Extreme points of the unit ball and isometries of noncommutative quasi-Banach Marcinkiewicz spaces

Abstract: We characterize extreme points of the (positive) unit ball of Marcinkiewicz spaces with respect to their natural quasi-norms. Having these results at hand, we show that every positive isometry on a noncommutative Marcinkiewicz space is of elementary form. In particular, a linear mapping on a noncommutative Marcinkiewicz space over an atomic von Neumann algebra with all atoms having the same trace is a positive surjective isometry if and only if it is the restriction of a Jordan $*$-isomorphism which preserves the singular value functions. We also study (not necessarily positive) surjective isometries on weak $L_p$-spaces affiliated a $σ$-finite non-atomic von Neumann algebra for $1<p<\infty$, Marcinkiewicz sequence spaces, and weak $\ell_p$ operator ideals, $$p\>0$$, as well as one-parameter groups of isometries on the latter two.