Welcome to the Harris Geospatial product documentation center. Here you will find reference guides, help documents, and product libraries.


  >  Docs Center  >  IDL Reference  >  Advanced Math and Stats  >  IMSL_BESSK_EXP

IMSL_BESSK_EXP

IMSL_BESSK_EXP

The IMSL_BESSK_EXP function evaluates the exponentially scaled modified Bessel function of the third kind of orders zero and one.



Syntax


Result = IMSL_BESSK_EXP(order, x [, /DOUBLE])

Return Value


The value of the exponentially scaled Bessel function exK0(x) or exK1(x)

Arguments


order

Order of the function. The order must be either zero or one.

x

Argument for which the function value is desired.

Keywords


DOUBLE

If present and nonzero, double precision is used.

Discussion


If the argument order is zero, the Bessel function K0(x) is defined to be:

If order is one, the value of the Bessel function K1(x):

The argument x must be greater than zero for the result to be defined.

Example


The expression:

is computed directly by calling IMSL_BESSK_EXP, and indirectly by calling IMSL_BESSK. The absolute difference is printed. For large x, the internal scaling provided by IMSL_BESSK_EXP avoids underflow that may occur in IMSL_BESSK.

ans = IMSL_BESSK_EXP(0, 0.5)  
error = ABS(ans - (EXP(0.5))*IMSL_BESSK(0, 0.5))
PRINT, ans
   1.52411
PRINT, ';Error =', error
Error = 1.1920929e-07

Errors


Fatal Errors

MATH_SMALL_ARG_OVERFLOW—The argument x must be large enough (x > max
(1/b, s) where s is the smallest representable positive number and b is the largest representable number) that K1(x) does not overflow.

Version History


6.4
Introduced



© 2018 Harris Geospatial Solutions, Inc. |  Privacy Policy
My Account    |    Store    |    Contact Us