- #1
PatsyTy
- 30
- 1
Homework Statement
Solve
\begin{equation*}
36x^2y''+(5-9x^2)y=0
\end{equation*}
using the Frobenius method
Homework Equations
Assume a solution of the form
\begin{equation*}
y=\sum_{n=0}^{\infty}{a_nx^{n+s}}
\end{equation*}
then
\begin{equation*}
y''=\sum_{n=0}^{\infty}{(n+s)(n+s-1)a_nx^{n+s-2}}
\end{equation*}
The Attempt at a Solution
I substitute the expanded forms of ##y## and ##y''## into the D.E
\begin{equation*}
36x^2 \sum_{n=0}^{\infty}{(n+s)(n+s-1)a_nx^{n+s-2}}+5 \sum_{n=0}^{\infty}{a_nx^{n+s}} -9x^2 \sum_{n=0}^{\infty}{a_nx^{n+s}} =0
\end{equation*}
and put the terms of ##x## into the sums
\begin{equation*}
36 \sum_{n=0}^{\infty}{(n+s)(n+s-1)a_nx^{n+s}}+5 \sum_{n=0}^{\infty}{a_nx^{n+s}} -9\sum_{n=0}^{\infty}{a_nx^{n+s+2}} =0
\end{equation*}
I then modify the last term so that the ##x## factor is ##x^{n+s}##
\begin{equation*}
36 \sum_{n=0}^{\infty}{(n+s)(n+s-1)a_nx^{n+s}}+5 \sum_{n=0}^{\infty}{a_nx^{n+s}} -9\sum_{n=2}^{\infty}{a_{n-2}x^{n+s}} =0
\end{equation*}
This is where I get confused, the last term starts at ##n=2##, all the examples I have seen have their highest starting value for a sum equal to ##n=1##, in those cases I would comine the terms with the sum starting at ##n=0## and take out the first term of ##n=0## resulting in the indicial equation multiplied by ##a_o## which I would use to solve the D.E. Now however I have an extra term, just to be clear below is how I have found the indicial equation:
\begin{equation*}
\sum_{n=0}^{\infty}{[36(n+s)(n+s-1)+5]a_nx^{n+s}}-9\sum_{n=2}^{\infty}{a_{n-2}x^{n+s}}=0 \\
[36(s)(s-1)+5]a_0x^s+\sum_{n=1}^{\infty}{[36(n+s)(n+s-1)+5]a_nx^{n+s}}-9\sum_{n=2}^{\infty}{a_{n-2}x^{n+s}}=0
\end{equation*}
So I have my indicial equation but I can't combine the second and third terms together because their initial values don't match. I'm not sure if I should pull out the ##n=1## term on the second term so I can create a recursion relation to use however I am not sure what to do with that term I pull out.
Also a somewhat unrelated question, in this course we are also working with the Legendre D.Es and Bessel's equations, would I be able to solve both of these using the forbenius method? Or do I need some other techniques?
Any help would be appreciated!
Last edited: