On exit: contains the value of the spline at the point , for .
10: WRK() – REAL (KIND=nag_wp) arrayWorkspace
11: IWRK() – INTEGER arrayWorkspace
12: IFAIL – INTEGERInput/Output
On entry: IFAIL 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.
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 or , explanatory error messages are output on the current error message unit (as defined by X04AAF).
Errors or warnings detected by the routine:
On entry, the knots in array LAMDA, or those in array MU, are not in nondecreasing order, or , or .
On entry, at least one of the prescribed points lies outside the rectangle defined by , and , .
The method used to evaluate the B-splines is numerically stable, in the sense that each computed value of can be regarded as the value that would have been obtained in exact arithmetic from slightly perturbed B-spline coefficients. See Cox (1978) for details.
8 Further Comments
Computation time is approximately proportional to the number of points, , at which the evaluation is required.
This program reads in knot sets and , and a set of bicubic spline coefficients . Following these are a value for and the coordinates , for , at which the spline is to be evaluated.