On the apolar algebra of a product of linear forms

Abstract

Apolarity is a important tool in commutative algebra and algebraic geometry which studies a form, f, by the action of polynomial differential operators on f. The quotient of all polynomial differential operators by those which annihilate f is called the apolar algebra of f. In general, the apolar algebra of a form is useful for determining its Waring decomposition, which consists of writing the form as a sum of powers of linear forms with as few summands as possible. In this article we study the apolar algebra of a product of linear forms, which generalizes the case of monomials and connects to the geometry of hyperplane arrangements. In the first part of the article we provide a bound on the Waring rank of a product of linear forms under certain genericity assumptions; for this we use the defining equations of so-called star configurations due to Geramita, Harbourne, and Migliore. In the second part of the article we use the computer algebra system Bertini, which operates by homotopy continuation methods, to solve certain rank equations for catalecticant matrices. Our computations suggest that, up to a change of variables, there are exactly six homogeneous polynomials of degree six in three variables which factor completely as a product of linear forms defining an irreducible multi-arrangement and whose apolar algebras have dimension six in degree three. As a consequence of these calculations, we find six cases of such forms with cactus rank six, five of which also have Waring rank six. Among these are products defining subarrangements of the braid and Hessian arrangements.