Pythagoras Numbers for Ternary Forms

We study the Pythagoras numbers $py(3,2d)$ of real ternary forms, defined for each degree $2d$ as the minimal number $r$ such that every degree $2d$ ternary form which is a sum of squares can be written as the sum of at most $r$ squares of degree $d$ forms. Scheiderer showed that $d+1\le py(3,2d)\le d+2$. We show that $py(3,2d)=d+1$ for $2d=8,10,12$. The main technical tool is Diesel’s characterization of height 3 Gorenstein algebras.