arXiv subjects
Publications and source records attributed to Lev Sakhnovich.
In this note, we formulate the notions of the direct and inverse problems and contact points for the Feynman diagrams. For the electron-photon interaction case, the solutions of these direct and inverse problems are presented. The interrelations between Feynman diagrams and the classical colored graphs are discussed.
First, we consider generalized wave and scattering operators and derive modifications of commutation relations (between scattering operators and unperturbed operators) when the corresponding deviation factors behave as $\exp\{i t {\mathcal C}_{\pm}\}$ for $t\to \pm \infty$. Then, we construct so called secondary generalized scattering operators for the related case of linear divergence in QED, which gives a positive answer (in that case) to the well-known problem of J. R. Oppenheimer regarding scattering operators in QED: "Can the procedure be freed of the expansion in $\varepsilon$ and carried out rigorously?"
In the present article, we assume that the first approximation of the scattering operator is given and that it has the logarithmic divergence. This first approximation allows us to construct the so called deviation factor. Using the deviation factor, we regularize all terms of the scattering operator's approximations. The infrared and ultraviolet cases as well as concrete examples are considered. Thus, for a wide range of cases, we provide a positive answer to the well-known problem of J. R. Oppenheimer regarding scattering operators in QED: ``Can the procedure be freed of the expansion in $\varepsilon$ and carried out rigorously?"
Ordinary differential equations of the first order on the torus have been investigated in detail by H. Poincaré and A. Denjoy. The long-standing problem of generalising these results for the equations of the order $k>1$ (or for the systems of equations) is both important and difficult, requiring an essentially new approach. (See, e.g., an interesting old paper by P. Bohl.) In this paper, we propose a new (non-Hamiltonian) and promising approach. We use Hamiltonians, that is, ordinary differential systems of equations of the first order, only for heuristics. In the main scheme and corresponding proofs we do not use these systems. Instead of differential systems, we study sets of continuous vector functions $ϕ(t,η)$ satisfying important conditions, which follow from the analogy with the solutions in the case $k=1$. Some of our results are new even in the case $k=1.$
The oscillations described by periodic functions play an important role in many areas. In the present paper, we study periodic functions which belong to the class of the $n$-member chains. The self-intersection and local singular points of these periodic functions are constructed. We consider several classical curves in two and three dimensions. We also introduce and study two new classes of periodic functions: the class of periodic helices and the class of S-torus knots. In the last section, we construct folds for the $2$-member chains. For these purposes, we derive and use several results on trigonometric formulas.
We present some basic inequalities between the classical and quantum values of free energy, entropy and mean energy. We investigate the transition from the deterministic case (classical mechanics) to the probabilistic case (quantum mechanics). In the first part of the paper, we assume that the reduced Planck constant $\hbar$, the absolute temperature $T$, the frequency of an oscillator $ω$, and the degree of freedom of a system $N$ are fixed. This approach to the problem of comparing quantum and classical mechanics is new (see [35]--[37]). In the second part of the paper, we simultaneously derive the semiclassical limits for four cases, that is, for $\hbar{\to}0$, $T{\to}\infty$, $ω{\to}0$, and $N{\to}\infty$. We note that only the case $\hbar{\to}0$ is usually considered in quantum mechanics (see [21]). The cases $T{\to}\infty$ and $ω{\to}0$ in quantum mechanics were initially studied by M. Planck and by A. Einstein, respectively.
In the present paper, we study the limit sets of the almost periodic functions $f(x)$. It is interesting that the values $r=\inf|f(x)|$ and $R=\sup|f(x)|$ may be expressed in the exact form. We show that the ring $r\leq |z|\leq R$ is the limit set of the almost periodic function $f(x)$ (under some natural conditions on $f$). The exact expression for $r$ coincides with the well known partition problem formula and gives a new analytical method of solving the corresponding partition problem. Several interesting examples are considered. For instance, in the case of the five numbers, the well-known Karmarkar--Karp algorithm gives the value $m=2$ as the solution of the partition problem in our example, and our method gives the correct answer $m=0.$ The figures presented in Appendix illustrate our results.
In the present paper, we consider the integral operator, which acts in Hilbert space and has sine kernel. This operator generates two operator identities and two corresponding canonical differential systems. We find the asymptotics of the corresponding resolvent and Hamiltonians. We use both the method of operator identities and the theory of random matrices.
The area related to M. Livšic's characteristic matrix functions is too vast to be discussed in one paper and we selected for this article the problems which are close to our scientific interests. We discuss M.Livšic's results connected with characteristic matrix functions and various important developments including factorization of the transfer operator function, inverse problems, triangular models, reduction of the operators to the simplest form as well as applications to Wiener--Masani prediction theory, to random matrices, and to Riemann--Hilbert problems.
In this paper, we consider two types of the scattering problems (relativistic case), namely, the stationary scattering problem, where the distance $r$ tends to infinity, and the dynamical scattering problem, where the time $t$ tends to infinity. Using our results on Lippmann-Schwinger equation in the relativistic case, we found the connection between the stationary scattering problem (the scattering amplitude) and the dynamical scattering problem (the scattering operator). This result is the quantum mechanical analog of the ergodic formulas in the classical mechanics.
Stationary scattering problem (when the distance $r$ tends to infinity) and dynamical scattering problem (when the time $t$ tends to infinity) are considered for the 3D Schrödinger equation. A simple interconnection between the scattering amplitude (stationary case) and scattering operator (dynamical case) is given in the paper. This result is a quantum mechanical analog of the ergodic formulas in the classical mechanics.
The classical Lippmann-Schwinger equation (LS equation) plays an important role in the scattering theory for the non-relativistic case (Schrödinger equation). In our previous paper arXiv:1801.05370, we consider the relativistic analogue of the Lippmann-Schwinger equation (RLS equation). We represent the corresponding equation in the integral form. In the present paper, we use the corresponding integral equation and investigate the scattering problems for both stationary and dynamical cases. Our approach allows us to develop an RLS equation theory which is comparable in its completeness with the theory of the LS equation. In particular, we consider the eigenfunction expansion associated with the relativistic Dirac equation. We note that the works on the theory of the LS equation serve as a model for us.
We consider the divergences in quantum electrodynamics. Our approach is based on ideas from the theory of generalized wave operators. In particular, we use the concept of the deviation factor. The deviation factor characterizes the deviations of the initial and final waves from the free waves. The approach is demonstrated on important examples.
The classical Lippmann-Schwinger equation plays an important role in the scattering theory (non-relativistic case, Schrödinger equation). In the present paper we consider the relativistic analogue of the Lippmann-Schwinger equation. We represent the corresponding equation in the integral form. Using this integral equation we investigate the stationary scattering problems (relativistic case, Dirac equation). We consider the dynamical scattering problems (relativistic case, Dirac equation) as well.
We assume that the unperturbed operators $A_0$ are known. Then, the fact that the scattering operators $S$ and the unperturbed operators $A_0$ are pairwise permutable provides some important information about the structure of the scattering operators. Using this information and the ideas from the theory of generalized wave operators, we present a new approach to the divergence problems in quantum electrodynamics. We show that the so called infrared divergences appeared because the deviations of the initial and final waves from the free waves were not taken into account.
We characterize the set of rectangular Weyl matrix functions corresponding to Dirac systems with locally square-integrable potentials on a semi-axis and demonstrate a new way to recover the locally square-integrable potential from the Weyl function. Important interconnections between our approach and accelerants of convolution operators are discussed as well.
Ideas and results of the generalized wave operator theory for dynamical and stationary cases are developed further and exact expressions for generalized scattering operators are obtained for wide classes of differential equations. New results on the structure of the generalized scattering operators are derived. Interesting interrelations between dynamical and stationary cases are found for radial Schrödinger and Dirac equations, and for Dirac-type equations as well. For some important examples we explain why the well-known divergences in the higher order approximations of the scattering matrices do not appear in the "generalized wave operator" approach.
We have constructed a concrete example of a non-factorable positive operator. As a result, for the well-known problems (Ringrose, Kadison and Singer problems) we replace existence theorems by concrete examples.