5.9 Derivatives or Integrals of a Chebyshev-approximated Function 195 5.9 Derivatives or Integrals of a Chebyshev-approximated Function If you have obtained the Chebyshev coefficients that approximate a function in a certain range(e.g.,from chebft in $5.8),then it is a simple matter to transform them to Chebyshev coefficients corresponding to the derivative or integral of the function.Having done this,you can evaluate the derivative or integral just as if it were a function that you had Chebyshev-fitted ab initio. The relevant formulas are these:If ci,i=0,...,m-1 are the coefficients 81 that approximate a function f in equation (5.8.9),C;are the coefficients that approximate the indefinite integral of f,and c are the coefficients that approximate the derivative of f,then 意蓉贫 C=4-1-94+1 (i>0) (5.9.1) 2i RECIPES c-1=c+1+2ic (i=m-1,m-2,,1) (5.9.2) 兰 Press. Equation(5.9.1)is augmented by an arbitrary choice of Co,corresponding to an arbitrary constant of integration.Equation(5.9.2),which is a recurrence,is started with the values cm=c1=0,corresponding to no information about the m +1st Chebyshev coefficient of the original function f. Here are routines for implementing equations(5.9.1)and(5.9.2). SCIENTIFIC void chder(float a,float b,float c[],float cder[],int n) Given a,b,c[..n-1].as output from routine chebft $5.8.and given n,the desired degree of approximation (length of c to be used),this routine returns the array cder [o..n-1],the Chebyshev coefficients of the derivative of the function whose coefficients are c. int j; float con; cder[n-1]=0.0; n-1 and n-2 are special cases. cder[n-2]=2*(n-1)*c[n-1]; for(j=n-3;j>=0;j--) Numerical Recipes 021 43108 cder[j]=cder[j+2]+2*(j+1)*c[j+1]; Equation (5.9.2) con=2.0/(b-a); for (j=0;j<n;j++) Normalize to the interval b-a (outside cder[i]*con; Software. void chint(float a,float b,float c[],float cint ]int n) Given a,b,c[O..n-1],as output from routine chebft $5.8,and given n,the desired degree of approximation (length of c to be used).this routine returns the array cint [0..n-1],the Chebyshev coefficients of the integral of the function whose coefficients are c.The constant of integration is set so that the integral vanishes at a. int ji float sum=0.0,fac=1.0,con; con=0.25*(b-a); Factor that normalizes to the interval b-a
5.9 Derivatives or Integrals of a Chebyshev-approximated Function 195 Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copyin Copyright (C) 1988-1992 by Cambridge University Press. Programs Copyright (C) 1988-1992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0-521-43108-5) g of machinereadable files (including this one) to any server computer, is strictly prohibited. To order Numerical Recipes books or CDROMs, visit website http://www.nr.com or call 1-800-872-7423 (North America only), or send email to directcustserv@cambridge.org (outside North America). 5.9 Derivatives or Integrals of a Chebyshev-approximated Function If you have obtained the Chebyshev coefficients that approximate a function in a certain range (e.g., from chebft in §5.8), then it is a simple matter to transform them to Chebyshev coefficients corresponding to the derivative or integral of the function. Having done this, you can evaluate the derivative or integral just as if it were a function that you had Chebyshev-fitted ab initio. The relevant formulas are these: If ci, i = 0,...,m − 1 are the coefficients that approximate a function f in equation (5.8.9), Ci are the coefficients that approximate the indefinite integral of f, and c i are the coefficients that approximate the derivative of f, then Ci = ci−1 − ci+1 2i (i > 0) (5.9.1) c i−1 = c i+1 + 2ici (i = m − 1, m − 2,..., 1) (5.9.2) Equation (5.9.1) is augmented by an arbitrary choice of C 0, corresponding to an arbitrary constant of integration. Equation (5.9.2), which is a recurrence, is started with the values c m = c m−1 = 0, corresponding to no information about the m + 1st Chebyshev coefficient of the original function f. Here are routines for implementing equations (5.9.1) and (5.9.2). void chder(float a, float b, float c[], float cder[], int n) Given a,b,c[0..n-1], as output from routine chebft §5.8, and given n, the desired degree of approximation (length of c to be used), this routine returns the array cder[0..n-1], the Chebyshev coefficients of the derivative of the function whose coefficients are c. { int j; float con; cder[n-1]=0.0; n-1 and n-2 are special cases. cder[n-2]=2*(n-1)*c[n-1]; for (j=n-3;j>=0;j--) cder[j]=cder[j+2]+2*(j+1)*c[j+1]; Equation (5.9.2). con=2.0/(b-a); for (j=0;j<n;j++) Normalize to the interval b-a. cder[j] *= con; } void chint(float a, float b, float c[], float cint[], int n) Given a,b,c[0..n-1], as output from routine chebft §5.8, and given n, the desired degree of approximation (length of c to be used), this routine returns the array cint[0..n-1], the Chebyshev coefficients of the integral of the function whose coefficients are c. The constant of integration is set so that the integral vanishes at a. { int j; float sum=0.0,fac=1.0,con; con=0.25*(b-a); Factor that normalizes to the interval b-a
196 Chapter 5. Evaluation of Functions for(j=1;j<=n-2;j+){ cint [j]=con*(c[j-1]-c[j+1])/j Equation (5.9.1) sum +fac*cint[j]; Accumulates the constant of integration. fac -fac; Vill equal士1. cint [n-1]=con*c[n-2]/(n-1); Special case of (5.9.1)for n-1. sum +fac*cint [n-1]; c1nt[0]=2.0*gum; Set the constant of integration Clenshaw-Curtis Quadrature 83 Since a smooth function's Chebyshev coefficients c decrease rapidly,generally expo- 鱼 nentially,equation(5.9.1)is often quite efficient as the basis for a quadrature scheme.The routines chebft and chint,used in that order,can be followed by repeated calls to chebev if f(x)dr is required for many different values of r in the range a <<b. If only the single definite integral f()dr is required,then chint and chebev are replaced by the simpler formula,derived from equation (5.9.1), f(z)dr=(-a)5oo--c-...-7 111 1 2k+1(2k-2k-·. (5.9.3) America where the c's are as returned by chebft.The series can be truncated when czk becomes Press. negligible,and the first neglected term gives an error estimate. This scheme is known as Clenshaw-Curtis guadrature [1].It is often combined with an 9 adaptive choice of N,the number of Chebyshev coefficients calculated via equation (5.8.7), which is also the number of function evaluations of f().If a modest choice of N does not give a sufficiently small czk in equation (5.9.3),then a larger value is tried.In this SCIENTIFIC adaptive case,it is even better to replace equation (5.8.7)by the so-called "trapezoidal"or Gauss-Lobatto (84.5)variant, 景（贷】(） 2 j=0,.,N-1 (5.9.4) k=0 where (N.B.!)the two primes signify that the first and last terms in the sum are to be multiplied by 1/2.If N is doubled in equation (5.9.4),then half of the new function evaluation points are identical to the old ones,allowing the previous function evaluations to be reused.This feature,plus the analytic weights and abscissas (cosine functions in 5.9.4),give Clenshaw-Curtis quadrature an edge over high-order adaptive Gaussian quadrature (cf.84.5), Numerical Recipes 10-521 43106 which the method otherwise resembles. Ifyour problem forces you to large values of N,you should be aware that equation(5.9.4) can be evaluated rapidly,and simultaneously for all the values of j,by a fast cosine transform. (outside (See $12.3,especially equation 12.3.17.)(We already remarked that the nontrapezoidal form (5.8.7)can also be done by fast cosine methods,cf.equation 12.3.22.) 首 Software. CITED REFERENCES AND FURTHER READING: Goodwin,E.T.(ed.)1961,Modern Computing Methods,2nd ed.(New York:Philosophical Li- bray),pp.78-79. Clenshaw,C.W.,and Curtis,A.R.1960,Numerische Mathematik,vol.2,pp.197-205.[1]
196 Chapter 5. Evaluation of Functions Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copyin Copyright (C) 1988-1992 by Cambridge University Press. Programs Copyright (C) 1988-1992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0-521-43108-5) g of machinereadable files (including this one) to any server computer, is strictly prohibited. To order Numerical Recipes books or CDROMs, visit website http://www.nr.com or call 1-800-872-7423 (North America only), or send email to directcustserv@cambridge.org (outside North America). for (j=1;j<=n-2;j++) { cint[j]=con*(c[j-1]-c[j+1])/j; Equation (5.9.1). sum += fac*cint[j]; Accumulates the constant of integration. fac = -fac; Will equal ±1. } cint[n-1]=con*c[n-2]/(n-1); Special case of (5.9.1) for n-1. sum += fac*cint[n-1]; cint[0]=2.0*sum; Set the constant of integration. } Clenshaw-Curtis Quadrature Since a smooth function’s Chebyshev coefficients ci decrease rapidly, generally exponentially, equation (5.9.1) is often quite efficient as the basis for a quadrature scheme. The routines chebft and chint, used in that order, can be followed by repeated calls to chebev if x a f(x)dx is required for many different values of x in the range a ≤ x ≤ b. If only the single definite integral b a f(x)dx is required, then chint and chebev are replaced by the simpler formula, derived from equation (5.9.1), b a f(x)dx = (b − a) 1 2 c0 − 1 3 c2 − 1 15 c4 −···− 1 (2k + 1)(2k − 1) c2k −··· (5.9.3) where the ci’s are as returned by chebft. The series can be truncated when c2k becomes negligible, and the first neglected term gives an error estimate. This scheme is known as Clenshaw-Curtis quadrature [1]. It is often combined with an adaptive choice of N, the number of Chebyshev coefficients calculated via equation (5.8.7), which is also the number of function evaluations of f(x). If a modest choice of N does not give a sufficiently small c2k in equation (5.9.3), then a larger value is tried. In this adaptive case, it is even better to replace equation (5.8.7) by the so-called “trapezoidal” or Gauss-Lobatto (§4.5) variant, cj = 2 N N k=0 f cos πk N cos πjk N j = 0,...,N − 1 (5.9.4) where (N.B.!) the two primes signify that the first and last terms in the sum are to be multiplied by 1/2. If N is doubled in equation (5.9.4), then half of the new function evaluation points are identical to the old ones, allowing the previous function evaluations to be reused. This feature, plus the analytic weights and abscissas (cosine functions in 5.9.4), give Clenshaw-Curtis quadrature an edge over high-order adaptive Gaussian quadrature (cf. §4.5), which the method otherwise resembles. If your problem forces you to large values of N, you should be aware that equation (5.9.4) can be evaluated rapidly, and simultaneously for all the values of j, by a fast cosine transform. (See §12.3, especially equation 12.3.17.) (We already remarked that the nontrapezoidal form (5.8.7) can also be done by fast cosine methods, cf. equation 12.3.22.) CITED REFERENCES AND FURTHER READING: Goodwin, E.T. (ed.) 1961, Modern Computing Methods, 2nd ed. (New York: Philosophical Library), pp. 78–79. Clenshaw, C.W., and Curtis, A.R. 1960, Numerische Mathematik, vol. 2, pp. 197–205. [1]
5.10 Polynomial Approximation from Chebyshev Coefficients 197 5.10 Polynomial Approximation from Chebyshev Coefficients You may well ask after reading the preceding two sections,"Must I store and evaluate my Chebyshev approximation as an array of Chebyshev coefficients for a transformed variable y?Can't I convert the ck's into actual polynomial coefficients in the original variable z and have an approximation of the following form?" f(x (5.10.1) nted for Yes,you can do this (and we will give you the algorithm to do it),but we caution you against it:Evaluating equation(5.10.1),where the coefficient g's reflect an underlying Chebyshev approximation,usually requires more significant figures than evaluation of the Chebyshev sum directly (as by chebev).This is because the Chebyshev polynomials themselves exhibit a rather delicate cancellation:The leading coefficient of T(),for example,is 2-1;other coefficients of T()are (Nort serve even bigger:yet they all manage to combine into a polynomial that lies between+1. America computer, make one paper e University Press. THE Only when m is no larger than 7 or 8 should you contemplate writing a Chebyshev ART fit as a direct polynomial,and even in those cases you should be willing to tolerate two or so significant figures less accuracy than the roundoff limit of your machine. You get the g's in equation(5.10.1)from the c's output from chebft(suitably ictly proh Programs truncated at a modest value ofm)by calling in sequence the following two procedures: #include "nrutil.h" to dir void chebpc(f1oatc],float d[☐，intn) Chebyshev polynomial coefficients.Given a coefficient array c[..n-1],this routine generates coefficient array d[.1]such that)-co/2.The method OF SCIENTIFIC COMPUTING(ISBN is Clenshaw's recurrence (5.8.11),but now applied algebraically rather than arithmetically. int k,j; float sv,*dd; dd=vector(0,n-1); v@cambridge.org 胸之 1988-1992 by Numerical Recipes for(j=0;j=1;j--)[ for (k=n-j;k>=1;k--){ sv=d[k]; Software. d[k]=2.0*dk-1]-dd[k]; dd[k]=sv; (outside North America) sv=d[o]; d[o]=-dd[o]+c[j] dd[o]=sv; for(j=n-1;j>=1;j-) d[j]=d[j-1]-dd[j]; d[0]=-dd[0]+0.5*c[0]; free_vector(dd,0,n-1);
5.10 Polynomial Approximation from Chebyshev Coefficients 197 Permission is granted for internet users to make one paper copy for their own personal use. Further reproduction, or any copyin Copyright (C) 1988-1992 by Cambridge University Press. Programs Copyright (C) 1988-1992 by Numerical Recipes Software. Sample page from NUMERICAL RECIPES IN C: THE ART OF SCIENTIFIC COMPUTING (ISBN 0-521-43108-5) g of machinereadable files (including this one) to any server computer, is strictly prohibited. To order Numerical Recipes books or CDROMs, visit website http://www.nr.com or call 1-800-872-7423 (North America only), or send email to directcustserv@cambridge.org (outside North America). 5.10 Polynomial Approximation from Chebyshev Coefficients You may well ask after reading the preceding two sections, “Must I store and evaluate my Chebyshev approximation as an array of Chebyshev coefficients for a transformed variable y? Can’t I convert the ck’s into actual polynomial coefficients in the original variable x and have an approximation of the following form?” f(x) ≈ m −1 k=0 gkxk (5.10.1) Yes, you can do this (and we will give you the algorithm to do it), but we caution you against it: Evaluating equation (5.10.1), where the coefficient g’s reflect an underlying Chebyshev approximation, usually requires more significant figures than evaluation of the Chebyshev sum directly (as by chebev). This is because the Chebyshev polynomials themselves exhibit a rather delicate cancellation: The leading coefficient of Tn(x), for example, is 2n−1; other coefficients of Tn(x) are even bigger; yet they all manage to combine into a polynomial that lies between ±1. Only when m is no larger than 7 or 8 should you contemplate writing a Chebyshev fit as a direct polynomial, and even in those cases you should be willing to tolerate two or so significant figures less accuracy than the roundoff limit of your machine. You get the g’s in equation (5.10.1) from the c’s output from chebft (suitably truncated at a modest value of m) by calling in sequence the following two procedures: #include "nrutil.h" void chebpc(float c[], float d[], int n) Chebyshev polynomial coefficients. Given a coefficient array c[0..n-1], this routine generates a coefficient array d[0..n-1] such that n-1 k=0 dkyk = n-1 k=0 ckTk(y) − c0/2. The method is Clenshaw’s recurrence (5.8.11), but now applied algebraically rather than arithmetically. { int k,j; float sv,*dd; dd=vector(0,n-1); for (j=0;j=1;j--) { for (k=n-j;k>=1;k--) { sv=d[k]; d[k]=2.0*d[k-1]-dd[k]; dd[k]=sv; } sv=d[0]; d[0] = -dd[0]+c[j]; dd[0]=sv; } for (j=n-1;j>=1;j--) d[j]=d[j-1]-dd[j]; d[0] = -dd[0]+0.5*c[0]; free_vector(dd,0,n-1); }
