Actually it just the parameterization that is causing trouble near k=0
let u = (x-z)/a
then the problematic part of your function is
(1- k*u)^(1/k)
take the log to get
log(1-k*u)/k
= -(k*u +k^2*u^2/2 + ...)/k
= -(u +k u^2/2 + ..)
so your function is exp(-u - ku^2/2 - ...)
and use that for k<eps for some small eps. You can add the right number of
terms and fix the size of eps
so that the function behaves well.
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