Interpolation of subspaces of infinite codimension

Abstract

Given three normed spaces $X\_0,X\_1,\widetilde{X\_1}$, with $\widetilde{X\_1}$ a closed subspace of $X\_1$ with same norm, how does $[X\_0,X\_1]\_\theta$ compare to $[X\_0 , X\_1 ]\_θ\,? This classical problem of interpolation theory has been mostly considered in the case of finite codimensional subspaces. We show here that quotient J norms are a useful tool which allows to tackle several relevant cases.