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Chapter Contents
Chapter Introduction
NAG Toolbox

## Purpose

nag_quad_1d_gauss_wres (d01tb) returns the weights and abscissae appropriate to a Gaussian quadrature formula with a specified number of abscissae. The formulae provided are for Gauss–Legendre, rational Gauss, Gauss–Laguerre and Gauss–Hermite.

## Syntax

[weight, abscis, ifail] = d01tb(key, a, b, n)
[weight, abscis, ifail] = nag_quad_1d_gauss_wres(key, a, b, n)

## Description

nag_quad_1d_gauss_wres (d01tb) returns the weights and abscissae for use in the Gaussian quadrature of a function f(x)$f\left(x\right)$. The quadrature takes the form
 n S = ∑ wif(xi) i = 1
$S=∑i=1nwif(xi)$
where wi${w}_{i}$ are the weights and xi${x}_{i}$ are the abscissae (see Davis and Rabinowitz (1975), Fröberg (1970), Ralston (1965) or Stroud and Secrest (1966)).
Weights and abscissae are available for Gauss–Legendre, rational Gauss, Gauss–Laguerre and Gauss–Hermite quadrature, and for a selection of values of n$n$ (see Section [Parameters]).
 b S ≃ ∫ f(x)dx a
$S≃∫abf(x)dx$
where a$a$ and b$b$ are finite and it will be exact for any function of the form
 2n − 1 f(x) = ∑ cixi. i = 0
$f(x)=∑i=0 2n-1cixi.$
 ∞ a S ≃ ∫ f(x)dx (a + b > 0)  or  S ≃ ∫ f(x)dx (a + b < 0) a − ∞
$S≃∫a∞f(x) dx (a+b> 0) or S≃∫-∞a f(x) dx (a+b< 0)$
and will be exact for any function of the form
 2n + 1 f(x) = ∑ (ci)/((x + b)i) = ( ∑ i = 02n − 1c2n + 1 − i(x + b)i)/((x + b)2n + 1). i = 2
$f(x)=∑i=2 2n+1ci(x+b)i=∑i=0 2n-1c2n+1-i(x+b)i(x+b)2n+1.$
 ∞ a S ≃ ∫ f(x)dx (b > 0)  or  S ≃ ∫ f(x)dx (b < 0) a − ∞
$S≃∫a∞f(x) dx (b> 0) or S≃∫-∞a f(x) dx (b< 0)$
and will be exact for any function of the form
 2n − 1 f(x) = e − bx ∑ cixi. i = 0
$f(x)=e-bx∑i=0 2n-1cixi.$
 + ∞ S ≃ ∫ f(x)dx − ∞
$S≃∫-∞ +∞ f(x) dx$
and will be exact for any function of the form
 2n − 1 f(x) = e − b(x − a)2 ∑ cixi (b > 0). i = 0
$f(x)=e-b (x-a) 2∑i=0 2n-1cixi (b>0).$
 ∞ a S ≃ ∫ e − bxf(x)dx (b > 0)  or  S ≃ ∫ e − bxf(x)dx (b < 0) a − ∞
$S≃∫a∞e-bxf(x) dx (b> 0) or S≃∫-∞a e-bxf(x) dx (b< 0)$
and will be exact for any function of the form
 2n − 1 f(x) = ∑ cixi. i = 0
$f(x)=∑i=0 2n-1cixi.$
 + ∞ S ≃ ∫ e − b(x − a)2f(x)dx − ∞
$S≃∫-∞ +∞ e-b (x-a) 2f(x) dx$
and will be exact for any function of the form
 2n − 1 f(x) = ∑ cixi. i = 0
$f(x)=∑i=0 2n-1cixi.$
Note:  the Gauss–Legendre abscissae, with a = 1$a=-1$, b = + 1$b=+1$, are the zeros of the Legendre polynomials; the Gauss–Laguerre abscissae, with a = 0$a=0$, b = 1$b=1$, are the zeros of the Laguerre polynomials; and the Gauss–Hermite abscissae, with a = 0$a=0$, b = 1$b=1$, are the zeros of the Hermite polynomials.

## References

Davis P J and Rabinowitz P (1975) Methods of Numerical Integration Academic Press
Fröberg C E (1970) Introduction to Numerical Analysis Addison–Wesley
Ralston A (1965) A First Course in Numerical Analysis pp. 87–90 McGraw–Hill
Stroud A H and Secrest D (1966) Gaussian Quadrature Formulas Prentice–Hall

## Parameters

### Compulsory Input Parameters

1:     key – int64int32nag_int scalar
key = 0${\mathbf{key}}=0$
Gauss–Legendre quadrature on a finite interval, using normal weights.
key = 3${\mathbf{key}}=3$
Gauss–Laguerre quadrature on a semi-infinite interval, using normal weights.
key = -3${\mathbf{key}}=-3$
key = 4${\mathbf{key}}=4$
Gauss–Hermite quadrature on an infinite interval, using normal weights.
key = -4${\mathbf{key}}=-4$
key = -5${\mathbf{key}}=-5$
Constraint: key = 0${\mathbf{key}}=0$, 3$3$, -3$-3$, 4$4$, -4$-4$ or -5$-5$.
2:     a – double scalar
3:     b – double scalar
The quantities a$a$ and b$b$ as described in the appropriate sub-section of Section [Description].
Constraints:
• Rational Gauss: a + b0.0${\mathbf{a}}+{\mathbf{b}}\ne 0.0$;
• Gauss–Laguerre: b0.0${\mathbf{b}}\ne 0.0$;
• Gauss–Hermite: b > 0${\mathbf{b}}>0$.
4:     n – int64int32nag_int scalar
n$n$, the number of weights and abscissae to be returned.
Constraint: n = 1${\mathbf{n}}=1$, 2$2$, 3$3$, 4$4$, 5$5$, 6$6$, 8$8$, 10$10$, 12$12$, 14$14$, 16$16$, 20$20$, 24$24$, 32$32$, 48$48$ or 64$64$.
Note: if n > 0$n>0$ and is not a member of the above list, the maxmium value of n$n$ stored below n$n$ will be used, and all subsequent elements of abscis and weight will be returned as zero.

None.

None.

### Output Parameters

1:     weight(n) – double array
The n weights.
2:     abscis(n) – double array
The n abscissae.
3:     ifail – int64int32nag_int scalar
${\mathrm{ifail}}={\mathbf{0}}$ unless the function detects an error (see [Error Indicators and Warnings]).

## Error Indicators and Warnings

Errors or warnings detected by the function:

Cases prefixed with W are classified as warnings and do not generate an error of type NAG:error_n. See nag_issue_warnings.

W ifail = 1${\mathbf{ifail}}=1$
The n-point rule is not among those stored.
W ifail = 2${\mathbf{ifail}}=2$
Underflow occurred in calculation of normal weights.
W ifail = 3${\mathbf{ifail}}=3$
No nonzero weights were generated for the provided parameters.
ifail = 11${\mathbf{ifail}}=11$
Constraint: key = 0${\mathbf{key}}=0$, 3$3$, -3$-3$, 4$4$, -4$-4$ or -5$-5$.
ifail = 12${\mathbf{ifail}}=12$
The value of a and/or b is invalid for the chosen key. Either:
• Constraint: |a + b| > 0.0$|{\mathbf{a}}+{\mathbf{b}}|>0.0$.
• Constraint: |b| > 0.0$|{\mathbf{b}}|>0.0$.
• Constraint: b > 0.0${\mathbf{b}}>0.0$.
ifail = 14${\mathbf{ifail}}=14$
Constraint: n > 0${\mathbf{n}}>0$.

## Accuracy

The weights and abscissae are stored for standard values of a and b to full machine accuracy.

Timing is negligible.

## Example

```function nag_quad_1d_gauss_wres_example
key = int64(-3);
a = 0;
b = 1;
n = int64(6);
[weight, abscis, ifail] = nag_quad_1d_gauss_wres(key, a, b, n)

function [fv, iflag, user] = f(x, nx, iflag, user)
fv = sin(x)./x.*log(10*(1-x));
```
```

weight =

0.5735
1.3693
2.2607
3.3505
4.8868
7.8490

abscis =

0.2228
1.1889
2.9927
5.7751
9.8375
15.9829

ifail =

0

```
```function d01tb_example
key = int64(-3);
a = 0;
b = 1;
n = int64(6);
[weight, abscis, ifail] = d01tb(key, a, b, n)

function [fv, iflag, user] = f(x, nx, iflag, user)
fv = sin(x)./x.*log(10*(1-x));
```
```

weight =

0.5735
1.3693
2.2607
3.3505
4.8868
7.8490

abscis =

0.2228
1.1889
2.9927
5.7751
9.8375
15.9829

ifail =

0

```