# Find two linearly independent power series solutions of $y''-xy'+y=0$

I am trying to find two linearly independent power series solutions of $y''-xy'+y=0$. Upon solving for $C_{n+2}$ I got the following recursion: $$C_{n+2} = \frac{nC_n-C_n}{(n+2)(n+1)}$$ After that, I found a series of $C$s for $n=0,1,2,3,...$

Theses are some of them: $$C_2 = \frac{-1*C_0}{2!}$$ $$C_3 = 0$$ $$C_4 = \frac{-1*C_0}{4!}$$ $$C_5 = 0$$ $$C_6 = \frac{-1*3*C_0}{6!}$$ $$C_7 = 0$$ $$C_6 = \frac{-1*3*5*C_0}{8!}$$

Now I know that one of the power series solution is $n$ because the odd terms are just $0$. Now, I have difficulty finding the solution for the even terms because the pattern is not obvious. Please someone help me.

• Should the recursion be $C_{n+2}=\dfrac{n C_n-n}{(n+1)(n+2)}$? – Semiclassical May 23 '17 at 3:12
• @Semiclassical It should be $$C_{n+2} = \frac{nC_n-C_n}{(n+2)(n+1)}$$ – VadaCurry May 23 '17 at 3:52

As you say, one solution is $y_1(x)=c_1 x$. The other solution has no nice expression, as far as I can tell. According to Wolfram Alpha, it is $$y_2(x)=c_2 \sqrt{-x^2}\left( 2i \sqrt π\left( \text{Erfi}\left(\sqrt{\frac{x^2}2}\right) + 1\right) - \frac{2i \sqrt2 \,e^{x^2/2}}{\sqrt{x^2}} - 2 \sqrt π\right)$$

• A cute integral form is given by $y_2(x)=c_2 x\int_x^\infty e^{-t^2/2}\frac{dt}{t^2}$. (To obtain this, let $y(x)=x \int w(x)\,dx$ to get a first-order ODE in $w(x)$.) – Semiclassical May 23 '17 at 4:05
• Nice. $\ \ \ \ \$ – Martin Argerami May 23 '17 at 5:13

Hint

In numerators, you face terms looking like $$P_n=1\cdot 3 \cdot 5 \cdots (2n-1)$$ Insert the even numbers to rewrite $$P_n=1\cdot 3 \cdot 5 \cdots (2n-1) = \frac{ 1 \cdot 2 \cdot 3 \cdot 4 \cdots (2n-1)\cdot (2n) }{2\cdot 4 \cdot 6 \cdots (2n)}$$ Now take out the factor $2$ from each term in the denominator to get $$P_n=\frac{(2n)!}{2^n\, n!}$$

I am sure that you can take it from here (not missing the fact that $y=kx$ is a solution).