# Approximate Solutions to Non-Linear Differential Equations

1. Feb 20, 2009

### nassboy

I'm interested in coming up with an function that approximates the solution to a non-linear differential equation. (There is no known closed form solution)

The equation is Y''=(1/Y)(Y')^2-Y*A+Y^2*A where "A" is a constant, and Y' and Y'' are the first and second derivatives with respect to x.

How would I look for approximations to the answer in the literature? Is this equation a member of a certain class of differential equations?

Is there a technique known to work well?

Any help would be appreciated!

2. Feb 20, 2009

### HallsofIvy

Since the independent variable does not appear explicitely in this equation, it looks like a candidate for "quadrature". Taking x to be the independent variable, let z= dy/dx. Then d2y/dx2= dz/dx= (dz/dy)(dy/dx)= zdz/dx. Now the equation becomes
$$z\frac{dz}{dy}= \frac{z^2}{y}- Ay+ Ay^2$$
a separable first order equation.

Another typical method for non-linear equations is "perturbations". The "WKB approximation" in quantum mechanics is a type of perburbation method.

3. Feb 20, 2009

### nassboy

4. Feb 21, 2009

### nassboy

If I work the problem using this form $$z\frac{dz}{dy}= \frac{z^2}{y}- Ay+ Ay^2$$

, I'm still left with an integral that has no solution when I try to integrate a second time!

5. Feb 21, 2009

### arildno

Well, set $$u=\frac{1}{2}z^{2}$$, rewriting the equation as:
$$\frac{du}{dy}-\frac{2}{y}u=-Ay+Ay^{2}$$
Multiplying this by the integrating factor y^-2, we get:
$$\frac{d}{dy}(\frac{1}{y^{2}}u)=-\frac{A}{y}+A$$,
or, by integrating:
$$u=Cy^{2}+Ay^{3}-Ay^{2}\ln(y)$$
Thus, we get:
$$\frac{dy}{dx}=\pm\sqrt{2(Cy^{2}+Ay^{3}-Ay^{2}\ln(y))}$$

which is separable, if not easy to solve..

6. Feb 21, 2009

### nassboy

Thanks for the help!

[TEX]\int \frac{1}{\sqrt{2\left(C Y^2+A Y^3-A Y^2 \text{Log}[Y]\right)}} [/TEX]

(I hope that is right, the latex preview isn't working for me)

After separating the variables, one is left with the integral above which has no closed form solution. What is the best way to approximate the integral, keeping in mind that I want to solve algebraically for y in the end.

Last edited: Feb 21, 2009