NAG CL Interface
s18dcc (bessel_k_complex)
1
Purpose
s18dcc returns a sequence of values for the modified Bessel functions for complex , non-negative and , with an option for exponential scaling.
2
Specification
void |
s18dcc (double fnu,
Complex z,
Integer n,
Nag_ScaleResType scal,
Complex cy[],
Integer *nz,
NagError *fail) |
|
The function may be called by the names: s18dcc, nag_specfun_bessel_k_complex or nag_complex_bessel_k.
3
Description
s18dcc evaluates a sequence of values for the modified Bessel function , where is complex, , and is the real, non-negative order. The -member sequence is generated for orders , . Optionally, the sequence is scaled by the factor .
The function is derived from the function CBESK in
Amos (1986).
Note: although the function may not be called with less than zero, for negative orders the formula may be used.
When is greater than , extra values of are computed using recurrence relations.
For very large or , argument reduction will cause total loss of accuracy, and so no computation is performed. For slightly smaller or , the computation is performed but results are accurate to less than half of machine precision. If is very small, near the machine underflow threshold, or is too large, there is a risk of overflow and so no computation is performed. In all the above cases, a warning is given by the function.
4
References
Amos D E (1986) Algorithm 644: A portable package for Bessel functions of a complex argument and non-negative order ACM Trans. Math. Software 12 265–273
5
Arguments
-
1:
– double
Input
-
On entry: , the order of the first member of the sequence of functions.
Constraint:
.
-
2:
– Complex
Input
-
On entry: the argument of the functions.
Constraint:
.
-
3:
– Integer
Input
-
On entry: , the number of members required in the sequence .
Constraint:
.
-
4:
– Nag_ScaleResType
Input
-
On entry: the scaling option.
- The results are returned unscaled.
- The results are returned scaled by the factor .
Constraint:
or .
-
5:
– Complex
Output
-
On exit: the required function values: contains
, for .
-
6:
– Integer *
Output
-
On exit: the number of components of
cy that are set to zero due to underflow. If
and
, elements
are set to zero. If
,
nz simply states the number of underflows, and not which elements they are.
-
7:
– NagError *
Input/Output
-
The NAG error argument (see
Section 7 in the Introduction to the NAG Library CL Interface).
6
Error Indicators and Warnings
- NE_ALLOC_FAIL
-
Dynamic memory allocation failed.
See
Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
- NE_BAD_PARAM
-
On entry, argument had an illegal value.
- NE_COMPLEX_ZERO
-
On entry, .
- NE_INT
-
On entry, .
Constraint: .
- NE_INTERNAL_ERROR
-
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact
NAG for assistance.
See
Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
- NE_NO_LICENCE
-
Your licence key may have expired or may not have been installed correctly.
See
Section 8 in the Introduction to the NAG Library CL Interface for further information.
- NE_OVERFLOW_LIKELY
-
No computation because .
No computation because is too large.
- NE_REAL
-
On entry, .
Constraint: .
- NE_TERMINATION_FAILURE
-
No computation – algorithm termination condition not met.
- NE_TOTAL_PRECISION_LOSS
-
No computation because .
No computation because .
- NW_SOME_PRECISION_LOSS
-
Results lack precision because .
Results lack precision because .
7
Accuracy
All constants in s18dcc are given to approximately digits of precision. Calling the number of digits of precision in the floating-point arithmetic being used , then clearly the maximum number of correct digits in the results obtained is limited by . Because of errors in argument reduction when computing elementary functions inside s18dcc, the actual number of correct digits is limited, in general, by , where represents the number of digits lost due to the argument reduction. Thus the larger the values of and , the less the precision in the result. If s18dcc is called with , then computation of function values via recurrence may lead to some further small loss of accuracy.
If function values which should nominally be identical are computed by calls to
s18dcc with different base values of
and different
n, the computed values may not agree exactly. Empirical tests with modest values of
and
have shown that the discrepancy is limited to the least significant
–
digits of precision.
8
Parallelism and Performance
s18dcc is not threaded in any implementation.
The time taken for a call of
s18dcc is approximately proportional to the value of
n, plus a constant. In general it is much cheaper to call
s18dcc with
n greater than
, rather than to make
separate calls to
s18dcc.
Paradoxically, for some values of
and
, it is cheaper to call
s18dcc with a larger value of
n than is required, and then discard the extra function values returned. However, it is not possible to state the precise circumstances in which this is likely to occur. It is due to the fact that the base value used to start recurrence may be calculated in different regions for different
n, and the costs in each region may differ greatly.
Note that if the function required is
or
, i.e.,
or
, where
is real and positive, and only a single function value is required, then it may be much cheaper to call
s18acc,
s18adc,
s18ccc or
s18cdc, depending on whether a scaled result is required or not.
10
Example
The example program prints a caption and then proceeds to read sets of data from the input data stream. The first datum is a value for the order
fnu, the second is a complex value for the argument,
z, and the third is a character value
used as a flag
to set the argument
scal. The program calls the function with
to evaluate the function for orders
fnu and
, and it prints the results. The process is repeated until the end of the input data stream is encountered.
10.1
Program Text
10.2
Program Data
10.3
Program Results