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k-Extreme Points in Symmetric Spaces of Measurable Operators

Let $\mathcal{M}$ be a semifinite von Neumann algebra with a faithful, normal, semifinite trace $τ$ and $E$ be a strongly symmetric Banach function space on $[0,τ(1))$. We show that an operator $x$ in the unit sphere of $E\left(\mathcal{M},τ\right)$ is $k$-extreme, $k\in\mathbb N$, whenever its singular value function $μ(x)$ is $k$-extreme and one of the following conditions hold (i) $μ(\infty,x)=\lim_{t\to\infty}μ(t,x)=0$ or (ii) $n(x)\mathcal{M} n(x^*)=0$ and $|x|\geq μ(\infty,x)s(x)$, where $n(x)$ and $s(x)$ are null and support projections of $x$, respectively. The converse is true whenever $\mathcal{M}$ is non-atomic. The global $k$-rotundity property follows, that is if $\mathcal{M}$ is non-atomic then $E$ is $k$-rotund if and only if $E\left(\mathcal{M},τ\right)$ is $k$-rotund. As a consequence of the noncommutive results we obtain that $f$ is a $k$-extreme point of the unit ball of the strongly symmetric function space $E$ if and only if its decreasing rearrangement $μ(f)$ is $k$-extreme and $|f|\geq μ(\infty,f)$. We conclude with the corollary on orbits $Ω(g)$ and $Ω'(g)$. We get that $f$ is a $k$-extreme point of the orbit $Ω(g)$, $g\in L_1+L_{\infty}$, or $Ω'(g)$, $g\in L_1[0,α)$, $α<\infty$, if and only if $μ(f)=μ(g)$ and $|f|\geq μ(\infty,f)$. From this we obtain a characterization of $k$-extreme points in Marcinkiewicz spaces.