根と係数の関係
{ α + β + γ + δ = − b a α β + α γ + α δ + β γ + β δ + γ δ = c a α β γ + α β δ + α γ δ + β γ δ = − d a α β γ δ = e a {\displaystyle {\begin{cases}\alpha +\beta +\gamma +\delta =-{\cfrac {b}{a}}\\[7pt]\alpha \beta +\alpha \gamma +\alpha \delta +\beta \gamma +\beta \delta +\gamma \delta ={\cfrac {c}{a}}\\[7pt]\alpha \beta \gamma +\alpha \beta \delta +\alpha \gamma \delta +\beta \gamma \delta =-{\cfrac {d}{a}}\\[7pt]\alpha \beta \gamma \delta ={\cfrac {e}{a}}\end{cases}}}
が四次の場合として成り立つ。