On the K-divisibility constant for some special finite-dimensional Banach couples

Abstract

We prove new estimates of the K -divisibility constants for some special Banach couples.In particular, we prove that the K -divisibility constant for a couple of the form (U ⊕ V , U )where U and V are non-trivial Hilbert spaces equals 2/ √ 3. We also prove estimates for the K -divisibility constant of the two-dimensional version of the couple (L 2 , L ∞ ), proving in particular that this couple is not exactly K -divisible. There are also several auxiliary results, including some estimates for relative Calderón constants for finite-dimensional couples.