Sum of squares length of real forms

Abstract

For $n,d\ge 1$ let $p(n,2d)$ denote the smallest number $p$ such that every sum of squares of forms of degree $d$ in $\mathbb{R}[x_1,\dots ,x_n]$ is a sum of $p$ squares. We establish lower bounds for these numbers that are considerably stronger than the bounds known so far. Combined with known upper bounds they give $p(3,2d)\in \{d+1,d+2\}$ in the ternary case. Assuming a conjecture of Iarrobino-Kanev on dimensions of tangent spaces to catalecticant varieties, we show that $p(n,2d)\sim const\cdot d^{(n-1)/2}$ for $d\to \infty$ and all $n\ge 3$. For ternary sextics and quaternary quartics we determine the exact value of the invariant, showing $p(3,6)=4$ and $p(4,4)=5$.