# Chapter 11Chebyshev Approximations

This chapter describes the routines for computing Chebyshev approximations to univariate functions provided by the PLT Scheme Science Collection. A Chebyshev approximation is a trucvation of the series

where the Chebyshev polymonials Tn(x) = cos(n arccos x) provides an orthogonal basis of polynomials in the interval [ - 1,1] with the weight function 1 / (1 - x2)1/2. The first few Chebyshev ploymonials are, T0(x) = 1, T1(x) = x, T2(x) = 2 x2 - 1. For more information see Abramowitz and Stegan, Chapter 22.

The functions described in this chapter are defined in the chebyshev.ss file in the science collection and are made available using the form:

(require (planet "chebyshev.ss" ("williams" "science.plt")))


## 11.1  The chebyshev-series Structure

chebyshev-series

 Structure: chebyshev-series Contract: (struct chebyshev-series ((coefficient (vectorof real?)) (order natural-number?) (lower real?) (upper real?)))
This structure defines a Chebyshev Series.
 coefficients - a vector of length order containing the coefficients for the Chebyshev series. order - The order of the Chebyshev series. lower - The lower bound on the interval over which the Chebyshev series is defined. upper - The upper bound on the interval over which the Chebyshev series is defined.

The approximations are made over the range [lower, upper] using order + 1 terms, including coefficient[0]. The series is computed using the following convention,

which is needed when accessing the coefficients directly.

## 11.2  Creation and Calculation of Chebyshev Series

make-chebyshev-series-order

 Function: (make-chebyshev-series-order order) Contract: (-> natural-number? chebyshev-series?)
This function returns a newly created Chebyshev series with the given order.

chebyshev-series-init

 Function: (chebyshev-series-init cs func a b) Contract: (-> chebyshev-series? procedure? real? real? void?)
This function computes the Chebyshev approximation cs for the function func over the range (a, b) to the previously specified order. The computation of the Chebyshev approximation is an O(n2) process and requires n function evaluations.

## 11.3  Chebyshev Series Evaluation

chebyshev-eval

 Function: (chebyshev-eval cs x) Contract: (-> chebyshev-series? real? real?)
This function evaluates the Chebyshev series cs at the given point x.

chebyshev-eval-n

 Function: (chebyshev-eval-n cs n x) Contract: (-> chebyshev-series? natural-number? real? real?)
This function evaluates the Chebyshev series cs at the given point x to (at most) the given order n.

## 11.4  Examples

Example: The following program computes Chebyshev approximations to a step function. This is an extremely difficult approximation to make, due to the discontinuity, and was chosed as an example where approximation error is visible. For smooth functions the Chebyshev approximation converges extremely rapidly and errors would not be visible.

(require (planet "chebyshev.ss" ("williams" "science.plt")))
(require (lib "plot.ss" "plot"))

(define (f x)
(if (< x 0.5) .25 .75))

(define (chebyshev-example n)
(let ((cs (make-chebyshev-series-order 40))
(y-values '())
(y-cs-10-values '())
(y-cs-40-values '()))
(chebyshev-series-init cs f 0.0 1.0)
(do ((i 0 (+ i 1)))
((= i n) (void))
(let* ((x (exact->inexact (/ i n)))
(y (f x))
(y-cs-10 (chebyshev-eval-n cs 10 x))
(y-cs-40 (chebyshev-eval cs x)))
;(printf "~a ~a ~a ~a\n"
;        x y y-cs-10 y-cs-40)
(set! y-values (cons (vector x y) y-values))
(set! y-cs-10-values
(cons (vector x y-cs-10) y-cs-10-values))
(set! y-cs-40-values
(cons (vector x y-cs-40) y-cs-40-values))))
(printf "~a~n" (plot (mix (points (reverse y-values))
(points (reverse y-cs-10-values)))
(x-min 0) (x-max 1)
(y-min 0) (y-max 1)
(title "Chebyshev Series Order 10")))
(printf "~a~n" (plot (mix (points (reverse y-values))
(points (reverse y-cs-40-values)))
(x-min 0) (x-max 1)
(y-min 0) (y-max 1)
(title "Chebyshev Series Order 40")))))

(chebyshev-example 100)


Figures 71 and 72 show the resulting output plots.