On the asymptotic behavior of the Fourier coefficients of $(1-R\cos\theta)^{-3/2}$ Today in class I showed some ways for dealing with the classical integral $\int_{0}^{2\pi}\frac{d\theta}{(A+B\cos\theta)^2}$ under the constraints $A>B>0$, including


*

*Symmetry and the tangent half-angle substitution;

*Relating the integral to the area enclosed by an ellipse, via the polar form of an ellipse with respect to a focus and the formula $\text{Area}=\pi a b =\frac{1}{2}\int_{0}^{2\pi}\rho(\theta)^2\,d\theta$; 

*Computing the geometric-like series $\sum_{n\geq 0} r^n \sin(n\theta)$ and applying Parseval's identity to it;

*Applying Feynman's trick (differentiation under the integral sign) to $\int_{0}^{2\pi}\frac{d\theta}{1-R\cos\theta}$ which is an elementary integral due to point 1.


I finished the lesson by remarking that the point $4.$ allows to compute $\int_{0}^{2\pi}\frac{d\theta}{\left(1-R\cos\theta\right)^3}$ almost without extra efforts, while the $L^2$ machinery (point 3.) does not seem to grant the same. Apparently I dug my own grave, since someone readily asked (with the assumption $R\in(-1,1)$)

What is the asymptotic behavior of the coefficients $c_n$ in
  $$ \frac{1}{\left(1-R\cos\theta\right)^{3/2}}= c_0+\sum_{n\geq 1}c_n \cos(n\theta) $$
  ? (Q2) Do we get something interesting by following the "unnatural" approach of applying Parseval's identity to such Fourier (cosine) series?

At the moment I replied that the Paley-Wiener theorem ensures an exponential decay of the $c_n$s, and with just a maybe to the second question. Later I figured out a pretty technical proof (through hypergeometric functions) of 
$$ |c_n| \sim K_R\cdot\sqrt{n}\cdot\left(\frac{|R|}{1+\sqrt{1-R^2}}\right)^n \quad \text{as }n\to +\infty$$
and the fact that $c_n$ is given by a linear combination of complete elliptic integrals of the first and second kind. (Q1) I would like to know if there is a more elementary way for deriving the previous asymptotic behaviour. And the outcome of (Q2).

I don't think this is more elementary, but it may be a little more straightforward.  Start with Heine's toroidal identity,
$$ \frac{1}{\sqrt{z-\cos\,t}} = \frac{\sqrt{2}}{\pi} \sum_{m=-\infty}^\infty\,Q_{m-1/2}(z)\,\exp{(i \, m\, t}), \, \, |z|\ge1,  \, 0\le t \le 2\pi \, .$$
Splitting the summation into positive and negative $m$ and using the fact that for the toroidal function (Gradshteyn & Rhyzhik 8.737.4) 
$Q_{-(m-1/2)-1}(z) = Q_{m-1/2}(z)$ for integer $m$ then one obtains
$$ \frac{1}{\sqrt{z-\cos\,t}} = \frac{\sqrt{2}}{\pi}\Big(2 \sum_{m=1}^\infty\,Q_{m-1/2}(z)\,\cos{( m\, t)}+  Q_{-1/2}(z)\Big) .$$
Differentiate both sides with respect to $z$ and use G&R 8.706.1 (with the fact that $z>1$ to get the phase correct) to get
$$ (z-\cos\,t)^{-3/2} = \frac{-2\sqrt{2}}{\pi\,\sqrt{z^2-1}}\Big(2 \sum_{m=1}^\infty\,Q_{m-1/2}^{\,1}(z)\,\cos{( m\, t)}+  Q_{-1/2}^{\,1}(z)\Big) .$$
From the problem's statement, with $z=1/R,$ we see that the associated toroidal function $Q_{m-1/2}^{\,1}(z)$ must be asymptotically estimated.  G&R 8.777.2 has a hypergeometric representation that has an asymptotic nature for large $m,$
$$Q_{m-1/2}^{\,1}(z)=-2\sqrt{\pi}\,\frac{\Gamma(m+3/2)}{\Gamma(m+1)} 
\,\frac{\sqrt{z^2-1}}{\zeta^{\,m+3/2}} F\big(3/2,m+3/2; m+1; 1/\zeta^2\big)$$
where $\zeta = z+\sqrt{z^2-1} > 1.$  For convenience set $y=1/\zeta^2$ and expand the ration of Pochhamer symbols in the hypergeometric function as
$$\frac{(m+3/2)_k}{(m+1)_k} \sim 1 + \frac{k}{2m} + \, ...  \,\, m\to \infty.$$ Therefore
$$F\big(3/2,m+3/2; m+1; 1/\zeta^2\big)\sim \sum_{k=0}^\infty \dfrac{(3/2)_k}{k!} \big(1+\frac{k}{2m}\big)y^k= (1-y)^{-3/2}\Big(1+\frac{3y}{4m(1-y)} \Big).
$$
For this to be a useful asymptotic expression, $y$ needs to bounded away from 1, which implies $z$ must be bounded from 1, and $R$ must be bounded from 1.
Writing 
$$ (z-\cos\,t)^{-3/2} =d_0(z)+\sum_{m=1}^\infty d_m(z)\,\cos{(m\,t)} $$
then, using only the first term of the expression above and the first in the asymptotic expansion of the ratio of gamma functions, 
$$ d_m(z) \sim \frac{8\sqrt{2\, m}}{\sqrt{\pi}} (z+\sqrt{z^2-1})^{-3/2}(1-y)^{3/2}$$
In the proposer's notation it is clear to see that
$$c_m = R^{-3/2}d_m(z \to 1/R)$$ and, with some simplification one finds
$$c_m \sim \frac{4}{\sqrt{\pi}} \,\sqrt{m}\, (1-R^2)^{-3/4}
\Big(\frac{R}{1+\sqrt{1-R^2}}\Big)^m .$$
This expression was derived for $z>1$ implying $0\le R<1.$  To extend to
 $-1\le R \le 0,$ simply let $t \to t+\pi$ and a factor of $(-1)^m$ is introduced into the summand.  It is then clear that the $(-1)^m$ can be incorporated into the final expression as the proposer did, by making the unsigned $R$ an absolute value.
I have checked many of the intermediate steps in Mathematica, but to do so one must use $Q_{m-1/2}^{\,1}(z)=$ LegendreQ[m-1/2, 1, 3, z].
