Fence Posets, Good Gradings and Frobenius Maximal Parabolics

Let $\mathfrak{L}$ be a Frobenius maximal parabolic subalgebra of ${\mathfrak{s}\mathfrak{l}}_n$. For any $F\in {\mathfrak{L}}^{\ast }$ for which the Kirillov form $B_F(x,y)=F([x,y])$ is non-degenerate, let $\widehat{F}$ denote the associated principal element. We prove that the multiplicities of the eigenvalues of ${\mathrm{ad}}_{\widehat{F}}$ on $\mathfrak{L}$ form a unimodal sequence symmetric about $\frac{1}{2}$. We also prove that the multiplicities of the eigenvalues of ${\mathrm{ad}}_{\widehat{F}}$ on ${\mathfrak{g}\mathfrak{l}}_n$ form a unimodal sequence symmetric about $0$. The proof relates the ranked meander associated to $\mathfrak{L}$ to the order ideals of a related fence poset through Panyushev reduction. The known unimodality of the rank polynomial of the fence poset implies that of the meander, which in turn determines a good grading of ${\mathfrak{g}\mathfrak{l}}_n$ in the sense of Elashvili and Kac. We prove that this grading coincides with that induced by the principal element and that the pyramid associated to this grading may be filled in such a way that its good element $e$ lies in $\mathfrak{L}$. The two unimodality results then follow from the injectivity properties of ${\mathrm{ad}}_e$ coming from the good grading and the duality induced by the bilinear form $B_F$.