![Open Access](https://prespacetime.com/lib/pkp/templates/images/icons/fulltext_open_medium.gif)
![Restricted Access](https://prespacetime.com/lib/pkp/templates/images/icons/fulltext_restricted_medium.gif)
Variational Principle for py''+qy'+ry=ϕ
Abstract
The Lagrangian associated to py''+qy'+ry=phi accepts a variational symmetry which allows to obtain the complete solution of this differential equation if we know one solution for the corresponding homogeneous equation.