Nontrivial isometries on sp(α)

Abstract

sp(α) is a Banach space of sequences x with ‖x‖=(∑i=0∞|xi|p+α∑i=0∞|xi+1−xi|p)1/p. For 1<p<∞, p≠2, 0<α<∞, α≠1, there are no nontrivial surjective isometries in sp(α). It has been conjectured that there are no nontrivial isometries. This note gives two distinct counterexamples to this conjecture and a partial affirmative answer for the case of isometries with finite codimension.