Difference between revisions of "Population growth models"

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<math> \frac{\Delta y}{\Delta t} = \alpha\cdot y </math>.
<math> \frac{\Delta y}{\Delta t} = \alpha\cdot y </math>.
With <math>\Delta \to 0</math> we get
With <math>\Delta t\to 0</math> we get
<math> \frac{d y}{d t} = \alpha\cdot y </math>, i.e. <math> y' = \alpha\cdot y </math>.
<math> \frac{d y}{d t} = \alpha\cdot y </math>, i.e. <math> y' = \alpha\cdot y </math>.

Revision as of 20:13, 12 May 2009

Exponential population growth model

In time [math] \Delta t[/math] the population consisting of [math]y[/math] elements grows by [math]\alpha\cdot y [/math] elements: [math] \Delta y = \alpha\cdot y\cdot \Delta t [/math], that is [math] \frac{\Delta y}{\Delta t} = \alpha\cdot y [/math].

With [math]\Delta t\to 0[/math] we get [math] \frac{d y}{d t} = \alpha\cdot y [/math], i.e. [math] y' = \alpha\cdot y [/math].

The initial population is [math]y(0)= s[/math].

The red line shows the exact solution of the differential equation [math]y(t)=s\cdot e^{\alpha t}[/math]. The blue line is the simulation with [math]\Delta t = 0.1[/math].

Other models

The JavaScript code

<jsxgraph height="500" width="600" board="board"  box="box1">
brd = JXG.JSXGraph.initBoard('box1', {originX: 10, originY: 250, unitX: 40, unitY: 20, axis:true});
var t = brd.createElement('turtle',[4,3,70]);
var s = brd.createElement('slider', [[0,-5], [10,-5],[-5,0.5,5]], {name:'s'});
var alpha = brd.createElement('slider', [[0,-6], [10,-6],[-1,0.2,2]], {name:'&alpha;'});
var e = brd.createElement('functiongraph', [function(x){return s.X()*Math.exp(alpha.X()*x);}],{strokeColor:'red'});

function clearturtle() {
function run() {
  delta = 0.1; // global
  x = 0.0;  // global
function loop() {
  var y = alpha.X()*t.pos[1]*delta;   // Exponential growth
  x += delta;
  if (x<20.0) {