We extend recent results by Pisier on K-subcouples, i.e. subcouples of an interpolation couple that preserve the K-functional (up to constants) and corresponding notions for quotient couples. Examples include interpolation (in the pointwise sense) and a reinterpretation of the Adamyan-Arov-Krein theorem for Hankel operators.
This work was done at the Mittag-Leffler Institute. I am particularly grateful to Richard Rochberg for helpful discussions.
"Interpolation of subcouples and quotient couples." Ark. Mat. 31 (2) 307 - 338, October 1993. https://doi.org/10.1007/BF02559489