Let $ G $ be a simple graph of $ \ell $ vertices $ \{ 1, \dots, \ell \} $ with edge set $ E_{ G} $. The graphical arrangement $ \mathcal{ A}_{ G} $ consists of hyperplanes $ \{ x_{ i}-x_{ j}=0\} $, where $ \{ i, j \} \in E_{ G} $. It is well known that three properties, chordality of $ G $, supersolvability of $ \mathcal{ A}_{ G} $, and freeness of $ \mathcal{ A}_{ G} $ are equivalent. Recently, Richard P. Stanley introduced $ \psi $-graphical arrangement $ \mathcal{ A}_{ G, \psi} $ as a generalization of graphical arrangements. Lili Mu and Stanley characterized the supersolvability of the $ \psi $-graphical arrangements and conjectured that the freeness and the supersolvability of $ \psi $-graphical arrangements are equivalent. In this paper, we will prove the conjecture.
Categories: math.CO