# Asymptotic behavior of coefficients

## Main Question or Discussion Point

The eigenvalue problem of Schroedinger equation can be solved in a variety of ways. The continued fraction method can be stated by the following recipe:

- represent the solution of the D.E. as a power series

- replace back this solution into the D.E.

- obtain a three term recurrence relation by aligning its indices;

- obtain the asymptotic behavior of the two coefficients -- the general asymptotic behavior is a combination of these terms

- check if the limit of the ratio of these coefficients as n tends to infinity is zero

- case positive, the solution used in the numerator is the minimal solution

- the quantization condition is given by c_1(lambda)=0;

- calculate the modulus of the ratio between successive terms of these solutions

- check the series convergence using these values (make a series expansion if necessary);

- if it converges then we can invoke Pincherle's theorem to create a continued fraction expansion and thereby a transcendental equation for finding the eigenvalues.

My question is:

Given the difference equation

$$a_{n+2}+A_n(\lambda)a_{n+1}+B_n(\lambda)a_n=0$$

where

$$A_n(\lambda)=-\frac{(n+1)(2\delta+\epsilon+3(n+\gamma))+Q}{s(n+2)(n+1+\gamma)}$$

and

$$B_n(\lambda)=\frac{(n+\alpha)(n+\beta)}{2(n+2)(n+1+\gamma)}$$

I know (see http://arxiv.org/abs/hep-th/0207133v2) that the asymptotic behavior of the coefficients is given by

$$a_n^{(1)}\sim 2^{-n}n^{-1-\lambda/2}\sum_{s=0}^{\infty}\frac{c_s^{(1)}}{n^s}$$

and

$$a_n^{(2)}\sim n^{-3}\sum_{s=0}^{\infty}\frac{c_s^{(2)}}{n^s}$$

I have to do the a similar calculation in my research project but I couldn't find out the procedure used to calculate the asymptotic expansion above. Please, can someone show me the steps to such a solution?

I tried to follow the text by Saber Elaydi, An Introduction to Difference Equations without success.

Last edited:

Related Quantum Physics News on Phys.org

Avodyne