  Let $\frb$ be a fixed Borel subalgebra of a finite-dimensional complex simple
Lie algebra $\frg$. The Shi bijection associates to every ad-nilpotent ideal
$\fri$ of $\frb$ a region $V_{\fri}$. In this paper, we show that $\fri$ is
abelian if and only if $V_{\fri}\cap 2A$ is empty, if and only if the volume of
$V_{\fri}\cap 2A$ equals to that of $A$, where $A$ is the fundamental alcove of
the affine Weyl group. For certain flag of abelian ideals, we record an
ascending property of their associated regions. We also determine the maximal
eigenvalue $m_{r-1}$ of the Casimir operator on $\wedge^{r-1} \frg$ and the
corresponding eigenspace $M_{r-1}$, where $r$ is the number of positive roots.
