NAG Library Manual, Mark 28.6
```/* nag_lapacklin_zgbcon (f07buc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.6, 2022.
*/

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

int main(void) {
/* Scalars */
Integer i, ipiv_len, j, kl, ku, n, pdab;
Integer exit_status = 0;
double anorm, rcond, sum;
NagError fail;
Nag_OrderType order;

/* Arrays */
Complex *ab = 0;
Integer *ipiv = 0;

#ifdef NAG_COLUMN_MAJOR
#define AB(I, J) ab[(J - 1) * pdab + kl + ku + I - J]
order = Nag_ColMajor;
#else
#define AB(I, J) ab[(I - 1) * pdab + kl + J - I]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapacklin_zgbcon (f07buc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%" NAG_IFMT "%" NAG_IFMT "%*[^\n] ", &n, &kl, &ku);
ipiv_len = n;
pdab = 2 * kl + ku + 1;

/* Allocate memory */
if (!(ab = NAG_ALLOC((2 * kl + ku + 1) * n, Complex)) ||
!(ipiv = NAG_ALLOC(ipiv_len, Integer))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read A from data file */
for (i = 1; i <= n; ++i) {
for (j = MAX(i - kl, 1); j <= MIN(i + ku, n); ++j)
scanf(" ( %lf , %lf )", &AB(i, j).re, &AB(i, j).im);
}
scanf("%*[^\n] ");
/* Compute norm of A */
anorm = 0.0;
for (j = 1; j <= n; ++j) {
sum = 0.0;
for (i = MAX(j - ku, 1); i <= MIN(j + kl, n); ++i)
/* nag_complex_abs (a02dbc).
* Modulus of a complex number
*/
sum = sum + nag_complex_abs(AB(i, j));
anorm = MAX(anorm, sum);
}
/* Factorize A */
/* nag_lapacklin_zgbtrf (f07brc).
* LU factorization of complex m by n band matrix
*/
nag_lapacklin_zgbtrf(order, n, n, kl, ku, ab, pdab, ipiv, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_zgbtrf (f07brc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Estimate condition number */
/* nag_lapacklin_zgbcon (f07buc).
* Estimate condition number of complex band matrix, matrix
* already factorized by nag_lapacklin_zgbtrf (f07brc)
*/
nag_lapacklin_zgbcon(order, Nag_OneNorm, n, kl, ku, ab, pdab, ipiv, anorm,
&rcond, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_zgbcon (f07buc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}
/* Print condition number */
/* nag_machine_precision (x02ajc).
* The machine precision
*/
if (rcond > nag_machine_precision)
printf("Estimate of condition number = %11.2e\n", 1.0 / rcond);
else
printf("A is singular to working precision\n");
END:
NAG_FREE(ab);
NAG_FREE(ipiv);
return exit_status;
}
```