# Rewriting a second order ODE as a first order ODE

I have the ODE

$$\left(\frac{y'}{x^2 \rho(x)} \right)' + \frac{4 \pi G \rho(x)}{x^2 P(x)}\, y - \left( \frac{F(x)}{x^2 \rho(x)} \right)' = 0$$

with the boundary conditions $$y(x=0)=0, y(x=1)=0$$. The independent variable is $$x$$, the variable I would like to solve for is $$y(x)$$, the functions $$\rho$$, $$P$$, and $$F$$ are all known as well as the constants $$G$$ and $$4\pi$$ (duh).

I would like to solve this equation numerically using scipy's sp.integrate.bvp_solve. For this I need to turn it into a first order system.

This is my attempt:

\begin{align} y_1' &= y_2 \\ y_2' &= \left(\frac{F}{x^2 \rho} \right)' - \frac{4 \pi G \rho}{x^2 P}\, y \end{align}

but I think that it is not correct because the first term in the original equation does not seem properly represented. Can someone please help?

• The LHside of your second ODE should be $\left( \frac{y_2}{x^2 \rho(x)}\right)^\prime$ instead of $y_2^\prime$; and of course you should have $y_1$ instead of $y$ in the RHside. Aug 7 '20 at 18:11
• @Pacciu thanks - that makes a lot more sense - however I am still stuck as at least the examples in scipy all have "pure" variables on the LHS. Can I maybe integrate the second ODE to get something "pure"? Or do you have advice on how to proceed from there? Aug 7 '20 at 18:17

## 1 Answer

The original ODE can be written also as

$$\left(\frac{y'-F(x)}{x^2 \rho(x)} \right)' + \frac{4 \pi G \rho(x)}{x^2 P(x)}\, y = 0$$

and making

$$\cases{y_1 = y\\ y_2=\frac{y_1'-F(x)}{x^2 \rho(x)} }$$

can be represented as

$$\cases{ y'_1 = x^2\rho(x) y_2+F(x)\\ y'_2 = -\frac{4\pi G \rho(x)}{x^2 P(x)}y_1 }.$$

• It works, it works, oh my gosh it works! You have made me a very happy person today. Thank you! Aug 8 '20 at 10:04