KELVIN_KEI0 Function
Evaluates the Kelvin function of the second kind, kei, of order zero.
Usage
result = KELVIN_KEI0(x)
Input Parameters
x—Argument for which the function value is desired.
Returned Value
result—The value of the Kelvin function of the second kind, kei, of order zero evaluated at x.
Input Keywords
Double—If present and nonzero, double precision is used.
Derivative—If present and nonzero, then the derivative of the Kelvin function of the second kind, kei, of order zero evaluated at x is computed.
Discussion
The modified Kelvin function kei0(x) is defined to be 
. The Bessel function K0(x) is defined as:
. The Bessel function K0(x) is defined as:
If the keyword Derivative is set, the function kei0'(x) is defined to be:
The function KELVIN_KEI0 is based on the work of Burgoyne (1963).
If x < 0, NaN (Not a Number) is returned. If x ≥ 119, zero is returned.
Example
In this example, kei0(0.4) and kei0'(0.6) are evaluated.
PRINT, KELVIN_KEI0(0.4)
; PV-WAVE prints: -0.703800
PRINT, KELVIN_KEI0(0.6, /DERIVATIVE)
; PV-WAVE prints: 0.348164
