NAG Library Manual, Mark 27.2
```/* nag_lapackeig_dspgvd (f08tcc) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.2, 2021.
*/

#include <math.h>
#include <nag.h>
#include <stdio.h>

int main(void) {
/* Scalars */
double anorm, bnorm, eps, rcond, rcondb, t1;
Integer i, j, n;
Integer exit_status = 0;
/* Arrays */
double *ap = 0, *bp = 0, *eerbnd = 0, *w = 0;
double dummy[1];
char nag_enum_arg[40];

/* Nag Types */
NagError fail;
Nag_OrderType order;
Nag_UploType uplo;

#ifdef NAG_COLUMN_MAJOR
#define A_UPPER(I, J) ap[J * (J - 1) / 2 + I - 1]
#define A_LOWER(I, J) ap[(2 * n - J) * (J - 1) / 2 + I - 1]
#define B_UPPER(I, J) bp[J * (J - 1) / 2 + I - 1]
#define B_LOWER(I, J) bp[(2 * n - J) * (J - 1) / 2 + I - 1]
order = Nag_ColMajor;
#else
#define A_UPPER(I, J) ap[(2 * n - I) * (I - 1) / 2 + J - 1]
#define A_LOWER(I, J) ap[I * (I - 1) / 2 + J - 1]
#define B_UPPER(I, J) bp[(2 * n - I) * (I - 1) / 2 + J - 1]
#define B_LOWER(I, J) bp[I * (I - 1) / 2 + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapackeig_dspgvd (f08tcc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n]");
scanf("%" NAG_IFMT "%*[^\n]", &n);
if (n < 0) {
printf("Invalid n\n");
exit_status = 1;
goto END;
;
}
scanf(" %39s%*[^\n]", nag_enum_arg);
/* nag_enum_name_to_value (x04nac).
* Converts NAG enum member name to value
*/
uplo = (Nag_UploType)nag_enum_name_to_value(nag_enum_arg);

/* Allocate memory */
if (!(ap = NAG_ALLOC(n * (n + 1) / 2, double)) ||
!(bp = NAG_ALLOC(n * (n + 1) / 2, double)) ||
!(eerbnd = NAG_ALLOC(n, double)) || !(w = NAG_ALLOC(n, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read the triangular parts of the matrices A and B from data file. */
if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i)
for (j = i; j <= n; ++j)
scanf("%lf", &A_UPPER(i, j));
scanf("%*[^\n]");
for (i = 1; i <= n; ++i)
for (j = i; j <= n; ++j)
scanf("%lf", &B_UPPER(i, j));
} else if (uplo == Nag_Lower) {
for (i = 1; i <= n; ++i)
for (j = 1; j <= i; ++j)
scanf("%lf", &A_LOWER(i, j));
scanf("%*[^\n]");
for (i = 1; i <= n; ++i)
for (j = 1; j <= i; ++j)
scanf("%lf", &B_LOWER(i, j));
}
scanf("%*[^\n]");

/* Compute the one-norms of the symmetric matrices A and B
* using nag_blast_dsp_norm (f16rdc).
*/
nag_blast_dsp_norm(order, Nag_OneNorm, uplo, n, ap, &anorm, &fail);
nag_blast_dsp_norm(order, Nag_OneNorm, uplo, n, bp, &bnorm, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_blast_dsp_norm (f16rdc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Solve the generalized symmetric eigenvalue problem A*B*x = lambda*x
* using nag_lapackeig_dspgvd (f08tcc).
* In the following call the 9th argument is set to n rather than 1 to
* avoid an incorrect error message in some vendor versions of LAPACK.
*/
nag_lapackeig_dspgvd(order, 2, Nag_EigVals, uplo, n, ap, bp, w, dummy, n,
&fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapackeig_dspgvd (f08tcc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print eigensolution */
printf("Eigenvalues\n   ");
for (j = 0; j < n; ++j)
printf(" %11.4f%s", w[j], j % 6 == 5 ? "\n" : "");
printf("\n");

/* Estimate the reciprocal condition number of the Cholesky factor of B.
* nag_lapacklin_dtpcon (f07ugc).
* Note that: cond(B) = 1/(rcond*rcond).
*/
nag_lapacklin_dtpcon(order, Nag_OneNorm, uplo, Nag_NonUnitDiag, n, bp, &rcond,
&fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dtpcon (f07ugc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print the reciprocal condition number of B */
rcondb = rcond * rcond;
printf("\nEstimate of reciprocal condition number for B\n     %11.1e\n",
rcondb);

/* Get the machine precision, using nag_machine_precision (x02ajc) */
eps = nag_machine_precision;
if (rcond < eps) {
printf("\nB is very ill-conditioned, error estimates have not been"
" computed\n");
goto END;
}

t1 = anorm * bnorm;
for (i = 0; i < n; ++i)
eerbnd[i] = eps * (t1 + fabs(w[i]) / rcondb);

/* Print the approximate error bounds for the eigenvalues */
printf("\nError estimates for the eigenvalues\n    ");
for (i = 0; i < n; ++i)
printf(" %11.1e%s", eerbnd[i], i % 6 == 5 ? "\n" : "");
printf("\n");

END:
NAG_FREE(ap);
NAG_FREE(bp);
NAG_FREE(eerbnd);
NAG_FREE(w);

return exit_status;
}
```