Waring decompositions of the product of two quadrics: the small rank cases

Abstract

In this paper we study forms of the type $(x_1^2+ \cdots +x_m^2)(y_1^2+ \cdots+y_n^2)$ using projections. For $m=1, m=2$, and for any $n$ we describe: the forbidden locus, the structure and the Hilbert function of all minimal apolar sets. In particular, we show that every minimal apolar ideal has the same Hilbert function. Further, we compute the cactus rank, a bound on the border rank, and the dimension of the Variety of Sums of Powers. For $m,n \geq 3,$ we provide new lower and upper bounds for the Waring rank.