A manifestly gauge-invariant formulation of quantum electrodynamics is constructed in which the basic dynamical variables are physically observable quantities. The theory is relativistically covariant, because the structure of the Poincaré group is built into it from the beginning.

The non-linear adiabatic response of some simple quantum systems to weak but finite driving forces is investigated. In particular, the effect of the electric field on the motion of a charged particle in a weak periodic potential, in a narrow one-dimensional energy band, and in a two-level...

The effective Hamiltonian for the degenerate energy-eigenvalue problem in adiabatic perturbation theory is cast in a form that permits an expansion in Feynman diagrams. By means of a block representation a resummation of these diagrams is carried out such that in the adiabatic limit no...

We consider noise in particle fluxes which undergo a reduction or a multiplication process, either direct or via branching. It is shown that these processes can be described with the method of addition of independent events, or with a collective procedure. Using the statistical rules for...

It is shown that the diffusion coefficient of clusters of hard rods in one dimension is exactly independent of the number of rods comprising the cluster. An expression for the autocorrelation function of the velocity of the cluster centre of mass is obtained from a simple stochastic model, and...

The generalized hydrodynamic structure of the charge-charge response function has been derived for two-component ionic conductors. The effects of resistive processes and of couplings to mass and heat fluctuations have been singled out. The limiting case of a symmetric molten salt and the long...

We present a macroscopic derivation of the Debije-Brinkman equation for the flow of a fluid through a polymer coil. By using an exact relation between the sedimentation coefficient and the permeability it is found that the permeability of the coil is strongly dependent on the nature of the...

A rigorous closed set of differential equations is derived for the correlations between x- and y-components of spins for the inhomogeneous XY-model with spin 12 in the high-temperature limit. The correlations can also be derived from a Lagrangian which describes the motion in a classical lattice...

Nonlinear equations governing the relaxation of some macroscopic quantities, a(r,t) are derived using Kubo's response theory, in isolated inhomogeneous systems. The resulting equations contain microcanonical time correlation functions. It is shown how to express these in terms of arbitrary...

The inadequacy of Sonine polynomials for a kinetic theory is discussed.