In this work, the linear and gauge-invariant theory of cosmological perturbations in a class of anisotropic and shear-free spacetimes is developed. After constructing an explicit set of complete eigenfunctions in terms of ...

We show that it is possible to extract useful information from the straightforward perturbation theory in classical mechanics. Although the secular terms make the perturbation series useless for large times, these expansions ...

We include the Roper excitation of the nucleon in a version of heavy-baryon chiral perturbation theory recently developed for energies around the delta resonance. We find significant improvement in the P(11) channel. (C) ...

We include the Roper excitation of the nucleon in a version of heavy-baryon chiral perturbation theory recently developed for energies around the delta resonance. We find significant improvement in the P(11) channel. (C) ...

Dressed perturbation theory (DPT) is a perturbative approach based on the idea of using dressed propagators, instead of free propagators like in ordinary perturbation theory. Quark and gluon self-energies are added to the ...

The local density of states (LDOS) is a distribution that characterizes the effects of perturbations on quantum systems. Recently, a semiclassical theory was proposed for the LDOS of chaotic billiards and maps. This theory ...

The basic field equations of a field theory are not always derivable from a Lagrangian. Lagrangian theories are perturbatively coherent, in the sense that they have well-defined vertices. Non-Lagrangian theories may be ...

This paper is concerned with a geometric study of singularly perturbed systems of ordinary differential equations expressed by (n-1)-parameter families of smooth vector fields on ℝl, where n ≥ 2. The inherent characteristic ...