# Non-linear second order DE, with no x term in it

Okay, I have a second order non linear de, which has no term containing the variable x. assuming $$y = f(x)$$ , the equation is

$$y'' - Ay' = \cos{y} - B\sin{y}$$

I tried solving it by substituting $$v = y'$$ and after that using the chain rule, to express it as a first order equation, but I don't seem to be getting anywhere....

Any help would be appreciated.

• How did you get this equation? Any physical background? Commented Aug 19, 2013 at 6:27
• There is no restriction on $A,B$, right? Commented Aug 19, 2013 at 17:07
• Are you looking for a general solution formula? Commented Sep 6, 2013 at 23:38
• one possibility is transform the rhs to $\sin(y-\alpha)$ where $\alpha=\tan^{-1}(1/B)$ and then change the variables to $\bar{y}=y-\alpha$ Commented Oct 10, 2013 at 21:02

$$\frac{d^2 y}{dx^2}-A\frac{dy}{dx}=\cos(y)-B\sin(y)$$ $\frac{dy}{dx}=v(y) \quad \to \quad \frac{d^2 y}{dx^2}= \frac{dv}{dy} \frac{dy}{dx} =\frac{dv}{dy}v(y)$ $$\frac{dv}{dy}v(y) –A\:v(y)=\cos(y)-B\sin(y)$$ $v(y)=A\:w(y) \quad \to \quad$ $$\frac{dw}{dy}w(y) –\:w(y)=\frac{\cos(y)-B\sin(y)}{A^2}$$ This is an Abel’s ODE of the second kind. The method of resolution is very complicated. See : http://arxiv.org/ftp/arxiv/papers/1503/1503.05929.pdf

In order to make the symbols more consistent with whose used in the paper, we change the notations : $y=X$ and $w=Y$, so that : $$Y(X)\frac{dY}{dX} –Y(X)=Q(X)$$ with $Q(X)=\frac{\cos(X)-B\sin(X)}{A^2}$

The solution (subject to some conditions, see in the paper referenced above) is provided on implicit form : $$Y(X)^3+p\:Y(X)+q=0$$ where $p$ and $q$ result from the calculus of :

$\psi=X\:Si(X)$ with the special function “Sin integral”

$c= -\frac{\sin(2X)}{4\psi^2} +\frac{\sin^2(X)}{\psi^2}\left(1+\frac{1}{2X} \right) -\frac{1}{\psi}\left( \cos(X)-\frac{\sin(X)}{X} \right)+\frac{\sin^3(X)}{2\psi^3}$

$b=3-c-\frac{4Q(X)}{X+\varphi}$ where $\varphi$ is an arbitrary constant of integration.

$p=-\frac{16}{3}+b$

$q=-\frac{128}{27}+\frac{4}{3}b+c$