LAGRANGE, Joseph Louis. Traité de la résolution des équations numériques de tous les degrés. 1826.

Third edition, enlarged with an analysis by Poinsot.

In this work, Lagrange demonstrates the need to find new techniques for solving equations beyond the fourth degree. Indeed, while auxiliary equations of degree 3 can be used to solve fourth-degree equations, for equations of degree 5 and above, the degree of the auxiliary equations increases.

Lagrange also presents methods of approximate solution in this work.