Symmetric rank of some reducible forms

Abstract: In this paper, we study the symmetric rank of products of linear forms and an irreducible quadratic form. The main result presents a new, non-trivial lower bound for the rank, and the arguments rely on the apolarity lemma. In the special case of degree 4, we give a list of normal forms for such quartics, and we apply our general result to compute the rank of almost all of them. These families of quartics provide examples of polynomials of generic, supergeneric and even maximal rank, as well as an unexpected number of decompositions.