There's one example on solving differential equations in math.js on the math.js homepage, but it's fairly complex and does not provide enough information for me personally to be able to apply math.js for other similar problems. So, what I'm trying to do is solving the Lotka–Volterra equations for predator-prey simulation. There are two equations in the system:
dx/dt = ax - bxy
dy/dt = cxy - y
Enconding this in math.js, I got
math.import({ndsolve:ndsolve});
const sim2 = math.parser();
sim2.eval("dxdt(x, y) = x - x * y");
sim2.eval("dydt(x, y) = x * y - y");
sim2.eval("dt = 1.0 s"); // Simulation timestep
sim2.eval("x0 = 0");
sim2.eval("y0 = 0");
sim2.eval("tfinal = 100 s"); // Simulation duration
sim2.eval("result_stage1 = ndsolve([dxdt, dydt], [x0, y0], dt, tfinal)");
where ndsolve is from the rocket trajectory example: http://mathjs.org/examples/browser/rocket_trajectory_optimization.html.html
function ndsolve(f, x0, dt, tmax) {
var n = f.size()[0]; // Number of variables
var x = x0.clone(); // Current values of variables
var dxdt = []; // Temporary variable to hold time-derivatives
var result = []; // Contains entire solution
var nsteps = math.divide(tmax, dt); // Number of time steps
for(var i=0; i<nsteps; i++) {
// Compute derivatives
for(var j=0; j<n; j++) {
dxdt[j] = f.get([j]).apply(null, x.toArray());
}
// Euler method to compute next time step
for(var j=0; j<n; j++) {
console.log(x.get([j]));
console.log(dt);
x.set([j], math.add(x.get([j]), math.multiply(dxdt[j], dt)));
}
result.push(x.clone());
}
return math.matrix(result);
}
However, running this code I get the error
Unexpected type of argument in function add (expected: number or Complex or BigNumber or Fraction, actual: Unit, index: 1)
What's the source of this error? Am I missing the correct units?
Grateful for any help.
