NAG Library Routine Document
S18AFF returns a value for the modified Bessel function , via the function name.
|REAL (KIND=nag_wp) S18AFF
S18AFF evaluates an approximation to the modified Bessel function of the first kind .
Note: , so the approximation need only consider .
The routine is based on three Chebyshev expansions:
. This approximation is used when
is sufficiently small for the result to be correct to machine precision
For large , the routine must fail because cannot be represented without overflow.
Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
- 1: X – REAL (KIND=nag_wp)Input
On entry: the argument of the function.
- 2: IFAIL – INTEGERInput/Output
must be set to
. If you are unfamiliar with this parameter you should refer to Section 3.3
in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value
is recommended. If the output of error messages is undesirable, then the value
is recommended. Otherwise, if you are not familiar with this parameter, the recommended value is
. When the value is used it is essential to test the value of IFAIL on exit.
unless the routine detects an error or a warning has been flagged (see Section 6
6 Error Indicators and Warnings
If on entry
, explanatory error messages are output on the current error message unit (as defined by X04AAF
Errors or warnings detected by the routine:
is too large. On soft failure the routine returns the approximate value of
at the nearest valid argument. See also the Users' Note
for your implementation.
Let and be the relative errors in the argument and result respectively.
is somewhat larger than the machine precision
is due to data errors etc.), then
are approximately related by:
shows the behaviour of the error amplification factor
is of the same order as machine precision
, then rounding errors could make
slightly larger than the above relation predicts.
For small , and there is no amplification of errors.
For large , and we have strong amplification of errors. However the routine must fail for quite moderate values of because would overflow; hence in practice the loss of accuracy for large is not excessive. Note that for large , the errors will be dominated by those of the standard function exp.
This example reads values of the argument from a file, evaluates the function at each value of and prints the results.
9.1 Program Text
Program Text (s18affe.f90)
9.2 Program Data
Program Data (s18affe.d)
9.3 Program Results
Program Results (s18affe.r)