Propagator moment expansion

The higher moments in the moment expansion of the propagator or the propagator matrix can become quite comphcated and approximations are necessary. The simplest approximation that yields useful results proceeds by approximating higher moments as powers of the first moment F = (X HX). Denote S = (X X) and obtain the approximation... 

We have used moment expansions in terms of nested commutators in the determination of propagators, and these concepts are useful also in this context. The notation... 

Making use of this superoperator formalism, the moment expansion of the polarization propagator can be written as... 

The second procedure, several aspects of which are reviewed in this paper, consists of directly computing the asymptotic value by employing newly-developed polymeric techniques which take advantage of the one-dimensional periodicity of these systems. Since the polarizability is either the linear response of the dipole moment to the field or the negative of the second-order term in the perturbation expansion of the energy as a power series in the field, several schemes can be proposed for its evaluation. Section 3 points out that several of these schemes are inconsistent with band theory summarized in Section 2. In Section 4, we present the main points of the polymeric polarization propagator approaches we have developed, and in Section 5, we describe some of their characteristics in applications to prototype systems. 

Consider a planar premixed flame front, such as that sketched in Figure 5.1.1. For the moment, we will be interested only in long length scales and we will treat the flame as an infinitely thin interface that transforms cold reactive gas, at temperature and density T p, into hot burnt gas at temperature and density T, A.-The flame front propagates at speed Sl into the xmbumt gas. We place ourselves in the reference frame of the front, so cold gas enters the front at speed = Su and because of thermal expansion, the hot gases leave the front at velocity 14 = Sl(Po/a)- The density ratio, Po/Pb, is roughly equal to the... 

We have seen above that calculation of the corrections of order a"(Za) m (n > 1) reduces to calculation of higher order corrections to the properties of a free electron and to the photon propagator, namely to calculation of the slope of the electron Dirac form factor and anomalous magnetic moment, and to calculation of the leading term in the low-frequency expansion of the polarization operator. Hence, these contributions to the Lamb shift are independent of any features of the bound state. A nontrivial interplay between radiative corrections and binding effects arises first in calculation of contributions of order a Za) m, and in calculations of higher order terms in the combined expansion over a and Za. 

Using this transformation, it has been shown in Refs. [54,72] that the effective-mode Hamiltonian Heg by itself reproduces the short-time dynamics of the overall system exactly. This is reflected by an expansion of the propagator, for which it can be shown that the first few terms of the expansion - relating to the first three moments of the overall Hamiltonian - are exactly reproduced by the reduced-dimensional Hamiltonian Heg. 

Successive orders H(n ) can be shown to correspond to successive orders in a moment (or cumulant) expansion of the propagator, which takes one to increasing times. Truncation of the chain at a given order n (i.e., 3 + 3n modes) leads to an approximate, lower-dimensional representation of the dynamical process, which reproduces the true dynamics up to a certain time. In Ref. [51], we have demonstrated explicitly that the nth-order (3n+3 mode) truncated HEP Hamiltonian exactly reproduces the first (2n + 3)rd order moments (cumulants) of the total Hamiltonian. A related analysis is given in Ref. [73],... 

For more complex models or for input distributions for which exact analytical methods are not applicable, approximate methods might be appropriate. Many approximation methods are based on Taylor series expansion solutions, in which the series is truncated depending on the desired amount of solution accuracy and whether one wishes to consider covariance among the input distributions (Hahn Shapiro, 1967). These methods often go by names such as generation of system moments , statistical error propagation , delta method and first-order methods , as discussed by Cullen Frey (1999). 

When the perturbing potential V is an external electric field and the property of interest A is the dipole moment, the coefficients, indicated in the expansion above by the double angle brackets, are referred to as polarization propagators. These functions are equivalent to the tensor components... 

While the 0 -theory discussed in section 3.3 does not provide such averages it is essential that these can be performed in the framework of the MH model. With the effective Hamiltonian derived in section 2.4 it turns out that the moments correspond to the propagators of this theory with masses rk that reflect the fact that there is a distinct critical point associated to each moment, i.e. there is no multicritical point as in the spin models with finite numbers of components and as suggested by the d > 4 interpretation of the (j> polymer theory [39] in sect. 3.3. In extracting the scaling behavior of the moments gw or equivalently of the masses r the central quantities will be the terms linear in k in an expansion in A as suggested by Eq. (115). 

By applying the moment tensor analysis, kinematics of cracks can be analyzed (Ouyang, Landis et al. 1992). In the expansion test, which simulates crack propagation due to corrosion of reinforcing steel-bar, the moment tensor analysis was performed to identify cracking mechanisms. Here, crack modes of micro-cracks are classified into a tensile crack, shear crack and the mix-mode as illustrated in Fig. 10.27. 

Let us now consider the expansion process more carefully. From the point of impact of the detonation wave on the inner surface of the ring-shaped wall section, a shock wave propagates through the cylinder wall and reaches the outer cylinder surface only at a later point in time. Not earlier than that moment ( o), the cylinder wall starts to move as a whole at a free surface velocity. Accordingly, it is... 

Comparing this with the classical expansion of a time-dependent dipole moment in Eq. (7.18) we can identify the frequency-dependent polarizability tensor as a linear response function or polarization propagator... 

The first-order second moment method (FOSM) is the method adopted within the framework to propagate input parameter uncertainty through numerical models (26, 27). FOSM provides two moments, mean and variance of predicted variables. This method is based on Taylor series expansion, of which second-order and higher terms are truncated. The expected value of concentration, E[u] and its covariance, COV[u] are (25, 27),... 

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