MathGroup Archive 1989

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RE: BesselK[1/2,x]

> >From Jim_Wendel at Tue Mar 21 06:19:52 1989
> Subject: Re: Chebyshev "bug"
> ...
> Plot[BesselK[1/2,x],{x,20,25}] came out beautifully, values from 0 down to
> -0.0000175 on the vertical axis.
Except that BesselK[1/2,x] is positive for x > 0.
Abramowitz & Stegun, pp. 444, give a formula for
BesselK[1/2,x] in terms of Exp[-x] namely

	10.2.17.   sqrt(Pi/2/x) K(1/2,x) = Pi/2/x exp(-x)

from which you can deduce this.
I tried the Plot on our Mac II (version 1.1 of Mathematica)
and it gives random noise beyond x = 21.

mbmonagan at

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