e02 Chapter Contents
e02 Chapter Introduction
NAG C Library Manual

# NAG Library Function Documentnag_1d_cheb_deriv (e02ahc)

## 1  Purpose

nag_1d_cheb_deriv (e02ahc) determines the coefficients in the Chebyshev series representation of the derivative of a polynomial given in Chebyshev series form.

## 2  Specification

 #include #include
 void nag_1d_cheb_deriv (Integer n, double xmin, double xmax, const double a[], Integer ia1, double *patm1, double adif[], Integer iadif1, NagError *fail)

## 3  Description

nag_1d_cheb_deriv (e02ahc) forms the polynomial which is the derivative of a given polynomial. Both the original polynomial and its derivative are represented in Chebyshev series form. Given the coefficients ${a}_{\mathit{i}}$, for $\mathit{i}=0,1,\dots ,n$, of a polynomial $p\left(x\right)$ of degree $n$, where
 $px=12a0+a1T1x-+⋯+anTnx-$
the function returns the coefficients ${\stackrel{-}{a}}_{\mathit{i}}$, for $\mathit{i}=0,1,\dots ,n-1$, of the polynomial $q\left(x\right)$ of degree $n-1$, where
 $qx=dpx dx =12a-0+a-1T1x-+⋯+a-n-1Tn-1x-.$
Here ${T}_{j}\left(\stackrel{-}{x}\right)$ denotes the Chebyshev polynomial of the first kind of degree $j$ with argument $\stackrel{-}{x}$. It is assumed that the normalized variable $\stackrel{-}{x}$ in the interval $\left[-1,+1\right]$ was obtained from your original variable $x$ in the interval $\left[{x}_{\mathrm{min}},{x}_{\mathrm{max}}\right]$ by the linear transformation
 $x-=2x-xmax+xmin xmax-xmin$
and that you require the derivative to be with respect to the variable $x$. If the derivative with respect to $\stackrel{-}{x}$ is required, set ${x}_{\mathrm{max}}=1$ and ${x}_{\mathrm{min}}=-1$.
Values of the derivative can subsequently be computed, from the coefficients obtained, by using nag_1d_cheb_eval2 (e02akc).
The method employed is that of Chebyshev series (see Chapter 8 of Modern Computing Methods (1961)), modified to obtain the derivative with respect to $x$. Initially setting ${\stackrel{-}{a}}_{n+1}={\stackrel{-}{a}}_{n}=0$, the function forms successively
 $a-i-1=a-i+1+2xmax-xmin 2iai, i=n,n-1,…,1.$

## 4  References

Modern Computing Methods (1961) Chebyshev-series NPL Notes on Applied Science 16 (2nd Edition) HMSO

## 5  Arguments

1:     nIntegerInput
On entry: $n$, the degree of the given polynomial $p\left(x\right)$.
Constraint: ${\mathbf{n}}\ge 0$.
2:     xmindoubleInput
3:     xmaxdoubleInput
On entry: the lower and upper end points respectively of the interval $\left[{x}_{\mathrm{min}},{x}_{\mathrm{max}}\right]$. The Chebyshev series representation is in terms of the normalized variable $\stackrel{-}{x}$, where
 $x-=2x-xmax+xmin xmax-xmin .$
Constraint: ${\mathbf{xmax}}>{\mathbf{xmin}}$.
4:     a[$\mathit{dim}$]const doubleInput
Note: the dimension, dim, of the array a must be at least $\left(1+\left({\mathbf{n}}+1-1\right)×{\mathbf{ia1}}\right)$.
On entry: the Chebyshev coefficients of the polynomial $p\left(x\right)$. Specifically, element $\mathit{i}×{\mathbf{ia1}}$ of a must contain the coefficient ${a}_{\mathit{i}}$, for $\mathit{i}=0,1,\dots ,n$. Only these $n+1$ elements will be accessed.
5:     ia1IntegerInput
On entry: the index increment of a. Most frequently the Chebyshev coefficients are stored in adjacent elements of a, and ia1 must be set to $1$. However, if, for example, they are stored in ${\mathbf{a}}\left[0\right],{\mathbf{a}}\left[3\right],{\mathbf{a}}\left[6\right],\dots \text{}$, then the value of ia1 must be $3$. See also Section 8.
Constraint: ${\mathbf{ia1}}\ge 1$.
6:     patm1double *Output
On exit: the value of $p\left({x}_{\mathrm{min}}\right)$. If this value is passed to the integration function nag_1d_cheb_intg (e02ajc) with the coefficients of $q\left(x\right)$, then the original polynomial $p\left(x\right)$ is recovered, including its constant coefficient.
7:     adif[$\mathit{dim}$]doubleOutput
Note: the dimension, dim, of the array adif must be at least $\left(1+\left({\mathbf{n}}+1-1\right)×{\mathbf{iadif1}}\right)$.
On exit: the Chebyshev coefficients of the derived polynomial $q\left(x\right)$. (The differentiation is with respect to the variable $x$.) Specifically, element $\mathit{i}×{\mathbf{iadif1}}$ of adif contains the coefficient ${\stackrel{-}{a}}_{\mathit{i}}$, for $\mathit{i}=0,1,\dots ,n-1$. Additionally, element $n×{\mathbf{iadif1}}$ is set to zero.
On entry: the index increment of adif. Most frequently the Chebyshev coefficients are required in adjacent elements of adif, and iadif1 must be set to $1$. However, if, for example, they are to be stored in ${\mathbf{adif}}\left[0\right],{\mathbf{adif}}\left[3\right],{\mathbf{adif}}\left[6\right],\dots \text{}$, then the value of iadif1 must be $3$. See Section 8.
Constraint: ${\mathbf{iadif1}}\ge 1$.
9:     failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

## 6  Error Indicators and Warnings

On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.
NE_INT
On entry, ${\mathbf{ia1}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{ia1}}\ge 1$.
On entry, ${\mathbf{iadif1}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{iadif1}}\ge 1$.
On entry, ${\mathbf{n}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{n}}\ge 0$.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
NE_REAL_2
On entry, ${\mathbf{xmax}}=〈\mathit{\text{value}}〉$ and ${\mathbf{xmin}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{xmax}}>{\mathbf{xmin}}$.

## 7  Accuracy

There is always a loss of precision in numerical differentiation, in this case associated with the multiplication by $2i$ in the formula quoted in Section 3.

## 8  Further Comments

The time taken is approximately proportional to $n+1$.
The increments ia1, iadif1 are included as arguments to give a degree of flexibility which, for example, allows a polynomial in two variables to be differentiated with respect to either variable without rearranging the coefficients.

## 9  Example

Suppose a polynomial has been computed in Chebyshev series form to fit data over the interval $\left[-0.5,2.5\right]$. The following program evaluates the first and second derivatives of this polynomial at $4$ equally spaced points over the interval. (For the purposes of this example, xmin, xmax and the Chebyshev coefficients are simply supplied . Normally a program would first read in or generate data and compute the fitted polynomial.)

### 9.1  Program Text

Program Text (e02ahce.c)

None.

### 9.3  Program Results

Program Results (e02ahce.r)