Discriminants of derivatives and symmetric difference polynomials

Let $P$ be a monic polynomial of degree $n$ with roots $x_1,\ldots,x_n$. We study the discriminants of the derivatives $P^{(k)}$ as symmetric translation-invariant polynomials in the original roots. The general ``square-graph cone'' positivity problem was formulated by Alexandersson and Shapiro. The main result of this note proves this conjecture for the terminal cubic family $k=n-3$: we give an explicit positive square-graph expansion for $\disc(P^{(n-3)})$. We also record closed central-moment formulas for the terminal quadratic, cubic and quartic cases, introduce normalized terminal polynomials for all fixed terminal orders, and write down the next, quintic, terminal polynomial explicitly. These formulas turn the first open cases of the square-graph problem into concrete finite linear-algebraic certificate problems.