共查询到20条相似文献,搜索用时 31 毫秒
1.
Raychaudhuri's equation for the Brans-Dicke theory is used to relate /G to the deceleration and the density parameters. 相似文献
2.
The bifurcations of periodic orbits in a class of autonomous three-variable, nonlinear-differential-equation systems possessing a homoclinic orbit associated with a saddle focus with eigenvalues ( ±i , ), where ¦/¦ < 1 (Sil'nikov's condition), are studied in a two-parameter space. The perturbed homoclinic systems undergo a countable set of tangent bifurcations followed by period-doubling bifurcations leading to periodic orbits which may be attractors if ¦ /¦ < 1/2. The accumulation rate of the critical parameter values at the homoclinic system is exp(- 2¦/¦). A global mechanism for the onset of homoclinicity in strongly contractive flows is analyzed. Cusp bifurcations with bistability and hysteresis phenomena exist locally near the onset of homoclinicity. A countable set of these cusp bifurcations with scaling properties related to the eigenvalues ±i of the stationary state are shown to occur in infinitely contractive flows. In the two-parameter space, the periodic orbit attractor domain exhibits a spiral structure globally, around the set of homoclinic systems, in which all the different periodic orbits are continuously connected. 相似文献
3.
We consider translation-invariant attractive spin systems. Let T
,x
v
be the first time that the average spin inside the hypercube reaches the value x when the process is started from an invariant measure with density smaller than x. We obtain sufficient conditions for (1) ¦¦ –1 log T
,x
v
( x) in distribution as ¦¦ , and ¦¦ –1 log T
,x
v
( x) as ¦¦ , where ( x):= –lim
¦ ¦ –1 log {(average spin inside ) x. And (2) T
,x
v
/ ET
,x
v
converges to a unit mean exponential random variable as ¦¦ . Both (1) and (2) are proven under some type of rapid convergence to equilibrium. (1) is also proven without extra conditions for Ising models with ferromagnetic pair interactions evolving according to an attractive reversible dynamics; in this case is a thermodynamic function. We discuss also the case of finite systems with boundary conditions and what can be said about the state of the system at the time T
,x
v
.On leave from São Paulo University. 相似文献
4.
We study the integrated density of states H(
2) of a chain of harmonic oscillators with a binary random distribution of the masses. We show in particular that there is a dense set of values of the squared frequency for which the difference H(
2+)- H(
2) has a singularity of the type ¦ ¦ 2, multiplied by a periodic function of ln ¦ ¦, where the exponent and the period depend continuously on
2. In the region where < 1/2, H is not differentiate on a dense set of points. The same type of singularities is also present in the Lyapunov coefficient. 相似文献
5.
We consider an anharmonic crystal described by variables S
x
, x
d
, S
x
, with one-body interaction ¦ S
x
¦
and nearest neighbor (n.n.) two body interaction ¦ S
x
–S
y
¦ . We prove that, for
d
bounded, , where
is the correlation function for the free boundary condition Gibbs state in , >0 and are suitable constants independent of and . This generalizes previous results obtained in the case .Research partially supported by Consiglio Nazionale delle Ricerche. 相似文献
6.
Let G
a
be the free lattice field measure of mass m
0 on aZ
d
, and:
x
4
: be the corresponding fourth Wick power of the lattice field
x
. Let gC
0
(R
d
), g0, be a given function and a=a(a)a satisfy: lim
a0+a=0 and aZ
d
aZ
d
. We prove that if d3, or d=2 and lim
a0+
a|log a| 2=, then
satisfies the central limit theorem: there is V(a, a) with lim
a0+
V(a, a)= such that the distribution of
under G
a
is convergent to the standard normal distribution, as a0 +. 相似文献
7.
In this paper we present the results of a search for the charmed strange baryon
c
+
in the final states 0++K – and –++. The experiment was performed using the magnetic spectrometer BIS-2 with a hydrogen target located in the neutron beam of the Serpukhov accelerator. A narrow peak in the 0++K – state is observed at a mass of 2440 and possibly also of 2310 MeV/c 2, corresponding to signals for the
c
+
0++K – and
c
+
0++K – ( 0 0) decays respectively. The statistics obtained for the –++ state is too low to make any conclusion.We are grateful to K. Hiller, F. Mandl, M. Markytan and J.MacNaughton for useful discussions and valuable remarks. 相似文献
8.
Every convex subset of a locally convex Hausdorff space ( X, ) is equipped with the ( -algebra generated by its -relatively open subsets. Within the set () of probability measures on two particular convex subsets are considered: (a) the set
s
() of probability measures with a separable support, and (b) the set
c
() of probability measures with a compact convex support. If is the base of a cone in X, then there exists an affine barycenter map from
c
() onto whose composition with the natural embedding of in
c
() yields the identity map on , and every -continuous affine transformation of can be represented by an affine transformation of
c
() that is induced by a Markov kernel. If ( X, ) is a Banach space and is a closed, bounded, generating cone base in X that is contained in a hyperplane, then analogous results are obtained with respect to
s
(). Since the state spaces considered in noncommutative measure theory are cone bases and every change in time of an empirical system can be thought of as an affine transformation of the associated state space (Schrödinger picture), the existence of these representation theorems implies that the time evolution of general empirical systems can be described by dynamical concepts borrowed from classical probability theory. 相似文献
10.
Nuclear magnetic resonance on oriented nuclei has been detected for the first time via the destruction of the anisotropy of characteristic L x-rays. The new method can be applied to isomeric states which decay only via highly converted transitions, for which the standard NMR-ON technique — detection of NMR via the anisotropy of -rays — is not applicable. The X-NMR-ON technique has been used to measure the magnetic hyperfine splitting of 193mpt (I =1322 +; E=149.8 keV; T 1/2= 4.3 d) to be ¦ g
NB HF/h¦=111.3 (3) MHz. with the known hyperfine field of –1280(27) kG the magnetic moment of 193mpt is deduced to be ¦ ¦=0.7417(14) N. This magnetic moment differs strongly from the known magnetic moments of the 13/2 + isomeric states in Hg and Pb and 195mPt. 相似文献
11.
Starting from Schrödinger equations with SU(2) group-theoretic potentials, we consider a general family of kinks labeled by two (half-)integers ( l, n) with ¦ n¦ l. A particular choice of n=0, l= L ( L positive integer) leads to a general L-family, where L=1 corresponds to sine-Gordon theory, while L=2 represents the (
4) 1+1 model. The (
6) 1+1 model can also be recovered with l=3/2, n=–1/2, a particular case of theories labeled by l and n such that l-n=2 which possess simple kink solutions. We also discuss one-loop order corrections to the kink masses in supersymmetric versions of the L-family. As a byproduct, we obtain the SUSY renormalization of the so-called parameter in sine-Gordon theory. 相似文献
12.
We study the asymptotic behavior of the charge fluctuations ( Q
– ( Q
) 2 in infinite classical systems of charged particles, and show, under certain clustering assumptions, that if the charge fluctuations are not extensive, then they are necessarily of the order of the surface ¦¦. Moreover, when the canonical sum rules that are typical for equilibrium states of particles interacting with long-range forces hold true, we prove a central limit theorem for the normalized charge variable ¦¦ –1/2(( Q
– ( Q
)) in two and three dimensions. In one dimension, the probability distribution of the charge itself converges. The latter case is illustrated by the example of the one-dimensional Coulomb gas. 相似文献
13.
A recent analysis by Richard Price of spherical collapse with small nonspherical perturbations is here generalized to the case of an electrically charged collapsing star (0¦ Q¦- M). The perturbations are confined to a scalar field generated by a nonspherical distribution of scalar charge in the star. As in the electrically neutral case, the scalar perturbations are probably a prototype for all others — electromagnetic, gravitational, and higherspin. The collapse is shown to produce a Reissner-Nordström black hole, and the scalar-field perturbations are shown to radiate completely away; but they die out more slowly the larger is the star's electric charge. For charge ¦ Q¦ M, the -pole part of the perturbation at fixed r and late times is dominated by a tail that dies out as t
–(2+ 2). But for ¦ Q¦= M, the primary outgoing waves emitted from the star's surface are everywhere larger than the tail. At fixed r and late times they die as t –(+2). Also, a calculation of the redshift shows that a collapsing star becomes black more slowly the larger is the star's electric charge.Supported in part by the National Science Foundation (GP-27304, GP-28027, GP-19887). 相似文献
14.
The stationary states of electrovac fields for which the geodesic eigenrays of both the Maxwell and gravitational field coincide are investigated. The fact that the Kerr-Newman solutions belong to this class lends physical interest to the fields considered here. Particular attention is devoted to fields with shearing eigenrays since principal null congruences do not coincide with eigenrays in this case, and so earlier approaches to the problem fail. By the generalisation of a theorem on the corresponding vacuum case, it is proven that no spherical solutions exist in the shearing class, with the exception of the fields with 2¦G O¦ 2-¦H O¦ 2=O. The metrics with O, admitted by the theorem, can either be calculated from the corresponding vacuum solutions by a relatively simple procedure, or, if not, we list them explicitly in this paper. 相似文献
15.
We consider percolation on the sites of a graph G, e.g., a regular d-dimensional lattice. All sites of G are occupied (vacant) with probability p (respectively, q=1–p), independently of each other. W denotes the cluster of occupied sites containing a fixed site (which will usually be taken to be the origin) and W the cardinality of W. The percolation probability is the probability that # W=, i.e., (p)=P
p{# W=}. Some critical values of p,p
H and p
T, are defined, respectively, as the smallest value of p for which (p)> 0, and for which the expectation of # W is infinite. Formally, p
H=inf { p(p)>0} and p
T=inf{ p E
p{# W}=}. We show for fairly general graphs Gthat if p
T, thenP
P{#W n} decreases exponentially inn. For the special casesG =G
0= the simple quadratic lattice andG
1= the graph which corresponds to bond-percolation on 2, we obtain upper and lower bounds for(p) of the formC¦p¦-P
H¦, and bounds forEp{#W} of the formC¦p–p
H¦–. We also investigate smoothness properties of (p)=E
p{number of clusters per site} =E
p {(#W)–1; (#W) 1}. This function was introduced by Sykes and Essam, who assumed that (·) has exactly one singularity, namely, atp=p
H. For the graphsG
0 andG
1, (i.e., site or bond percolation on 2) we show that (p) is analytic atp p
H and has two continuous derivatives atp=p
H. The emphasis is on rigorous proofs.Research supported by the NSF through a grant to Cornell University. 相似文献
16.
The typical fluctuation of the net electric charge Q contained in a subregion of an infinitely extended equilibrium Coulomb system is expected to grow only as S, where S is the surface area of . For some cases it has been previously shown that Q/S has a Gaussian distribution as ¦ ¦. Here we study the probability law for larger charge fluctuations (large-deviation problem). We discuss the case when both ¦ ¦ and Q are large, but now with Q of an order larger than S. For a given value of Q, the dominant microscopic configurations are assumed to be those associated with the formation of a double electrical layer along the surface of . The probability law for Q is then determined by the free energy of the double electrical layer. In the case of a one-component plasma, this free energy can be computed, for large enough Q, by macroscopic electrostatics. There are solvable two-dimensional models for which exact microscopic calculations can be done, providing more complete results in these cases. A variety of behaviors of the probability law are exhibited. 相似文献
17.
The magnetic moments of the proton drip-line nuclei 13O( I
= 3/2 –, T
1/2 = 8.6 ms) and 9C( I
= 3/2 –, T
1/2 = 126 ms) have been determined for the first time through the combined techniques of polarized radioactive nuclear beams and -NMR detection. The observed magnetic moments are ¦ ( 13O)¦ = 1.3891 ±0.0003
N and ¦ ( 9C)¦ = 1.3914 ±0.0005
N. Spin expectation values are deduced to be 0.76 and 1.44 for 13O and 9C, respectively. While the of 13O is consistent with the systematics from isospin T= 1/2 mirror pairs, the of 9C is unusually large, even far larger than the single particle value, = 1. 相似文献
18.
Closed form expressions for all partial waves of the pure Coulomb off-shell T-matrix p¦ T c,l.( k
2) ¦ p are obtained. All singularities appear through simple special functions, which makes it possible to study the analytic properties of p¦ T c,l(k 2) ¦p as a function not only of one of the momenta p, p but even of both of them. With use of the renormalization procedure found by Zorbas the transition to half-and on-shell values is performed reproducing known expressions. By the same method simple expressions for the partial waves of the Coulomb wave function in the momentum representation are found. 相似文献
19.
We consider the length of an occupied crossing of a box of size [0, n]×[0, 3 n]
D–1 (in the short direction) in standard (Bernoulli) bond percolation on
D
at criticality. Let ¦ s
n¦ be the length of the shortest such crossing. It is believed that ¦ s
n¦ 1+c in some sense for some c>0. Here we show that if the correlation length ( p) satisfies ( p)( p
c}– p)
– for some <1, then with a probability tending to 1, ¦ s
n¦>/ C
1
n
1/(log n) –(1–)/. The assumption ( p) C
3( p
c– p) – with <1 has been rigorously established (1,2) for large D, but cannot hold (3) for D=2. In the latter case, let ¦ l
n¦ be the length of the lowest occupied crossing of the square [0, n] 2. We outline a proof of P
pc(¦l n¦ n
1+c) n
– for some c, >0. We also obtain a result about the length of optimal paths in first-passage percolation. 相似文献
20.
We show with simple arguments that, as a consequence of the Poisson equation, the correlations of a charged system at equilibrium decay faster than any inverse power, if they are integrable and monotonous at infinity. For all other longrange systems (with potential ( x) b¦ x¦ –s
, ¦x¦ , 0 v,s} 2), the decay is bounded below by an inverse power.Partially supported by the Swiss National Foundation for Scientific Research. 相似文献
|