Sample theory proposition
Symmetric cubics from power sums
Let a monic cubic have roots r₁, r₂, r₃. If the first two power sums are known, then
e₂ = ((r₁+r₂+r₃)² − (r₁²+r₂²+r₃²))/2.
The packet derives this identity, identifies the exceptional information still needed to reconstruct the cubic, and shows how a root value supplies it.