Approximations of Strongly Singular Evolution Equations

The problem of specification of self-adjoint operators corresponding to singular bilinear forms is very important for applications, such as quantum field theory and theory of partial differential equations with coefficient functions being distributions. In particular, the formal expression $-\Delta + g\delta({\bold x})$ corresponds to a non-trivial self-adjoint operator $\hat{H}$ in the space $L^2({\Bbb R}^d)$ only if $d\le 3$. For spaces of larger dimensions (this corresponds to the strongly singular case), the construction of $\hat{H}$ is much more complicated: first one should consider the space $L^2({\Bbb R}^d)$ as a subspace of a wider Pontriagin space, then one implicitly specifies $\hat{H}$. It is shown in this paper that Schrodinger, parabolic and hyperbolic equations containing the operator $\hat{H}$ can be approximated by explicitly defined systems of evolution equations of a larger order. The strong convergence of evolution operators taking the initial condition of the Cauchy problem to the solution of the Cauchy problem is proved.Comment: 30 pages, no figures, AMSTe

Semiclassical Mechanics of Constrained Systems

Semiclassical mechanics of systems with first-class constraints is developed. Starting from the quantum theory, one investigates such objects as semiclassical states and observables, semiclassical inner product, semiclassical gauge transformations and evolution. Quantum mechanical semiclassical substitutions (not only the WKB-ansatz) can be viewed as "composed semiclassical states" being infinite superpositions of wave packets with minimal uncertainties of coordinates and momenta ("elementary semiclassical states"). Each elementary semiclassical state is specified by a set (X,f) of classical variables X (phase, coordinates, momenta) and quantum function f ("shape of the wave packet" or "quantum state in the background X"). A notion of an elemantary semiclassical state can be generalized to the constrained systems, provided that one uses the refined algebraic quantization approach based on modifying the inner product rather than on imposing the constrained conditions on physical states. The inner product of physical states is evaluated. It is obtained that classical part of X the semiclassical state should belong to the constrained surface; otherwise, the semiclassical state (X,f) will have zero norm for all f. Even under classical constraint conditions, the semiclassical inner product is degenerate. One should factorize then the space of semiclassical states. Semiclassical gauge transformations and evolution of semiclassical states are studied. The correspondence with semiclassical Dirac approach is discussed.Comment: 46 pages, LaTeX, no figure

Group Transformations of Semiclassical Gauge Systems

Semiclassical systems being symmetric under Lie group are studied. A state of a semiclassical system may be viewed as a set (X,f) of a classical state X and a quantum state f in the external classical background X. Therefore, the set of all semiclassical states may be considered as a bundle ("semiclassical bundle"). Its base {X} is the set of all classical states, while a fibre is a Hilbert space of quantum states in the external background X. Symmetry transformation of a semiclassical system may be viewed as an automorphism of the semiclassical bundle. Automorphism groups can be investigated with the help of sections of the bundle: to any automorphism of the bundle one assigns a transformation of section of the bundle. Infinitesimal properties of transformations of sections are investigated; correspondence between Lie groups and Lie algebras is discussed. For gauge theories, some points of the semiclassical bundle are identified: a gauge group acts on the bundle. For this case, only gauge-invariant sections of the semiclassical bundle are taken into account.Comment: 18 pages, LaTe

An Axiomatic Approach to Semiclassical Field Perturbation Theory

Semiclassical perturbation theory is investigated within the framework of axiomatic field theory. Axioms of perturbation semiclassical theory are formulated. Their correspondence with LSZ approach and Schwinger source theory is studied. Semiclassical S-matrix, as well as examples of decay processes, are considered in this framework.Comment: 32 pages, LaTeX, margins are corrected due to problems with viewing the PostScript fil

An Axiomatic Approach to Semiclassical Perturbative Gauge Field Theories

Different approaches to axionatic field theory are investigated. The main notions of semiclassical theory are the following: semiclassical states, Poincare transformations, semiclassical action form, semiclassical gauge equivalence and semiclassical field. If the manifestly covariant approach is used, the notion of semiclassical state is related to Schwinger sourse, while the semicalssical action is presented via the R-function of Lehmann, Symanzik and Zimmermann. Semiclassical perturbation theory is constructed. Its relation with the S-matrix theory is investigated. Semiclassical electrodynamics and non-Abelian gauge theories are studied, making us of the Gupta-Bleuler and BRST approaches.Comment: 54 page

Refined Algebraic Quantization of Constrained Systems with Structure Functions

The method of refined algebraic quantization of constrained systems which is based on modification of the inner product of the theory rather than on imposing constraints on the physical states is generalized to the case of constrained systems with structure functions and open gauge algebras. A new prescription for inner product for the open-algebra systems is suggested. It is illustrated on a simple example. The correspondence between refined algebraic and BRST-BFV quantizations is investigated for the case of nontrivial structure functions.Comment: 10 pages, LaTe

States and Observables in Semiclassical Field Theory: a Manifestly Covariant Approach

A manifestly covariant formulation of quantum field Maslov complex-WKB theory (semiclassical field theory) is investigated for the case of scalar field. The main object of the theory is "semiclassical bundle". Its base is the set of all classical states, fibers are Hilbert spaces of quantum states in the external field. Semiclassical Maslov states may be viewed as points or surfaces on the semiclassical bundle. Semiclassical analogs of QFT axioms are formulated. A relationship between covariant semiclassical field theory and Hamiltonian formulation is discussed. The constructions of axiomatic field theory (Schwinger sources, Bogoliubov $S$-matrix, Lehmann-Symanzik-Zimmermann $R$-functions) are used in constructing the covariant semiclassical theory. A new covariant formulation of classical field theory and semiclassical quantization proposal are discussed.Comment: 20 pages, LaTeX, margins are corrected due to problems with viewing PostScript fil

Large-N Theory from the Axiomatic Point of View

The state space and observables for the leading order of the large-N theory are constructed. The obtained model ("theory of infinite number of fields") is shown to obey Wightman-type axioms (including invariance under boost transformations) and to be nontrivial (there are scattering processes, bound states, unstable particles etc). The considered class of exactly solvable relativistic quantum models involves good examples of theories containing such difficulties as volume divergences associated with the Haag theorem, Stueckelberg divergences and infinite renormalization of the wave function.Comment: 46 pages, LaTe

Exactly Solvable Quantum Mechanical Models with Infinite Renormalization of the Wave Function

The main difficulty of quantum field theory is the problem of divergences and renormalization. However, realistic models of quantum field theory are renormalized within the perturbative framework only. It is important to investigate renormalization beyond perturbation theory. However, known models of constructive field theory do not contain such difficulties as infinite renormalization of the wave function. In this paper an exactly solvable quantum mechanical model with such a difficulty is constructed. This model is a simplified analog of the large-N approximation to the $\Phi\phi^a\phi^a$-model in 6-dimensional space-time. It is necessary to introduce an indefinite inner product to renormalize the theory. The mathematical results of the theory of Pontriagin spaces are essentially used. It is remarkable that not only the field but also the canonically conjugated momentum become well-defined operators after adding counterterms.Comment: 13 pages, LaTe

BRST-BFV, Dirac and Projection Operator Quantizations: Correspondence of States

The correspondence between BRST-BFV, Dirac and projection operator approaches to quantize constrained systems is analyzed. It is shown that the component of the BFV wave function with maximal number of ghosts and antighosts in the Schrodinger representation may be viewed as a wave function in the projection operator approach. It is shown by using the relationship between different quantization techniques that the Marnelius inner product for BRST-BFV systems should be in general modified in order to take into account the topology of the group; the Giulini-Marolf group averaging prescription for the inner product is obtained from the BRST-BFV method. The relationship between observables in different approaches is also found.Comment: LaTeX, 7 pages; some remarks and references are adde