Moments of volumes of lower-dimensional random simplices are not monotone

Abstract: In a $d$-dimensional convex body $K$, for $n \leq d+1$, random points $X_0, \dots, X_{n-1}$ are chosen according to the uniform distribution in $K$. Their convex hull is a random $(n-1)$-simplex with probability $1$. We denote its $(n-1)$-dimensional volume by $V_{K[n]}$. The $k$-th moment of the $(n-1)$-dimensional volume of a random $(n-1)$-simplex is monotone under set inclusion, if $K \subseteq L$ implies that the $k$-th moment of $V_{K[n]}$ is not larger than that of $V_{L[n]}$. Extending work of Rademacher [On the monotonicity of the expected volume of a random simplex. Mathematika 58 (2012), 77--91] and Reichenwallner and Reitzner [On the monotonicity of the moments of volumes of random simplices. Mathematika 62 (2016), 949--958], it is shown that for $n \leq d$, the moments of $V_{K[n]}$ are not monotone under set inclusion.