Invertibility of operators in spaces of real interpolation.
A Carlson type inequality with blocks and interpolation
An inequality, which generalizes and unifies some recently proved Carlson type inequalities, is proved. The inequality contains a certain number of “blocks” and it is shown that these blocks are, in a sense, optimal and cannot be removed or essentially changed. The proof is based on a special equivalent representation of a concave function (see [6, pp. 320-325]). Our Carlson type inequality is used to characterize Peetre’s interpolation functor (see [26]) and its Gagliardo closure on couples of...
Structure of the Hardy operator related to Laguerre polynomials and the Euler differential equation.
We present a direct proof of a known result that the Hardy operator Hf(x) = 1/x ∫ f(t) dt in the space L = L(0, ∞) can be written as H = I - U, where U is a shift operator (Ue = e, n ∈ Z) for some orthonormal basis {e}. The basis {e} is constructed by using classical Laguerre polynomials. We also explain connections with the Euler differential equation of the first order y' - 1/x y = g and point out some generalizations to the case with weighted L (a, b) spaces.
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