# Math Help - Second order non linear inhomogeneous differential equation.. :(

1. ## Second order non linear inhomogeneous differential equation.. :(

Solve

y''= x^3/y

I tried by using let u=y'

then I get

u'=x^3/y .. now something tells me this is not going to be an easy ride home.

2. Lets write the second order DE as...

$y^{''} y = x^{3}$ (1)

... and suppose that the solution is analytic in $x=0$ so that is...

$y(x)= a_{0} + a_{1} x + a_{2} x^{2} + \dots + a_{n} x^{n} + \dots$ (2a)

$y^{'} (x) = a_{1} + 2 a_{2} x + 3 a_{3} x^{2} + \dots + n a_{n} x^{n-1} + \dots$ (2b)

$y^{''} (x) = 2 a_{2} + 6 a_{3} x + 12 a_{4} x^{2} + \dots + n(n-1) a_{n} x^{n-2} + \dots$ (2c)

The $a_{n}$ are found inserting the (2a) and (2c) in (1), once we know the 'initial conditions' $y(0)=a_{0}$ and $y^{'} (0) = a_{1}$ ...

$2 a_{0} a_{2} =0 \rightarrow a_{2} = 0$

$6 a_{0} a_{3} + 2 a_{1} a_{2} = 0 \rightarrow a_{3}=0$

$12 a_{0} a_{4} + 6 a_{1} a_{3} + 2a_{2}^{2}= 0 \rightarrow a_{4}=0$

$20 a_{0} a_{5} + 12 a_{1} a_{4} + 8a_{2} a_{3}= 1 \rightarrow a_{5}= \frac{a_{0}}{20}$

$30 a_{0} a_{6} + 20 a_{1} a_{5} + 12a_{2} a_{4} + 6 a_{3}^{2} = 0 \rightarrow a_{6}= - \frac{a_{1}}{30}$

... and so one...

Kind regards

$\chi$ $\sigma$

3. Why assume x=0?

4. Originally Posted by Khonics89
Why assume x=0?

Because then you get a single equation with the two unknown functions, u and y.

And he did NOT "assume x= 0", he assumed the solution was analytic at x= 0.