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- A note on Riesz fractional integrals in the limiting case alpha(x)p(x) a parts per thousand nPublication . Samko, StefanWe show that the Riesz fractional integration operator I (alpha(center dot)) of variable order on a bounded open set in Omega aS, a"e (n) in the limiting Sobolev case is bounded from L (p(center dot))(Omega) into BMO(Omega), if p(x) satisfies the standard logcondition and alpha(x) is Holder continuous of an arbitrarily small order.
- Embeddings of variable Hajlasz-Sobolev spaces into holder spaces of variable orderPublication . Almeida, Alexandre; Samko, StefanPointwise estimates in variable exponent Sobolev spaces on quasi-metric measure spaces are investigated. Based on such estimates, Sobolev embeddings into Holder spaces with variable order are obtained. This extends some known results to the variable exponent setting. (C) 2008 Elsevier Inc. All rights reserved.
- Fractional integrals and hypersingular integrals in variable order Holder spaces on homogeneous spacesPublication . Samko, Natasha; Samko, Stefan; Vakulov, BorisWe consider non-standard Holder spaces H(lambda(.))(X) of functions f on a metric measure space (X, d, mu), whose Holder exponent lambda(x) is variable, depending on x is an element of X. We establish theorems on mapping properties of potential operators of variable order alpha(x), from such a variable exponent Holder space with the exponent lambda(x) to another one with a "better" exponent lambda(x) + alpha(x), and similar mapping properties of hypersingular integrals of variable order alpha(x) from such a space into the space with the "worse" exponent lambda(x) - alpha(x) in the case alpha(x) < lambda(x). These theorems are derived from the Zygmund type estimates of the local continuity modulus of potential and hypersingular operators via such modulus of their densities. These estimates allow us to treat not only the case of the spaces H(lambda(.))(X), but also the generalized Holder spaces H(w(.,.))(X) of functions whose continuity modulus is dominated by a given function w(x, h), x is an element of X, h > 0. We admit variable complex valued orders alpha(x), where R alpha(x) may vanish at a set of measure zero. To cover this case, we consider the action of potential operators to weighted generalized Holder spaces with the weight alpha(x).
- BMO-VMO results for fractional integrals in variable exponent Morrey spacesPublication . Rafeiro, Humberto; Samko, StefanWe prove the boundedness of the fractional integration operator of variable order alpha(x) in the limiting Sobolev case alpha(x)p(x) = n - lambda(x) from variable exponent Morrey spaces L-p(.),L-lambda(.) (Omega) into BMO (Omega), where Omega is a bounded open set. In the case alpha(x) (math) const, we also show the boundedness from variable exponent vanishing Morrey spaces VLp(.),lambda (.) (Omega) into VMO (Omega). The results seem to be new even when p and A are constant. (C) 2019 Elsevier Ltd. All rights reserved.
- Mixed norm spaces of analytic functions as spaces of generalized fractional derivatives of functions in hardy type spacesPublication . Karapetyants, Alexey; Samko, StefanThe aim of the paper is twofold. First, we present a new general approach to the definition of a class of mixed norm spaces of analytic functions A(q;X)(D), 1 <= q < infinity on the unit disc D. We study a problem of boundedness of Bergman projection in this general setting. Second, we apply this general approach for the new concrete cases when X is either Orlicz space or generalized Morrey space, or generalized complementary Morrey space. In general, such introduced spaces are the spaces of functions which are in a sense the generalized Hadamard type derivatives of analytic functions having l(q) summable Taylor coefficients.
- Characterization of the variable exponent Bessel potential spaces via the Poisson semigroupPublication . Rafeiro, Humberto; Samko, StefanUnder the standard assumptions on the variable exponent p(x) (log- and decay conditions), we give a characterization of the variable exponent Bessel potential space B(alpha)[L(p(-))(R(n))] in terms of the rate of convergence of the Poisson semigroup P(t). We show that the existence of the Riesz fractional derivative D(alpha) f in the space L(p(-))(R(n)) is equivalent to the existence of the limit 1/epsilon(alpha)(I - P(epsilon))(alpha) f. In the pre-limiting case sup(x) p(x) < n/alpha we show that the Bessel potential space is characterized by the condition parallel to(I - P(epsilon))(alpha) f parallel to p((.)) <= C epsilon(alpha). (C) 2009 Elsevier Inc. All rights reserved.
- Boundedness of the Bergman projection and some properties of Bergman type spacesPublication . Karapetyants, Alexey; Rafeiro, Humberto; Samko, StefanWe give a simple proof of the boundedness of Bergman projection in various Banach spaces of functions on the unit disc in the complex plain. The approach of the paper is based on the idea of Zaharyuta and Yudovich (Uspekhi Mat Nauk 19(2):139-142, 1964) where the boundedness of the Bergman projection in Lebesgue spaces was proved using Calderon-Zygmund operators. We exploit this approach and treat the cases of variable exponent Lebesgue space, Orlicz space and variable exponent generalized Morrey spaces. In the case of variable exponent Lebesgue space the boundedness result is known, so in that case we provide a simpler proof, whereas the other cases are new. The major idea of this paper is to show that the approach can be applied to a wide range of function spaces. We also study the rate of growth of functions near the boundary in spaces under consideration and their approximation by mollified dilations.
- Hardy type inequalityin variable lebesgue spacesPublication . Rafeiro, Humberto; Samko, StefanWe prove that in variable exponent spaces where L-p(.)(Omega), where p(.) satisfies the log-condition and Omega is a bounded domain in R-n with the property that R-n\(Omega) over bar has the cone property, the validity of the Hardy type inequality parallel to 1/delta(x)(alpha)integral(Omega)phi(y)/vertical bar x-y vertical bar(n-alpha)dy parallel to(p(.)) <= C parallel to phi parallel to(p(.)), 0 < alpha < min (1, n/p(+)), where delta(x) is approximately equal to dist(x, partial derivative Omega), is equivalent to a certain property of the domain Omega expressed in terms of alpha and chi(Omega).