An integral operator induced by a Zygmund function

A function f:mathbb {R}rightarrow mathbb {R} is: almost continuous in the sense of Stallings, fin textrm{AC}, if each open set Gsubset mathbb {R}^2 containing the graph of f contains also the graph of a continuous function g:mathbb {R}rightarrow mathbb {R}; Sierpiński–Zygmund, fin textrm{SZ} (or, more generally, fin textrm{SZ}(textrm{Bor})), provided its restriction frestriction M is discontinuous (not Borel, respectively) for any Msubset mathbb {R} of cardinality continuum. It is known that an example of a Sierpiński–Zygmund almost continuous function f:mathbb {R}rightarrow mathbb {R} (i.e., an fin textrm{SZ}cap textrm{AC}) cannot be constructed in ZFC; however, an fin textrm{SZ}cap textrm{AC} exists under the additional set-theoretical assumption {{,textrm{cov},}}(mathcal {M})=mathfrak {c}, that is, that mathbb {R} cannot be covered by less than mathfrak {c}-many meager sets. The primary purpose of this paper is to show that the existence of an fin textrm{SZ}cap textrm{AC} is also consistent with ZFC plus the negation of {{,textrm{cov},}}(mathcal {M})=mathfrak {c}. More precisely, we show that it is consistent with ZFC+{{,textrm{cov},}}(mathcal {M})<mathfrak {c} (follows from the assumption that {{,textrm{non},}}(mathcal {N})<{{,textrm{cov},}}(mathcal {N})=mathfrak {c}) that there is an fin textrm{SZ}(textrm{Bor})cap textrm{AC} and that such a map may have even stronger properties expressed in the language of Darboux-like functions. We also examine, assuming either {{,textrm{cov},}}(mathcal {M})=mathfrak {c} or {{,textrm{non},}}(mathcal {N})<{{,textrm{cov},}}(mathcal {N})=mathfrak {c}, the lineability and the additivity coefficient of the class of all almost continuous Sierpiński–Zygmund functions. Several open problems are also stated.

The Density of Translates of Zonal Kernels on Compact Homogeneous Spaces

In this survey we shall explore (one definition of) generalized trigonometric functions from different standpoints and illustrate the roles they play in various branches of mathematics. We start from the analytic point of view and for each p is an element of (1, infinity) introduce a function sin(p)(-1) by an integral formula, which is just an extension of the well known integral representation of arcsin, and then use it to define generalized sine, cosine and tangent functions (labelled sin(p), cos(p) and tan(p) respectively). Numerous properties of these functions, such as an identity of Pythagorean type, are exhibited. Then we consider the unit circle in R-2 with the l(p) norm and define generalized trigonometric functions as is done in the standard case when the 12 norm is used. We show that these functions coincide with those introduced earlier. In the third section we consider the integral operator T : L-P (I) -> L-P (I) given by T f(x) = integral(x)(0) f(t)dt, where I = (0,1), and look at the problem of finding an extremal function (an element of the unit sphere of L-P(I) at which the norm of T is attained). It turns out that the extremal functions are given by cos(p). The following section deals with the Dirichlet eigenvalue problem for the p-Laplacian on a bounded interval: all eigenfunctions are expressible by means of sin(p) functions, which corresponds exactly to the classical situation when p = 2. After establishing a connection with approximation theory, we conclude with a review of other ways in which the classical trigonometric functions have been generalized. In the literature a variety of different definitions of generalized trigonometric functions can be found (see [1], [14], [15], [8]): all extend the classical functions and preserve some of their properties. It becomes clear that no single definition preserves all the classical properties and that which definition is adopted depends on the applications envisaged. Our focus on a particular choice reflects our research interests. For an earlier survey (in Czech) see [6].

On the Polynomials Orthogonal on Regular Polygons

Several types of ℋ︁‐matrices were shown to provide a data‐sparse approximation of non‐local (integral) operators in FEM and BEM applications. The general construction is applied to the operators with asymptotically smooth kernel function provided that the Galerkin ansatz space has a hierarchical structure.The new class of ℋ︁‐matrices is based on the so‐called blended FE and polynomial approximations of the kernel function and leads to matrix blocks with a tensor‐product of block‐Toeplitz (block‐circulant) and rank‐k matrices. This requires the translation (rotation) invariance of the kernel combined with the corresponding tensor‐product grids. The approach allows for the fast evaluation of volume/boundary integral operators with possibly non‐smooth kernels defined on canonical domains/manifolds in the FEM/BEM applications. (Here and in the following, we call domains canonical if they are obtained by translation or rotation of one of their parts, e.g. parallelepiped, cylinder, sphere, etc.) In particular, we provide the error and complexity analysis for blended expansions to the Helmholtz kernel. Copyright © 2002 John Wiley &amp; Sons, Ltd.

The paper is devoted to the study of the function E γ ρ,μ(z) defined for complex ρ, μ, γ (Re(ρ) > 0) by which is a generalization of the classical Mittag-Leffler function E ρ,μ(z) and the Kummer confluent hypergeometric function Φ(γ, μ; z). The properties of E γ ρ,μ(z) including usual differentiation and integration, and fractional ones are proved. Further the integral operator with such a function kernel is studied in the space L(a, b). Compositions of the Riemann–Liouville fractional integration and differentiation operators with E γ ρ,μ,ω;a+ are established. An analogy of the semigroup property for the composition of two such operators with different indices is proved, and the results obtained are applied to construct the left inversion operator to the operator E γ ρ,μ,ω;a+. Since, for γ = 0, E 0 ρ,μ,ω;a+ coincides with the Riemann–Liouville fractional integral of order μ, the above operator and its inversion can be considered as generalized fractional calculus operators involving the generalized Mittag-Leffler function E γ ρ,μ(z) in the kernels. Similar assertions are presented for the integral operators containing the Mittag-Leffler and Kummer functions, E ρ,μ(z) and Φ(γ, μ; z), in the kernels, and applications are given to obtain solutions in closed form of the integral equations of the first kind.

Topological genericity of nowhere differentiable functions in the disc and polydisc algebras

We consider continuous functions f which map the open unit disk D into the Riemann sphere W. For a point ζ on the unit circle C, we say that χ is a chord at ζ if χ is a chord of C having one endpoint at ζ and that Δ is a Stolz angle at ζ if Δ is a Stolz angle with vertex ζ. Suppose S denotes either a chord at ζ, a Stolz angle at ζ, or D.

We investigate the growth rate of the Birkhoff sums $S_{n,\unicode[STIX]{x1D6FC}}f(x)=\sum _{k=0}^{n-1}f(x+k\unicode[STIX]{x1D6FC})$, where $f$ is a continuous function with zero mean defined on the unit circle $\mathbb{T}$ and $(\unicode[STIX]{x1D6FC},x)$ is a ‘typical’ element of $\mathbb{T}^{2}$. The answer depends on the meaning given to the word ‘typical’. Part of the work will be done in a more general context.

In this article we calculate the exact location in the Borel hierarchy ofUCF,UCF,the set of all continuous functions on the unit circle with uniformly convergent Fourier series. It turns out to be completeFσδ.F_{\sigma \delta }.Also we prove that anyGδσG_{\delta \sigma }set that includesUCFUCFmust contain a continuous function with divergent Fourier series.

We consider discrete one-dimensional Schrödinger operators whose potentials are generated by sampling along the orbits of a general hyperbolic transformation. Specifically, we show that if the sampling function is a non-constant Hölder continuous function defined on a subshift of finite type with a fully supported ergodic measure admitting a local product structure and a fixed point, then the Lyapunov exponent is positive away from a discrete set of energies. Moreover, for sampling functions in a residual subset of the space of Hölder continuous functions, the Lyapunov exponent is positive everywhere. If we consider locally constant or globally fiber bunched sampling functions, then the Lyapuonv exponent is positive away from a finite set. Moreover, for sampling functions in an open and dense subset of the space in question, the Lyapunov exponent is uniformly positive. Our results can be applied to any subshift of finite type with ergodic measures that are equilibrium states of Hölder continuous potentials. In particular, we apply our results to Schrödinger operators defined over expanding maps on the unit circle, hyperbolic automorphisms of a finite-dimensional torus, and Markov chains.

A continuous function φ on the unit circle is called badly approximable if ‖ φ − p ‖∞ ≧ ‖ φ |∞ for all polynomials p, where ‖ |∞ is the essential supremum norm. In (4), Poreda asked whether every continuous φ may be written φ = φW+φB, where φW is the uniform limit of polynomials (i.e. φW belongs to the disc algebra A) and φB is badly approximable. We call such a function φ decomposable. In (4), he characterised the badly approximable functions as those of constant non-zero modulus and negative winding number around the origin, i.e. ind (φ)&lt;0. (See (3) for two new proofs of this result.) We show that the answer to Poreda's question is no in general, but give a necessary and sufficient condition for a given φ to have such a decomposition. Then we apply this criterion to solve an interpolation problem.

On Hankel operators associated with a class of non-Toeplitz operators

e consider integral equations on the half-line of the form and the finite section approximation to x obtained by replacing the infinite limit of integration by the finite limit β. We establish conditions under which, if the finite section method is stable for the original integral equation (i.e. exists and is uniformly bounded in the space of bounded continuous functions for all sufficiently large β), then it is stable also for a perturbed equation in which the kernel k is replaced by k + h. The class of perturbations allowed includes all compact and some non-compact perturbations of the integral operator. Using this result we study the stability and convergence of the finite section method in the space of continuous functions x for which ()()()=−∫∞dttxt,sk)s(x0()syβxβx()sxsp+1 is bounded. With the additional assumption that ()(tskt,sk−≤ where ()()(),qsomefor,sassOskandRLkq11>+∞→=∈− we show that the finite-section method is stable in the weighted space for ,qp≤≤0 provided it is stable on the space of bounded continuous functions. With these results we establish error bounds in weighted spaces for x - xβ and precise information on the asymptotic behaviour at infinity of x. We consider in particular the case when the integral operator is a perturbation of a Wiener-Hopf operator and illustrate this case with a Wiener-Hopf integral equation arising in acoustics.

Gibbs–Wilbraham oscillation related to an Hermite interpolation problem on the unit circle