NAG Library Routine Document
E02BCF
1 Purpose
E02BCF evaluates a cubic spline and its first three derivatives from its B-spline representation.
2 Specification
INTEGER |
NCAP7, LEFT, IFAIL |
REAL (KIND=nag_wp) |
LAMDA(NCAP7), C(NCAP7), X, S(4) |
|
3 Description
E02BCF evaluates the cubic spline
and its first three derivatives at a prescribed argument
. It is assumed that
is represented in terms of its B-spline coefficients
, for
and (augmented) ordered knot set
, for
,
(see
E02BAF),
i.e.,
Here
,
is the number of intervals of the spline and
denotes the normalized B-spline of degree
(order
) defined upon the knots
. The prescribed argument
must satisfy
At a simple knot (i.e., one satisfying ), the third derivative of the spline is in general discontinuous. At a multiple knot (i.e., two or more knots with the same value), lower derivatives, and even the spline itself, may be discontinuous. Specifically, at a point where (exactly) knots coincide (such a point is termed a knot of multiplicity ), the values of the derivatives of order , for , are in general discontinuous. (Here ; is not meaningful.) You must specify whether the value at such a point is required to be the left- or right-hand derivative.
The method employed is based upon:
(i) |
carrying out a binary search for the knot interval containing the argument (see Cox (1978)), |
(ii) |
evaluating the nonzero B-splines of orders , , and by recurrence (see Cox (1972) and Cox (1978)), |
(iii) |
computing all derivatives of the B-splines of order by applying a second recurrence to these computed B-spline values (see de Boor (1972)), |
(iv) |
multiplying the fourth-order B-spline values and their derivative by the appropriate B-spline coefficients, and summing, to yield the values of and its derivatives. |
E02BCF can be used to compute the values and derivatives of cubic spline fits and interpolants produced by
E02BAF.
If only values and not derivatives are required,
E02BBF may be used instead of E02BCF, which takes about
longer than
E02BBF.
4 References
Cox M G (1972) The numerical evaluation of B-splines J. Inst. Math. Appl. 10 134–149
Cox M G (1978) The numerical evaluation of a spline from its B-spline representation J. Inst. Math. Appl. 21 135–143
de Boor C (1972) On calculating with B-splines J. Approx. Theory 6 50–62
5 Parameters
- 1: NCAP7 – INTEGERInput
On entry: , where is the number of intervals of the spline (which is one greater than the number of interior knots, i.e., the knots strictly within the range to over which the spline is defined).
Constraint:
.
- 2: LAMDA(NCAP7) – REAL (KIND=nag_wp) arrayInput
On entry: must be set to the value of the th member of the complete set of knots, , for .
Constraint:
the must be in nondecreasing order with .
- 3: C(NCAP7) – REAL (KIND=nag_wp) arrayInput
On entry: the coefficient
of the B-spline , for . The remaining elements of the array are not referenced.
- 4: X – REAL (KIND=nag_wp)Input
On entry: the argument at which the cubic spline and its derivatives are to be evaluated.
Constraint:
.
- 5: LEFT – INTEGERInput
On entry: specifies whether left- or right-hand values of the spline and its derivatives are to be computed (see
Section 3). Left- or right-hand values are formed according to whether
LEFT is equal or not equal to
.
If
does not coincide with a knot, the value of
LEFT is immaterial.
If , right-hand values are computed.
If
, left-hand values are formed, regardless of the value of
LEFT.
- 6: S() – REAL (KIND=nag_wp) arrayOutput
On exit: contains the value of the th derivative of the spline at the argument , for . Note that contains the value of the spline.
- 7: 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:
-
, i.e., the number of intervals is not positive.
Either
, i.e., the range over which
is defined is null or negative in length, or
X is an invalid argument, i.e.,
or
.
7 Accuracy
The computed value of
has negligible error in most practical situations. Specifically, this value has an
absolute error bounded in modulus by
, where
is the largest in modulus of
and
, and
is an integer such that
. If
and
are all of the same sign, then the computed value of
has
relative error bounded by
. For full details see
Cox (1978).
No complete error analysis is available for the computation of the derivatives of . However, for most practical purposes the absolute errors in the computed derivatives should be small.
The time taken is approximately linear in .
Note: the routine does not test all the conditions on the knots given in the description of
LAMDA in
Section 5, since to do this would result in a computation time approximately linear in
instead of
. All the conditions are tested in
E02BAF, however.
9 Example
Compute, at the arguments
,
,
,
,
,
,
,
the left- and right-hand values and first derivatives of the cubic spline defined over the interval having the interior knots
,
,
,
,
,
, the additional knots
,
,
,
,
,
,
,
, and the B-spline coefficients
,
,
,
,
,
,
,
,
,
.
The input data items (using the notation of
Section 5) comprise the following values in the order indicated:
|
|
, |
for |
, |
for |
, |
for |
This example program is written in a general form that will enable the values and derivatives of a cubic spline having an arbitrary number of knots to be evaluated at a set of arbitrary points. Any number of datasets may be supplied.
The only changes required to the program relate to the dimensions of the arrays
LAMDA and
C.
9.1 Program Text
Program Text (e02bcfe.f90)
9.2 Program Data
Program Data (e02bcfe.d)
9.3 Program Results
Program Results (e02bcfe.r)