Attribute VB_Name = "Module2"
Public V() As Double, vmini As Double, vmaxi As Double
Dim X As Double, r As Double, dr As Double
'***************************************************************************
'* FRACTALS: VERHULST DIAGRAM *
'* ----------------------------------------------------------------------- *
'* TUTORIAL: *
'* *
'* The dynamic process that allows to very simply simulate "chaos" has the *
'* following mathematical form: *
'* x = f (x , C ). *
'* n+1 n *
'* The only condition is that the relation between input x n and output *
'* x n+1 be non linear. If this iterative process starts with an arbitrary *
'* value x0, it will give a series of values: x1, x2,...,x n... The long *
'* term behaviour of this series is interesting. *
'* *
'* Let us consider the classical example of the growth of a population over*
'* several years. Let be an initial population x0 which after n years has *
'* become x n. The growth ratio is: *
'* r = ( x n+1 - x n ) / x n. *
'* If this ratio remains constant, the dynamic law is: *
'* *
'* x n+1 = ( 1 + r ) x n *
'* *
'* After n years, the population will be: x n = ( 1 + r ) x0. *
'* *
'* To clarify the situation, let us consider the case where x0 = 0,001. *
'* If the growth ratio equals 5%, the population will roughly double *
'* every 15 years. As a matter of fact: *
'* *
'* x0 = 0,001 x1 = 0,00105 x2 = 0,0011025 *
'* x3 = 0,00115763 x4 = 0,00121551 x5 = 0,00127628 *
'* x6 = 0,00134010 x7 = 0,00140710 x8 = 0,00147746 *
'* x9 = 0,00155133 x10 = 0,00162889 x11 = 0,00171034 *
'* x12 = 0,00179586 x13 = 0,00188565 x14 = 0,00197993 *
'* x15 = 0,00207893 ... *
'* *
'* This kind of growth is exponential. But this dynamic law is linear, *
'* which is not judicious. Actually, the real growth of a poulation is *
'* necessarily limited by the ressources of its habitat, which are not in *
'* infinite quantity, such as food, for example. The belgian Verhulst was *
'* one of the first to study this phenomenon in 1845. *
'* He advised to consider a variable growth ratio r, taking the form *
'* r = r - C x n. The dynamic law of growth then takes the form: *
'* *
'* x n+1 = ( 1 + r ) x n - C x² n *
'* *
'* By having C = r / X, the population increases up to the value X, then *
'* stabilizes at this value. At least, this remains true so long as the *
'* growth ratio is < 200 %. A human population has never such a high growth*
'* ratio, but in the case of insects, for example, this can be quite *
'* possible. For a growth ratio even higher, one can observe surprising *
'* results (see verhulst.pas program). *
'* *
'* The calculation begins at x0 = 0,1 X. *
'* *
'* Case r = 1.8 *
'* *
'* The response curve climbs up rapidly and after some oscillations reaches*
'* a limit that is an "attractor". *
'* *
'* Case r = 2.3 *
'* *
'* The curve oscilates rapidly between two levels that frame the value X. *
'* The suite has two attractors. *
'* *
'* Case r = 2.5 *
'* *
'* The suite has four attractors. *
'* *
'* Case r = 3.0 *
'* *
'* The numbers x jump from one value to another one, without having twice *
'* the same value. Such a behaviour can be qualified as "chaotic". *
'* *
'* See also program chaos.bas (Feigenbaum diagram). *
'* ----------------------------------------------------------------------- *
'* From "Graphisme dans le plan et dans l'espace avec Turbo Pascal 4.0 *
'* By R. Dony - MASSON, Paris 1990, pages 189-192 " [BIBLI 12]. *
'* *
'* Microsoft Visual Basic 4.0 release by J-P Moreau *
'* (Project must include verhulst1.frm and Graph2D.bas, *
'* Here scales in Ox and Oy are automatic). *
'* (www.jpmoreau.fr) *
'***************************************************************************
Sub Verhulst1()
Dim i As Integer
Dim Bord As Integer
Bord = 150
Form1.AutoRedraw = False
Form1.Cls
'init graph with auto scales (see graph2d.bas)
EchAuto = 1
InitWindow False, 10, 0, 170, 0, 1.4
Legends 0, 10, "VERHULST DIAGRAM", "X", "Y"
s$ = InputBox("Input value for R (maxi 3):", "DATA", "2.95")
r = Val(s$)
'main loop of Verhulst diagram
X = 1#: xx = 0.1
MoveXY 0#, xx
Do
X = X + 1#
xx = (1 + r) * xx - r * xx * xx
LineXY 0, X, xx
Loop Until X > 165
'print r value in bottom right corner
s$ = "R= " & Str(r)
Display 0, MaxX - 6 * Bord, MaxY - 3 * Bord, s$
'change label of button "Go"
Form1.Command2.Caption = "Continue"
End Sub