NAG Library Manual, Mark 27.2
```/* D02PR_P0W_F C++ Header Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.2, 2021.
*/

#include <iostream>
#include <math.h>
#include <nag.h>
#include <stdio.h>
using namespace std;

#ifdef __cplusplus
extern "C"
{
#endif
static void NAG_CALL f(void *&        ad_handle,
const double & t,
const Integer &n,
const double   y[],
double         yp[],
Integer        iuser[],
double         ruser[]);
#ifdef __cplusplus
}
#endif

int main(void)
{
const Integer n = 4, npts = 6;
const Integer liwsav = 130;
const Integer lrwsav = 350 + 32 * n;

Integer exit_status = 0;

double * rwsav = 0, *thresh = 0, *ynow = 0;
double * ypnow = 0, *y = 0, ruser[1];
Integer *iwsav = 0, iuser[1];

cout << "D02PR_P0W_F C++ Header Example Program Results\n\n";

thresh = new double[n];
ynow   = new double[n];
y      = new double[n];
ypnow  = new double[n];
iwsav  = new Integer[liwsav];
rwsav  = new double[lrwsav];

// Set initial conditions for ODE and parameters for the integrator.
Integer method = -3;
double  eps, tol, hstart, tend, tstart;
eps    = 0.7;
tstart = 0.0;
tol    = 1.0e-4;
tend   = 6.0 * nag_math_pi;
hstart = 0.0;
for (int k = 0; k < n; k++)
{
thresh[k] = 1.0e-10;
}

cout << "\n  Calculation with tol = " << tol << endl;

cout.setf(ios::fixed);
cout.setf(ios::right);
cout.precision(3);
cout << "\n    t         y1        y2" << endl;
cout.width(6);
cout << tstart;

y[0] = 1.0 - eps;
y[1] = 0.0;
y[2] = 0.0;
y[3] = sqrt((1.0 + eps) / (1.0 - eps));
for (int k = 0; k < n; k++)
{
cout.width(10);
cout << y[k];
}
cout << endl;

double tnow, twant, tinc;
tinc  = (tend - tstart) / ((double)npts);
twant = tstart + tinc;

// Initialize Runge-Kutta method for integrating ODE
Integer ifail = 0;

iwsav, rwsav, ifail);

tnow = tstart;
while (tnow < tend)
{
ifail = 0;
iwsav, rwsav, ifail);
if (tnow == twant)
{
cout.width(6);
cout << tnow;
for (int k = 0; k < n; ++k)
{
cout.width(10);
cout << ynow[k];
}
cout << endl;
twant = twant + tinc;
ifail = 0;
}
}

double  hnext, waste;
Integer fevals, stepcost, stepsok;
ifail = 0;
rwsav, ifail);
cout << "\n Cost of the integration in evaluations of f is " << fevals;
cout << endl;

delete[] thresh;
delete[] ynow;
delete[] y;
delete[] ypnow;
delete[] iwsav;
delete[] rwsav;
return exit_status;
}

static void NAG_CALL f(void *&        ad_handle,
const double & t,
const Integer &n,
const double   y[],
double         yp[],
Integer        iuser[],
double         ruser[])
{
double r;
r     = 1.0 / sqrt(y[0] * y[0] + y[1] * y[1]);
r     = r * r * r;
yp[0] = y[2];
yp[1] = y[3];
yp[2] = -y[0] * r;
yp[3] = -y[1] * r;
}
```