I am trying to find the power series solution of(adsbygoogle = window.adsbygoogle || []).push({});

[tex]y' = x^2y[/tex]

but dont know how to arrive at the answer of [tex]y = a_0exp(x^3/3)[/tex]. [I know that it's an easily solved separable equation, I'm just trying to figure out how to find the power series solution]

My solution so far:

Assume

[tex] y = \sum_{n=0}^\infty a_nx^n [/tex]

then

[tex] y' = \sum_{n=1}^\infty na_nx^{n-1} [/tex]

giving:

[tex] \sum_{n=1}^\infty na_nx^{n-1} = x^2 \sum_{n=0}^\infty a_nx^n [/tex]

[tex] \sum_{n=1}^\infty na_nx^{n-1} = \sum_{n=0}^\infty a_nx^{n+2} [/tex]

changing the index for the LHS to give [tex]x^n [/tex]

[tex] \sum_{n=0}^\infty (n + 1)a_{n + 1}x^n [/tex]

changing the index for the RHS to give [tex]x^n[/tex]:

[tex] \sum_{n=2}^\infty a_{n - 2}x^{n} [/tex]

Then taking the first two terms out of the LHS sum, so that both sums start from the same point:

[tex] a_1 + 2a_2x + \sum_{n=2}^\infty (n + 1)a_{n + 1}x^n = \sum_{n=2}^\infty a_{n - 2}x^{n} [/tex]

I don't know what to do after this (I'm not entirely sure if what I've done so far is right, either).

If theyterm in the initial equation didn't have the [tex]x^2[/tex] in front of it, it would be easy to equate the coefficients of [tex]x^n[/tex] to get the recursion formula. But having the terms [tex]a_1[/tex] and [tex]2a_2x[/tex] in front of the sum on the LHS throws me - can anyone explain clearly to me the correct steps required to solve the problem?

**Physics Forums - The Fusion of Science and Community**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Power series solution for 1st-order ODE

Loading...

Similar Threads - Power series solution | Date |
---|---|

Power series solution, differential equation question | Nov 12, 2015 |

Seek power series solutions of the given differential equation | Aug 31, 2015 |

Nonhomogeneous Power Series Solution | Mar 15, 2012 |

Factorial question in a power series solution | Oct 30, 2011 |

Power series solutions for ODEs. When are there how many of them? | Apr 2, 2011 |

**Physics Forums - The Fusion of Science and Community**