NAG FL Interface
s15asf (erf_​real_​vector)

1 Purpose

s15asf returns an array of values of the error function erfx

2 Specification

Fortran Interface
Subroutine s15asf ( n, x, f, ifail)
Integer, Intent (In) :: n
Integer, Intent (Inout) :: ifail
Real (Kind=nag_wp), Intent (In) :: x(n)
Real (Kind=nag_wp), Intent (Out) :: f(n)
C Header Interface
#include <nag.h>
void  s15asf_ (const Integer *n, const double x[], double f[], Integer *ifail)
The routine may be called by the names s15asf or nagf_specfun_erf_real_vector.

3 Description

s15asf calculates approximate values for the error function
erfx = 2π 0x e-t2 dt = 1-erfcx ,  
for an array of arguments xi, for i=1,2,,n.
Let x^ be the root of the equation erfcx-erfx=0 (then x^0.46875). For xx^ the value of erfx is based on the following rational Chebyshev expansion for erfx:
erfx xR,m x2 ,  
where R,m denotes a rational function of degree in the numerator and m in the denominator.
For x>x^ the value of erfx is based on a rational Chebyshev expansion for erfcx: for x^<x4 the value is based on the expansion
erfcx ex2 R,m x ;  
and for x>4 it is based on the expansion
erfcx ex2 x 1π + 1x2 R,m 1/x2 .  
For each expansion, the specific values of and m are selected to be minimal such that the maximum relative error in the expansion is of the order 10-d, where d is the maximum number of decimal digits that can be accurately represented for the particular implementation (see x02bef).
For xxhi there is a danger of setting underflow in erfcx (the value of xhi is given in the Users' Note for your implementation). For xxhi, s15asf returns erfx=1; for x-xhi it returns erfx=-1.

4 References

NIST Digital Library of Mathematical Functions
Cody W J (1969) Rational Chebyshev approximations for the error function Math.Comp. 23 631–637

5 Arguments

1: n Integer Input
On entry: n, the number of points.
Constraint: n0.
2: xn Real (Kind=nag_wp) array Input
On entry: the argument xi of the function, for i=1,2,,n.
3: fn Real (Kind=nag_wp) array Output
On exit: erfxi, the function values.
4: ifail Integer Input/Output
On entry: ifail must be set to 0, -1 or 1 to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of 0 causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of -1 means that an error message is printed while a value of 1 means that it is not.
If halting is not appropriate, the value -1 or 1 is recommended. If message printing is undesirable, then the value 1 is recommended. Otherwise, the value 0 is recommended. When the value -1 or 1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry ifail=0 or -1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
ifail=1
On entry, n=value.
Constraint: n0.
ifail=-99
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
ifail=-399
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
ifail=-999
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

See Section 7 in s15arf.

8 Parallelism and Performance

s15asf is not threaded in any implementation.

9 Further Comments

None.

10 Example

This example reads values of x from a file, evaluates the function at each value of xi and prints the results.

10.1 Program Text

Program Text (s15asfe.f90)

10.2 Program Data

Program Data (s15asfe.d)

10.3 Program Results

Program Results (s15asfe.r)