Essential points of conformal vector fields

An essential point of a conformal vector field ξ$\xi$ on a conformal manifold (M,c)$(M,c)$ is a point around which the local flow of ξ$\xi$ preserves no metric in the conformal class c$c$. It is well-known that a conformal vector field vanishes at each essential point. In this note we show that essential points are isolated. This is a generalization to higher dimensions of the fact that the zeros of a holomorphic function are isolated. As an application, we show that every connected component of the zero set of a conformal vector field is totally umbilical.     

One-harmonic invariant vector fields on three-dimensional Lie groups

We determine all left-invariant vector fields on three-dimensional Lie groups which define harmonic sections of the corresponding tangent bundles, equipped with the complete lift metric.     

Exploring invariant sets and invariant measures

We propose a method to explore invariant measures of dynamical systems. The method is based on numerical tools which directly compute invariant sets using a subdivision technique, and invariant measures by a discretization of the Frobenius-Perron operator. Appropriate visualization tools help to analyze the numerical results and to understand important aspects of the underlying dynamics. This will be illustrated for examples provided by the Lorenz system. (c) 1997 American Institute of Physics.     

Conformal invariant two and three-point functions for fields with arbitrary spin

The conformal invariant two and three-point functions for any “fundamental” fields with an arbitrary spin and scale dimensions are found in the Minkowsky x-space. The two-point functions for Dirac, symmetric and antisymmetric tensor fields are given. The three-point functions for two Dirac fields and one symmetrical tensor field, as well as any other field for which this function is nonvanishing, are given. In the case of conserved currents the Ward identities are considered.     

Vacuum type N space-times admitting homothetic vector fields with isolated fixed points

We consider vacuum space-times (M, g) which are of Petrov type N on an open dense subset ofM, and which admit (proper) homothetic vector fields with isolated fixed points. We prove that if such is the case then, at the fixed point, (M,g) is flat and the homothetic bivector,X _[a;b] , is necessarily simple-timelike. Furthermore, we prove that if the homothetic bivector remains simple-timelike in some neighbourhood of the fixed point then, around the fixed point, the space-time in question is a pp-wave. The paper ends with a local characterization and some examples of space-tunes satisfying these conditions.     

Translationally invariant states and the spectrum ideal in the algebra of test functions for quantum fields

A class of states on Borchers' tensor algebra is constructed. These states are invariant under the translation group and fulfill the spectrum condition. This leads to a characterization of the linear span of all such states in terms of a simple continuity property.     

Dimensions of invariant sets of expanding maps

We consider a compact invariant set of an expanding map of a manifoldM and give upper and lowerbounds for the Hausdorff Dimension dim _H (), and box dimensionsdim _B () and dim _B (). These bounds are given in terms of the topological entropy, topological pressure, and uniform Lyapunov exponents of the map.A measure-theoretic version of these results is also included.Part of this work was done when I was in the Department of Mathematics, University of Arizona.     

Gauge invariant fractional electromagnetic fields

Fractional derivatives and integrations of non-integers orders was introduced more than three centuries ago but only recently gained more attention due to its application on nonlocal phenomenas. In this context, several formulations of fractional electromagnetic fields was proposed, but all these theories suffer from the absence of an effective fractional vector calculus, and in general are non-causal or spatially asymmetric. In order to deal with these difficulties, we propose a spatially symmetric and causal gauge invariant fractional electromagnetic field from a Lagrangian formulation. From our fractional Maxwell?s fields arose a definition for the fractional gradient, divergent and curl operators.     

Shape invariant propagation of polychromatic fields

When a polychromatic field propagates the transverse distribution of the power spectrum generally changes from one plane to another. Conditions are found under which the spectrum distribution maintains the same shape in any cross-section. Examples of fields satisfying these conditions are given.     

Functional representations of conformally invariant free fields

Free fields in two-dimensional space-time are considered from the point of view of representation theory. Unitary representations of the conformal group are constructed in the representation space of equal-time commutation relations. The compatibility of the global conformal transformations with locality is discussed.     

Complex-dimensional invariant delta functions and lightcone singularities

Invariant delta functions (including imaginary-mass case) defined in a complexn-dimensional space-time are explicitly calculated in position space. It is proposed to define products of invariant delta functions in the ordinary Minkowski space by analytically continuing the correspondingn-dimensional ones ton=4. The (not only leading but also non-leading) lightcone singularities of [(x; m ²)]², (x; m ²)⁽¹⁾(x; m ²), and [⁽¹⁾(x; m ²)]² are shown to be unambiguously determined in this way.     

Identifying almost invariant sets in stochastic dynamical systems

We consider the approximation of fluctuation induced almost invariant sets arising from stochastic dynamical systems. The dynamical evolution of densities is derived from the stochastic Frobenius-Perron operator. Given a stochastic kernel with a known distribution, approximate almost invariant sets are found by translating the problem into an eigenvalue problem derived from reversible Markov processes. Analytic and computational examples of the methods are used to illustrate the technique, and are shown to reveal the probability transport between almost invariant sets in nonlinear stochastic systems. Both small and large noise cases are considered.     

Localization of compact invariant sets of the Lorenz system

The problem of finding domains in the state space of a nonlinear system which contain all compact invariant sets is considered. Such domains are computed for the Lorenz system by using different localizing functions.     

Local relativistic invariant flows for quantum fields

For quantum fields with trigonometric interaction in arbitrary space dimension we construct a representation of the Lorentz group by automorphisms on a Banach space generated by the Weyl algebra.     

From points to gauge fields

We show that several classes of free field theories with local gauge invariance (e.g. the Yang-Mills, Einstein and p-forms linearized actions) can be constructed from classical actions for a finite number of points by applying the BRST quantization initiated by Siegel. We briefly outline the generalization of this construction for strings.     

Set-based corral control in stochastic dynamical systems: making almost invariant sets more invariant

We consider the problem of stochastic prediction and control in a time-dependent stochastic environment, such as the ocean, where escape from an almost invariant region occurs due to random fluctuations. We determine high-probability control-actuation sets by computing regions of uncertainty, almost invariant sets, and Lagrangian coherent structures. The combination of geometric and probabilistic methods allows us to design regions of control, which provide an increase in loitering time while minimizing the amount of control actuation. We show how the loitering time in almost invariant sets scales exponentially with respect to the control actuation, causing an exponential increase in loitering times with only small changes in actuation force. The result is that the control actuation makes almost invariant sets more invariant.