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Interpolation with a parameter function. (English) Zbl 0619.46064
The (Lions-Peetre) real interpolation spaces $$\bar A{}_{\theta,q}$$ are defined by using the function norm $$\Phi (\phi)=(\int^{\infty}_{0}(\phi (t)/t^{\theta})^ qdt/t)^{1/q}$$. By replacing $$t^{\theta}$$ by a more general (parameter) function $$\rho =\rho (t)$$ we obtain the spaces $$\bar A{}_{\rho,q}$$. In this paper we shall point out the fact that most of the classical (and some new) theorems for the spaces $$\bar A{}_{\theta,q}$$ can be formulated also for the more general spaces $$\bar A{}_{\rho,q}$$. Sometimes we only need to adjust some recent results to the present situation but sometimes we must give separate proofs of our statements. Every result is given in a form which is very adjusted to immediate applications. This paper can be seen as a follow-up and unification of several results of this kind in the literature.

##### MSC:
 46M35 Abstract interpolation of topological vector spaces 46E30 Spaces of measurable functions ($$L^p$$-spaces, Orlicz spaces, Köthe function spaces, Lorentz spaces, rearrangement invariant spaces, ideal spaces, etc.)
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