sHello to everyone, i would like a suggestion on how solve this nonlinear differential equations:
$$y''+ a\ x\cos y=0 $$
where $a \in \mathbb{R}\ and \ \ y=y(x) $
I am aware that if a solution can be found it will be a series solution, for example, proceeding with the power series could lead to some result?
Or what replacement do I need to bring it back to the first order?
Thank you in advance for any idea or proposed solution.
P.S. I have not yet been able to find out if this differential equation has already been resolved in literature.