Differential equations

Display solutions of the ordinary differential equation

[math]\displaystyle{ y'= f(x,y) }[/math]

with initial value [math]\displaystyle{ (x_0,y_0) }[/math].

f(x,y)=

See also

• Systems of differential equations
• Lotka-Volterra equations
• Epidemiology: The SIR model
• Population growth models
• Autocatalytic process
• Logistic process
• Paul Pearson has written a very nice variation: Slope fields and solution curves (using the Runge-Kutta)

The underlying JavaScript code

<form>
f(x,y)=<input type="text" id="odeinput" value="(2-x)*y"><input type=button value="ok" onclick="doIt()">
</form>

var brd = JXG.JSXGraph.initBoard('jxgbox', {axis:true, boundingbox:[-11,11,11,-11]});
var N = brd.create('slider',[[-7,9.5],[7,9.5],[-15,10,15]], {name:'N'});
var P = brd.create('point',[0,1], {name:'(x_0,y_0)'});

function doIt() {
  var txt = JXG.GeonextParser.geonext2JS(document.getElementById("odeinput").value);
  f = new Function("x", "yy", "var y = yy[0]; var z = " + txt + "; return [z]");
  brd.update();
}

function ode() {
   return JXG.Math.Numerics.rungeKutta('heun', [P.Y()], [P.X(), P.X()+N.Value()], 200, f);
}

var g = brd.create('curve', [[0],[0]], {strokeColor:'red', strokeWidth:2});
g.updateDataArray = function() {
    var data = ode();
    var h = N.Value()/200;
    this.dataX = [];
    this.dataY = [];
    for(var i=0; i<data.length; i++) {
        this.dataX[i] = P.X()+i*h;
        this.dataY[i] = data[i][0];
    }
};
doIt();