NAG Library Manual, Mark 28.7
```/* nag_quad_md_simplex (d01pac) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.7, 2022.
*/

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

#ifdef __cplusplus
extern "C" {
#endif
static double NAG_CALL functn(Integer ndim, const double x[], Nag_Comm *comm);
#ifdef __cplusplus
}
#endif

int main(void) {
#define VERT(I, J) vert[(J - 1) * (ndim + 1) + I - 1]
/* Scalars */
Integer exit_status = 0;
Integer i, j, maxord, minord, mxord, ndim;
double esterr;
/* Arrays */
Integer iuser[1];
double *finvls = 0, *vert = 0;
/* Nag types */
NagError fail;
Nag_Comm comm;

INIT_FAIL(fail);

/* Skip heading in data file */
scanf("%*[^\n] ");
/* Input mxord and ndim */
scanf("%" NAG_IFMT " %" NAG_IFMT "%*[^\n] ", &mxord, &ndim);

if (!(finvls = NAG_ALLOC(mxord, double)) ||
!(vert = NAG_ALLOC(2 * (ndim + 1) * (ndim + 1), double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

for (i = 1; i <= ndim + 1; i++)
for (j = 1; j <= ndim; j++)
VERT(i, j) = 0.0;
for (j = 2; j <= ndim + 1; j++)
VERT(j, j - 1) = 1.0;

minord = 0;
iuser[0] = 0; /* Function counter */
comm.iuser = iuser;
for (maxord = 1; maxord <= mxord; maxord++) {
* Multidimensional quadrature over an n-simplex.
*/
nag_quad_md_simplex(ndim, vert, functn, &minord, maxord, finvls, &esterr,
&comm, &fail);
if (fail.code != NE_NOERROR) {
exit_status = 1;
goto END;
}

if (maxord == 1)
printf("maxord   Estimated      Estimated         Integrand\n"
"           value         accuracy        evaluations\n");
printf("%4" NAG_IFMT "%13.5f%16.3e%15" NAG_IFMT "\n", maxord,
finvls[maxord - 1], esterr, comm.iuser[0]);
}

END:
NAG_FREE(finvls);
NAG_FREE(vert);

return exit_status;
}

static double NAG_CALL functn(Integer ndim, const double x[], Nag_Comm *comm) {
comm->iuser[0]++;
return exp(x[0] + x[1] + x[2]) * cos(x[0] + x[1] + x[2]);
}
```