A hyperplane arrangement is a finite collection of distinct hyperplanes in a vector space or affine space. It is central if every hyperplane contains the origin. A central arrangement has an intersection lattice, ordered by reverse inclusion, which records its intersection pattern (Anghel and Stan 2026).
For a central arrangement over a field, the defining linear forms determine a module of polynomial derivations. The arrangement is a free arrangement if that module is free. Terao's conjecture asks whether this freeness is determined by the intersection lattice when the field is fixed. Graphical arrangements give a class arising from graphs.
