# JACOBI_EXACTNESS Exactness of Gauss-Jacobi Quadrature Rules

JACOBI_EXACTNESS is a C++ program which investigates the polynomial exactness of a Gauss-Jacobi quadrature rule for the interval [-1,1] with a weight function.

The Gauss-Jacobi quadrature rule is designed to approximate integrals on the interval [-1,1], with a weight function of the form (1-x)ALPHA * (1+x)BETA. ALPHA and BETA are real parameters that must be greater than -1.

Gauss-Jacobi quadrature assumes that the integrand we are considering has a form like:

```        Integral ( -1 <= x <= +1 ) (1-x)^alpha (1+x)^beta f(x) dx
```

For a Gauss-Jacobi rule, polynomial exactness is defined in terms of the function f(x). That is, we say the rule is exact for polynomials up to degree DEGREE_MAX if, for any polynomial f(x) of that degree or less, the quadrature rule will produce the exact value of

```        Integral ( -1 <= x <= +1 ) (1-x)^alpha (1+x)^beta f(x) dx
```

The program starts at DEGREE = 0, and then proceeds to DEGREE = 1, 2, and so on up to a maximum degree DEGREE_MAX specified by the user. At each value of DEGREE, the program generates the corresponding monomial term, applies the quadrature rule to it, and determines the quadrature error.

The program is very flexible and interactive. The quadrature rule is defined by three files, to be read at input, and the maximum degree top be checked is specified by the user as well.

Note that the three files that define the quadrature rule are assumed to have related names, of the form

• prefix_x.txt
• prefix_w.txt
• prefix_r.txt
When running the program, the user only enters the common prefix part of the file names, which is enough information for the program to find all three files.

For information on the form of these files, see the QUADRATURE_RULES directory listed below.

The exactness results are written to an output file with the corresponding name:

• prefix_exact.txt

### Usage:

jacobi_exactness prefix degree_max alpha beta
where
• prefix is the common prefix for the files containing the abscissa, weight and region information of the quadrature rule;
• degree_max is the maximum monomial degree to check. This would normally be a relatively small nonnegative number, such as 5, 10 or 15.
• alpha is the value of the exponent of (1-x) in the weight function; alpha should be a real number greater than -1.0.
• beta is the value of the exponent of (1+x) in the weight function; beta should be a real number greater than -1.0.

If the arguments are not supplied on the command line, the program will prompt for them.

### Languages:

JACOBI_EXACTNESS is available in a C++ version and a FORTRAN90 version and a MATLAB version.

### Related Data and Programs:

GEGENBAUER_EXACTNESS, a C++ program which tests the polynomial exactness of Gauss-Gegenbauer quadrature rules.

HERMITE_EXACTNESS, a C++ program which tests the polynomial exactness of Gauss-Hermite quadrature rules.

INT_EXACTNESS, a C++ program which tests the polynomial exactness of a quadrature rule for a finite interval.

INT_EXACTNESS_CHEBYSHEV1, a C++ program which tests the polynomial exactness of Gauss-Chebyshev type 1 quadrature rules.

INT_EXACTNESS_CHEBYSHEV2, a C++ program which tests the polynomial exactness of Gauss-Chebyshev type 2 quadrature rules.

INT_EXACTNESS_GEN_HERMITE, a C++ program which tests the polynomial exactness of a generalized Gauss-Hermite quadrature rule.

INT_EXACTNESS_GEN_LAGUERRE, a C++ program which tests the polynomial exactness of a generalized Gauss-Laguerre quadrature rule.

JACOBI_RULE, a C++ program which can generate a Gauss-Jacobi quadrature rule on request.

LAGUERRE_EXACTNESS, a C++ program which tests the polynomial exactness of Gauss-Laguerre quadrature rules for integration over [0,+oo) with density function exp(-x).

LEGENDRE_EXACTNESS, a C++ program which tests the monomial exactness of quadrature rules for the Legendre problem of integrating a function with density 1 over the interval [-1,+1].

### Reference:

1. Philip Davis, Philip Rabinowitz,
Methods of Numerical Integration,
Second Edition,
Dover, 2007,
ISBN: 0486453391,
LC: QA299.3.D28.
2. Shanjie Zhang, Jianming Jin,
Computation of Special Functions,
Wiley, 1996,
ISBN: 0-471-11963-6,
LC: QA351.C45.

### Examples and Tests:

JAC_O1_A0.5_B1.5 is a Gauss-Jacobi order 1 rule with ALPHA = 0.5, BETA = 1.5.

JAC_O2_A0.5_B1.5 is a Gauss-Jacobi order 2 rule with ALPHA = 0.5, BETA = 1.5.

JAC_O4_A0.5_B1.5 is a Gauss-Jacobi order 4 rule with ALPHA = 0.5, BETA = 1.5.

JAC_O8_A0.5_B1.5 is a Gauss-Jacobi order 8 rule with ALPHA = 0.5, BETA = 1.5.

JAC_O16_A0.5_B1.5 is a Gauss-Jacobi order 16 rule with ALPHA = 0.5, BETA = 1.5.

### List of Routines:

• MAIN is the main program for JACOBI_EXACTNESS.
• CH_CAP capitalizes a single character.
• CH_EQI is true if two characters are equal, disregarding case.
• CH_TO_DIGIT returns the integer value of a base 10 digit.
• FILE_COLUMN_COUNT counts the number of columns in the first line of a file.
• FILE_ROW_COUNT counts the number of row records in a file.
• JACOBI_INTEGRAL evaluates the integral of a monomial with Jacobi weight.
• R8_ABS returns the absolute value of an R8.
• R8_GAMMA evaluates Gamma(X) for a real argument.
• R8_HUGE returns a "huge" R8.
• R8_HYPER_2F1 evaluates the hypergeometric function 2F1(A,B,C,X).
• R8_PSI evaluates the function Psi(X).
• S_LEN_TRIM returns the length of a string to the last nonblank.
• S_TO_I4 reads an I4 from a string.
• S_TO_R8 reads an R8 from a string.
• S_TO_R8VEC reads an R8VEC from a string.
• S_WORD_COUNT counts the number of "words" in a string.
• TIMESTAMP prints the current YMDHMS date as a time stamp.
• TIMESTRING returns the current YMDHMS date as a string.

You can go up one level to the C++ source codes.

Last revised on 20 February 2008.