On entry: ifail must be set to , or to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of means that an error message is printed while a value of means that it is not.
If halting is not appropriate, the value or is recommended. If message printing is undesirable, then the value is recommended. Otherwise, the value is recommended. When the value or is used it is essential to test the value of ifail on exit.
On exit: unless the routine detects an error or a warning has been flagged (see Section 6).
6Error Indicators and Warnings
If and are the relative errors in the argument and the result, respectively, then in principle
That is, the relative error in the argument, , is amplified by a factor at least , in the result.
The equality should hold if is greater than the machine precision ( due to data errors etc.) but if is simply due to round-off in the machine representation it is possible that an extra figure may be lost in internal calculation round-off.
The behaviour of the amplification factor is shown in the following graph:
It should be noted that this factor is always less than or equal to one. For large we have the absolute error in the result, , in principle, given by
This means that eventually accuracy is limited by machine precision.
8Parallelism and Performance
s11abf is not threaded in any implementation.
This example reads values of the argument from a file, evaluates the function at each value of and prints the results.