NAG Library Routine Document

d02ubf (bvp_ps_lin_cgl_vals)


d02ubf evaluates a function, or one of its lower order derivatives, from its Chebyshev series representation at Chebyshev Gauss–Lobatto points on a,b. The coefficients of the Chebyshev series representation required are usually derived from those returned by d02uaf or d02uef.


Fortran Interface
Subroutine d02ubf ( n, a, b, q, c, f, ifail)
Integer, Intent (In):: n, q
Integer, Intent (Inout):: ifail
Real (Kind=nag_wp), Intent (In):: a, b, c(n+1)
Real (Kind=nag_wp), Intent (Out):: f(n+1)
C Header Interface
#include <nagmk26.h>
void  d02ubf_ (const Integer *n, const double *a, const double *b, const Integer *q, const double c[], double f[], Integer *ifail)


d02ubf evaluates the Chebyshev series
S x- = 12 c1 T0 x- + c2 T1 x- + c3T2 x- ++ cn+1 Tn x- ,  
or its derivative (up to fourth order) at the Chebyshev Gauss–Lobatto points on a,b. Here Tjx- denotes the Chebyshev polynomial of the first kind of degree j with argument x- defined on -1,1. In terms of your original variable, x say, the input values at which the function values are to be provided are
xr = - 12 b - a cos πr-1 /n + 1 2 b + a ,   r=1,2,,n+1 , ​  
where b and a are respectively the upper and lower ends of the range of x over which the function is required.
The calculation is implemented by a forward one-dimensional discrete Fast Fourier Transform (DFT).


Canuto C (1988) Spectral Methods in Fluid Dynamics 502 Springer
Canuto C, Hussaini M Y, Quarteroni A and Zang T A (2006) Spectral Methods: Fundamentals in Single Domains Springer
Trefethen L N (2000) Spectral Methods in MATLAB SIAM


1:     n – IntegerInput
On entry: n, where the number of grid points is n+1. This is also the largest order of Chebyshev polynomial in the Chebyshev series to be computed.
Constraint: n>0 and n is even.
2:     a – Real (Kind=nag_wp)Input
On entry: a, the lower bound of domain a,b.
Constraint: a<b.
3:     b – Real (Kind=nag_wp)Input
On entry: b, the upper bound of domain a,b.
Constraint: b>a.
4:     q – IntegerInput
On entry: the order, q, of the derivative to evaluate.
Constraint: 0q4.
5:     cn+1 – Real (Kind=nag_wp) arrayInput
On entry: the Chebyshev coefficients, ci, for i=1,2,,n+1.
6:     fn+1 – Real (Kind=nag_wp) arrayOutput
On exit: the derivatives Sq xi , for i=1,2,,n+1, of the Chebyshev series, S.
7:     ifail – IntegerInput/Output
On entry: ifail must be set to 0, -1 or 1. If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value -1 or 1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, if you are not familiar with this argument, the recommended value is 0. 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).

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:
On entry, n=value.
Constraint: n>0.
On entry, n=value.
Constraint: n is even.
On entry, a=value and b=value.
Constraint: a<b.
On entry, q=value.
Constraint: 0q4.
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.


Evaluations of DFT to obtain function or derivative values should be an order n multiple of machine precision assuming full accuracy to machine precision in the given Chebyshev series representation.

Parallelism and Performance

d02ubf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
d02ubf makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

Further Comments

The number of operations is of the order n logn  and the memory requirements are On; thus the computation remains efficient and practical for very fine discretizations (very large values of n).


See Section 10 in d02uef.