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The recurrence relation I ended up getting doesn't match with the final answer. Did a couple of revisions of the thing but nothing changes.

I get the recurrence relation as $${a_{r+2}=\frac{-[(r^2-2r+p)a_r+(r+1)(2r-1)a_{r+1}]}{(r+2)(r+1)}}$$

My attempt at a solution

The original equation can be written as $${y''-\frac{xy'}{x^{2}}+\frac{py}{x^{2}}=0}$$

Put $(x-1)=t$

Notice that $y''$ and $y'$ don't make a difference

Therefore we have $${y''-\frac{(t+1)y'}{(t+1)^{2}}+\frac{py}{(t+1)^{2}}=0} \qquad (1)$$

where $P(t)=-\frac1{(t+1)}$ and $Q(t)=\frac{p}{(t+1)^{2}}$, which have $t=0$ as an ordinary point

Now we can write $$y =\sum{a_rt^r},$$ $$y' =\sum{ra_rt^{r-1}},$$ $$y'' =\sum{(r-1)ra_rt^{r-2}}$$

Using these in the (1) we get $${\sum{(t+1)^{2}(r-1)ra_rt^{r-2}}-\sum{(t+1)ra_rt^{r-1}}+p\sum{a_rt^r}=0}$$ $$\implies{\sum{(t^2+2t+1)(r-1)ra_rt^{r-2}}-\sum{(t+1)ra_rt^{r-1}}+p\sum{a_rt^r}=0}$$ $${\implies\sum{(t^2+2t+1)(r-1)ra_rt^{r-2}}-\sum{(t+1)ra_rt^{r-1}}+p\sum{a_rt^r}=0}$$ $${\implies\sum{(r-1)ra_rt^{r}}+2\sum{(r-1)ra_rt^{r-1}}+\sum{(r-1)ra_rt^{r-2}}-\sum{ra_rt^{r}}-\sum{ra_rt^{r-1}}+p\sum{a_rt^r}=0}$$

Equating coefficients of $t^r$ on both sides, we have $${(r-1)ra_r+2{(r+1-1)(r+1)a_{r+1}}+(r+2-1)(r+2)a_{r+2}-ra_r-(r+1)a_{r+1}+pa_r=0}$$ $$\implies{((r-1)r-r+p)a_r+{(2r(r+1)-(r+1))a_{r+1}}+(r+1)(r+2)a_{r+2}=0}$$ $$\implies{(r^2-2r+p)a_r+{(r+1)(2r-1)a_{r+1}}+(r+1)(r+2)a_{r+2}=0}$$ $$\implies{(r+1)(r+2)a_{r+2}=-[(r^2-2r+p)a_r+{(r+1)(2r-1)a_{r+1}}]}$$

which gives the recurrence relation

$${a_{r+2}=\frac{-[(r^2-2r+p)a_r+(r+1)(2r-1)a_{r+1}]}{(r+1)(r+2)}}$$

However when I use the above mentioned relation for the solution, the coefficients don't come out to be correct as per the answer key.

The solution supposedly is http://s11.postimg.org/4zm9oouf6/IMG_20150708_190227.jpg

I can't post images yet, so the link.

1 Answers1

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We may notice that the original ODE is solved by $x^{\alpha}=\exp(\alpha \log x)$, provided that:

$$ \alpha(\alpha-1) -\alpha + p = 0,\tag{1} $$ so two independent solutions are given by $\exp\left[\left(1-\sqrt{1-p}\right) \log x\right]$ and $\exp\left[\left(1+\sqrt{1-p}\right)\log x\right]$. Assuming:

$$ y(x) = \sum_{n\geq 0}a_n (x-1)^n \tag{2}$$ we have:

$$\begin{eqnarray*} 0=x^2 y''(x) - x y(x) + p y(x) =\\ = (x-1)^2 y''(x) + 2(x-1)y''(x)+y''(x)-(x-1)y'(x)-y'(x)+py(x)\tag{3}\end{eqnarray*}$$

hence for any $n\geq 0$:

$$ n(n-1) a_n + 2n(n+1) a_{n+1}+(n+2)(n+1)a_{n+2}-na_n-(n+1)a_{n+1}+p a_n = 0$$

or:

$$ (n+2)(n+1) a_{n+2} + (2n^2+n-1)a_{n+1} + (n^2-2n+p)a_n = 0.\tag{4} $$

Jack D'Aurizio
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