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/* chbevl.c
*
* Evaluate Chebyshev series
*
*
*
* SYNOPSIS:
*
* int N;
* double x, y, coef[N], chebevl();
*
* y = chbevl( x, coef, N );
*
*
*
* DESCRIPTION:
*
* Evaluates the series
*
* N-1
* - '
* y = > coef[i] T (x/2)
* - i
* i=0
*
* of Chebyshev polynomials Ti at argument x/2.
*
* Coefficients are stored in reverse order, i.e. the zero
* order term is last in the array. Note N is the number of
* coefficients, not the order.
*
* If coefficients are for the interval a to b, x must
* have been transformed to x -> 2(2x - b - a)/(b-a) before
* entering the routine. This maps x from (a, b) to (-1, 1),
* over which the Chebyshev polynomials are defined.
*
* If the coefficients are for the inverted interval, in
* which (a, b) is mapped to (1/b, 1/a), the transformation
* required is x -> 2(2ab/x - b - a)/(b-a). If b is infinity,
* this becomes x -> 4a/x - 1.
*
*
*
* SPEED:
*
* Taking advantage of the recurrence properties of the
* Chebyshev polynomials, the routine requires one more
* addition per loop than evaluating a nested polynomial of
* the same degree.
*
*/
/* chbevl.c */
/*
* Cephes Math Library Release 2.0: April, 1987
* Copyright 1985, 1987 by Stephen L. Moshier
* Direct inquiries to 30 Frost Street, Cambridge, MA 02140
*/
#pragma once
#include "../config.h"
namespace xsf {
namespace cephes {
XSF_HOST_DEVICE double chbevl(double x, const double array[], int n) {
double b0, b1, b2;
const double *p;
int i;
p = array;
b0 = *p++;
b1 = 0.0;
i = n - 1;
do {
b2 = b1;
b1 = b0;
b0 = x * b1 - b2 + *p++;
} while (--i);
return (0.5 * (b0 - b2));
}
} // namespace cephes
} // namespace xsf
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