Algebras of singular integral operators on rearrangement-invariant spaces and Nikolski ideals.
In 1968, Gohberg and Krupnik found a Fredholm criterion for singular integral operators of the form aP + bQ, where a,b are piecewise continuous functions and P,Q are complementary projections associated to the Cauchy singular integral operator, acting on Lebesgue spaces over Lyapunov curves. We extend this result to the case of Nakano spaces (also known as variable Lebesgue spaces) with certain weights having finite sets of discontinuities on arbitrary Carleson curves.
Let be a separable Banach function space on the unit circle and let be the abstract Hardy space built upon . We show that the set of analytic polynomials is dense in if the HardyLittlewood maximal operator is bounded on the associate space . This result is specified to the case of variable Lebesgue spaces.
A classical theorem of Coifman, Rochberg, and Weiss on commutators of singular integrals is extended to the case of generalized Lp spaces with variable exponent.
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