Variational weak-coupling perturbation theory yields converging approximations, uniformly in the coupling strength. This allows us to calculate directly the coefficients of strong-coupling expansions. For the anharmonic oscillator we explain the physical origin of the empirically observed

The necessity of the asymptotic expansion in infinite dimensions arises from the pioneering work of the Chern-Simons theory by Witten [10] in 1989. However, the exponentia13rd term of the Chern-Simons theory is less likely to be handled by techniques known at present, so that we will challenge a

Abstract not found

Abstract. An estimate on the operator norm of an abstract fermionic renormalization group map is derived. This abstract estimate is applied in another paper to construct the thermodynamic Green’s functions of a two dimensional, weakly coupled fermion gas with an asymmetric Fermi curve. The estimate derived here is strong enough to control everything but the sum of all quartic contributions to the Green’s functions.

function moments

Abstract not found

The formulation of perturbation expansions for a quantum field theory of strongly interacting systems in a general non-equilibrium state is discussed. Non-vanishing initial correlations are included in the formulation of the perturbation expansion in terms of cumulants. The cumulants are shown

We consider a new perturbation scheme in nonabelian gauge theory. Pure Yang-Mills theory in three dimensions is taken as a concrete example. The zeroth-order in the perturbative expansion is given by BF theory coupled to a Sttickelberg-like field. The effective coupling for the expansion can

A complete perturbative expansion for the Hamiltonian describing the motion of a quantomechanical system constrained to move on an The quantomechanical description of constrained systems is extremely important in physics and since the early days of quantum mechanics several techniques have been

. In this paper, the analysis of the largest principal angle between these subspaces is performed in terms of the perturbation expansions of the corresponding orthogonal projectors. Applicable upper bounds are derived. The main attention is paid to the asymptotic case when the length of the time series tends