzyme profiles can be computed by solving a quadratic optimality
problem. The formulae show how enzymes can increase their efficiency by
oscillating in phase with their substrates and how cells can benefit from
adapting to external rhythms and from spontaneous, intrinsic enzyme rhythms.
Both types of behaviour may occur different parameter regions of the same
model. Optimal enzyme profiles are not passively adapted to existing substrate
rhythms, but shape them actively to create opportunities for further fitness
advantage: in doing so, they reflect the dynamic effects that enzymes can exert
in the network. The proposed theory combines the dynamics and economics of
metabolic systems and shows how optimal enzyme profiles are shaped by network
structure, dynamics, external rhythms, and metabolic objectives. It covers
static enzyme adaptation as a special case, reveals the conditions for
beneficial metabolic cycles, and predicts optimally combinations of gene
expression and posttranslational modification for creating enzyme rhythms
The Geometry of Small Causal Diamonds
The geometry of causal diamonds or Alexandrov open sets whose initial and
final events and respectively have a proper-time separation
small compared with the curvature scale is a universal. The corrections from
flat space are given as a power series in whose coefficients involve the
curvature at the centre of the diamond. We give formulae for the total 4-volume
of the diamond, the area of the intersection the future light cone of
with the past light cone of and the 3-volume of the hyper-surface of
largest 3-volume bounded by this intersection valid to .
The formula for the 4-volume agrees with a previous result of Myrheim.
Remarkably, the iso-perimetric ratio depends only on the energy density at the centre and is bigger
than unity if the energy density is positive. These results are also shown to
hold in all spacetime dimensions. Formulae are also given, valid to next
non-trivial order, for causal domains in two spacetime dimensions. We suggest a
number of applications, for instance, the directional dependence of the volume
allows one to regard the volumes of causal diamonds as an observable providing
a measurement of the Ricci tensor.Comment: 17 pages, no figures; Misprints in eqs.(62), (65), (66) and (81)
corrected; a new note on page 13 (with 2 new equations) adde
The Relevant Scale Parameter in the High Temperature Phase of QCD
We introduce the running coupling constant of QCD in the high temperature
phase, , through a renormalization scheme where the dimensional
reduction is optimal at the one-loop level. We then calculate the relevant
scale parameter, , which characterizes the running of
with , using the background field method in the static
sector. It is found that . We further verify that the coupling
is also optimal for lattice perturbative calculations. Our
result naturally explains why the high temperature limit of QCD sets in at
temperatures as low as a few times the critical temperature. In addition, our
agrees remarkably well with the scale parameter determined from the
lattice measurement of the spatial string tension of the SU(2) gauge theory at
high .Comment: 13 pages, RevTeX 3.0, no figures. Full postscript version available
via anonymous ftp (192.84.132.4) at
ftp://risc0.ca.infn.it/pub/private/lissia/infnca-th-94-24.p
Generalized Weyl solutions in d=5 Einstein-Gauss-Bonnet theory: the static black ring
We argue that the Weyl coordinates and the rod-structure employed to
construct static axisymmetric solutions in higher dimensional Einstein gravity
can be generalized to the Einstein-Gauss-Bonnet theory. As a concrete
application of the general formalism, we present numerical evidence for the
existence of static black ring solutions in Einstein-Gauss-Bonnet theory in
five spacetime dimensions. They approach asymptotically the Minkowski
background and are supported against collapse by a conical singularity in the
form of a disk. An interesting feature of these solutions is that the
Gauss-Bonnet term reduces the conical excess of the static black rings.
Analogous to the Einstein-Gauss-Bonnet black strings, for a given mass the
static black rings exist up to a maximal value of the Gauss-Bonnet coupling
constant . Moreover, in the limit of large ring radius, the suitably
rescaled black ring maximal value of and the black string maximal
value of agree.Comment: 43 pages, 14 figure
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