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Debye's Asymptotic Representation


Debye's asymptotic representation is an asymptotic expansion for a Hankel function of the first kind with nu approx x. For 1-nu/x>epsilon, nu/x=sinalpha, 1-(nu/x)>(3/x)nu^(1/2), and X=sqrt(-xcos(alpha/2)),

 H_nu^((1))(x)∼1/(sqrt(pi))exp{ix[cosalpha+(alpha-1/2pi)sinalpha]}[(e^(ipi/4))/X+(1/8+5/(24)tan^2alpha)(3e^(3pii/4))/(2X^3)+(3/(128)+(77)/(576)tan^alpha+(385)/(3456)tan^4alpha)(3·5e^(5pii/4))/(2^2X^5)+...].
(1)

For (nu/x)-1>epsilon, nu/x=coshsigma, |nu^2-x^2|^(1/2)>>1, |nu^2-x^2|^(3/2)nu^(-2)>>1, and X=sqrt(-xsinh(sigma/2)),

 H_nu^((1))(x)∼1/(sqrt(pi))exp[x(sigmacoshsigma-sinhsigma)][1/X+(1/8-5/(24)coth^2sigma)3/(2X^3)+(3/(128)-(77)/(576)coth^2sigma+(385)/(3456)coth^4sigma)(3·5)/(2^2X^5)+...].
(2)

Finally, for |x-nu|<<x^(1/3), x>>1, and x-nu=delta,

 H_nu^((1))∼(6^(1/3)e^(ipi/3))/(pisqrt(3))[Gamma(1/3)x^(-1/3)-6^(1/3)e^(ipi/3)deltaGamma(2/3)x^(-2/3)+(2/5delta-delta^3)Gamma(4/3)x^(-4/3)+(3/(140)-1/4delta^2+1/4delta^4)6^(1/3)e^(ipi/3)Gamma(5/3)x^(-5/3)+...].
(3)

See also

Hankel Function of the First Kind

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References

Itô, K. (Ed.). Encyclopedic Dictionary of Mathematics, 2nd ed., Vol. 4. Cambridge, MA: MIT Press, p. 1805, 1986.

Referenced on Wolfram|Alpha

Debye's Asymptotic Representation

Cite this as:

Weisstein, Eric W. "Debye's Asymptotic Representation." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DebyesAsymptoticRepresentation.html

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