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Bessel function series expansion 
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#1
Dec810, 11:40 AM

P: 172

1. The problem statement, all variables and given/known data
This is the how the question begins. 1. Bessel's equation is [tex]z^{2}\frac{d^{2}y}{dz^{2}} + z\frac{dy}{dz} + \left(z^{2} p^{2}\right)y = 0[/tex]. For the case [tex]p^{2} = \frac{1}{4}[/tex], the equation has two series solutions which (unusually) may be expressed in terms of elementary functions: [tex]J_{1/2} = \left(\frac{2}{\pi z}\right)^{1/2} sin z[/tex] [tex]J_{1/2} = \left(\frac{2}{\pi z}\right)^{1/2} cos z[/tex] [ The factors [tex]\left(\frac{2}{\pi z}\right)^{1/2}[/tex] are supefluous, but are included by convention, for reasons that are not relevant to the present purposes.] Clearly [tex]J_{1/2}[/tex] is singular at z = 0. Show that [tex]J_{1/2}(0) = 0[/tex]. 2...................(for later) 2. Relevant equations 3. The attempt at a solution I am going to assume the solutions [tex]J_{1/2}[/tex] and [tex]J_{1/2}[/tex] without worrying about why/how they come about. Obviously, when z = 0, [tex]cos z \neq 0 [/tex]. Therefore, [tex]\left(\frac{2}{\pi z}\right)^{1/2}[/tex] blows up and [tex]J_{1/2}[/tex] is singular at z = 0. On the other hand, if we draw separately the graphs of [tex]\left(\frac{2}{\pi z}\right)^{1/2}[/tex] and [tex] sin z [/tex] and then combine the two in a single graph of [tex]J_{1/2}[/tex], we find that it is sinusoidal with an amplitude given by [tex]\left(\frac{2}{\pi z}\right)^{1/2}[/tex]. This means that the curve oscillates as it moves towards z = 0 with an amplitude that tends to infinity as z tends to 0. How do I conclude from this that [tex]J_{1/2}(0) = 0[/tex]. 


#2
Dec810, 01:34 PM

P: 352

Compare the order to which [tex]\sin z[/tex] vanishes at 0 to the order of growth of [tex]\sqrt{2/(\pi z)}[/tex] at 0. From that you should be able to prove an estimate of the form [tex]J_{1/2}(z) = O(f(z))[/tex] as [tex]z \to 0[/tex] for an appropriately chosen bound [tex]f[/tex].



#3
Dec810, 01:58 PM

P: 172

Thanks for your help!
I don't understand what 'order' means in this context. How do we compare the orders? "an estimate of the form [tex]J_{1/2}(z) = O(f(z))[/tex] as [tex]z \to 0[/tex] for an appropriately chosen bound [tex]f[/tex].": I don't understand! 


#4
Dec810, 02:11 PM

P: 352

Bessel function series expansion
"Order" is short for "order of growth", which means a simple function (usually a power, log, or exponential function or simple combination of those functions) which you can use to "measure" the speed with which a given function vanishes or grows near a point. These orders are measured up to a constant factor.
For example, [tex]\csc x[/tex] grows like [tex]1/x[/tex] as [tex]x \to 0[/tex], that is, you can choose positive constants [tex]c, C[/tex] so that [tex]cx^{1} < \csc x < Cx^{1}[/tex] for [tex]x[/tex] close enough to zero. One common way to try to find an order of growth for a function is to expand it in a Taylor or Laurent series. For the meaning of [tex]O(f(z))[/tex] look here: Wikipedia article on BigO notation Using this notation you could express the example above as [tex]\csc x = \Theta(1/x)[/tex] as [tex]x \to 0[/tex]. The point of my hint is that, to prove that [tex]J_{1/2}(0) = 0[/tex], you should examine the orders of growth near zero of the functions whose product is [tex]J_{1/2}[/tex], to prove that the product can be estimated by a function you know vanishes at 0. 


#5
Dec810, 02:39 PM

P: 172

I will have a think about this method while we move on to the next part of the question.
2. By means of the substitution [tex]z = kx[/tex], show that [tex]J_{1/2}(kx)[/tex] and [tex]J_{1/2}(kx)[/tex] are solutions of the following equations: [tex]x^{2}\frac{d^{2}y}{dx^{2}} + x\frac{dy}{dx} + \left(k^{2}x^{2}  p^{2}\right)y = 0[/tex]. 3. Show that this equation, subject to boundary conditions [tex]y(0) = 0, y(1) = 0 [/tex], has eigenvalues [tex]k = n\pi, n = 1,2,3...[/tex] and eigenfunctions [tex]y_{n} = J_{1/2}(n\pi x)[/tex]. I am fine with 2, but din't know how to begin 3. 


#6
Dec810, 03:41 PM

HW Helper
P: 1,025

Try performing the limit calculation for a nore solid answer: show that [itex]\lim_{z\rightarrow 0} \left(\frac{2}{\pi z}\right)^{1/2} \sin z=1[/itex]



#7
Dec810, 04:29 PM

P: 172

[tex]\lim_{z\rightarrow 0} \left(\frac{\sin z}{z}\right)=1[/tex], so
[tex]\lim_{z\rightarrow 0} \left(\frac{2}{\pi z}\right)^{1/2} (\left \sin z \right)[/tex] = [tex]\lim_{z\rightarrow 0} \left(\frac{2z}{\pi}\right)^{1/2}\left(\frac{\sin z}{z}\right) [/tex] = [tex]\lim_{z\rightarrow 0} \left(\frac{2z}{\pi}\right)^{1/2} \lim_{z\rightarrow 0} \left(\frac{\sin z}{z}\right)[/tex] The first limit is 0 and the second is 1, so the final answer is 0: a contradiction!! Thoughts? 


#8
Dec910, 04:13 AM

P: 1,666

[tex]J_{1/2}(z)=\sqrt{\frac{2}{\pi z}}\sin(z)[/tex] is undefined at z=0. However, it's limit there is zero by L'Hospital's rule so that if we explicitly define: [tex]J_{1/2}(z)=\begin{cases} \sqrt{\frac{2}{\pi z}}\sin(z) & z>0 \\ 0 & z=0 \end{cases} [/tex] then [itex]J_{1/2}(z) [/itex] becomes continuous for real z greater than or equal to zero. and that thing about drawing separate graphs is not correct. If you plot the function, you should get a sinusoidal wave for positive real z. 


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