I open this good discussion, and hope can get a nice and strong idea in this domain of approximation of Hankel funciton and order truncation of infinite series. Thanks for all idea, can improve the discussion.
Using the asymptotics of Hankel function for large argument and large orders ( both together) and
find an order of truncation N of the obove series so that we can ensure an error bound of epsilon( epsilon very small given). abs(sum ( c[m]*HankelH1(m,x)*exp(I*m*theta), m=-infinity..infinity)-sum ( c[m]*HankelH1(m,x)*exp(I*m*theta), m=-N..N))<epsilon.
A first idea come in mind: the series converge, so that the general terms of this series converge to zero, and in particularity, abs(c[m]* HankelH1(m,x))<1:
then abs(c[m])<1/ abs(HankelH1(m,x)).
So we can ensure an error bound of epsilon on the coefficient c[m] by imposing (HankelH1(m,x))<epsilon this lead to abs(c[m])<epsilon.
I consider the case where m and x are very large, I can suppose for example m=x*(1+zeta), with 0<zeta<1. zeta parameter. So that our truncation N depend on zeta.
And then how can I find and approximation of the inverse of Hankel function for large argument and large order. using m=x*(1+zeta). I think this give us N the truncation order.
I hope get a good discussion in this subjet.
Of course maybe there are other strong idea to compute the truncation series.
I get the following error in the code:
Error, (in MultiSeries:-multiseries) unable to expand with respect to parameter
###### Code### and error
assume(0 <= x);
assume(0 <= zeta<1);