ORDINARY DIFFERENTIAL EQUATIONS POWERTOOL
Lesson 3 -- Application: Exponential and Logistic Growth
Prof. Douglas B. Meade
Industrial Mathematics Institute
Department of Mathematics
University of South Carolina
Columbia, SC 29208
URL: http://www.math.sc.edu/~meade/
E-mail: meade@math.sc.edu
Copyright 2001 by Douglas B. Meade
All rights reserved
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Outline for Lesson 3
3.A Exponential Growth and Decay
3.A-1 General Solution
3.A-2 Doubling Time
3.A-3 Half-Life
3.B Logistic Equation
3.B-1 General Solution
3.B-2 Carrying Capacity
Initialization
Warning, the name changecoords has been redefined
The general model for exponential growth and decay with rate constant , is
and initial condition
If is positive, the equation models exponential growth, whereas if is negative, it models exponential decay.
This equation is separable, and Maple agrees with this classification, as seen with
To find the general solution from first principles, divide through by to separate variables, and then integrate both sides of the resulting equation to obtain
Substitution of the initial condition leads to
and solving for gives
Of course, the same result could be obtained from
`Classification methods on request` `Methods to be used are: [separable]` `----------------------------` `* Tackling ODE using method: separable` `--- Trying classification methods ---` `trying separable` `<- separable successful`
and the final simplification
The doubling time for a process growing exponentially is the time needed for the quantity to double from its original size. Thus, the doubling time is found from the relationship
.
Implementing that for the solution found above leads to the equation
whose solution is
One of the important characteristics of the doubling time is that not only is it the time needed for the initial size to double, it is the time needed for the size to double at any point in an exponential process. The ratio of the solution at time and at time is given by
If is the true doubling time, then this ratio simplifies to 2, as is seen via
The half-life for a quantity that is decaying according to an exponential model is the time after which exactly half the original amount remains. The half life must obey the equation
Solving for leads to
Note that holds for a decaying process. Thus, the half-life for an exponential model with "growth" rate is the same as the doubling time for an exponential model with growth rate .
The general logistic equation is a modification of the exponential model in which the growth is tempered by the factor ( ). Therefore, the model consists of the ODE
with, for example, an initial condition of the form
Initially, when is small, the factor is essentially constant, so the population obeys an exponential growth law. As increases and approaches , the factor tends towards zero, so the rate of change, , approaches zero also. Thus, approaches a constant, and the growth is said to be self-limiting.
This ODE is easily separated by the obvious division, resulting in
Integrating from the initial time, 0, to any other time , yields
and solving for to obtain the explicit solution, gives
A quick check that this is, in fact, a solution to the original ODE shows
and, for the initial condition,
Before discussing the generic properties of the logistic model, it is instructive to use graphical methods to examine a specific example. For parameter values, choose
so the logistic equation becomes
Then, the direction field for this model is seen in Figure 3.1.
An equilibrium solution of the differential equation is any solution for which , identically. Hence, an equilibrium solution is a constant solution. Such constant solutions show up on the direction field as horizontal lines, corresponding to the constant value of .
Figure 3.1 suggests that might be an equilibrium solution. Closer inspection of Figure 3.1 suggests that might also be an equilibrium solution. These potential equilibrium solutions can be found analytically by solving the equation = 0 for its roots. The roots of the equation
are found to be
Initial conditions that will produce the equilibrium solutions are
Use of these initial conditions in the DEplot command yields Figure 3.2.
Figure 3.3 shows a sample of solutions with initial conditions between the two equilibria.
Note that all of these solutions are increasing and appear to approach as continues to increase.
Figure 3.4 shows a sample of solutions with initial conditions above the positive equilibrium.
These solutions are all decreasing and also appear to approach the as increases.
The composite plot is appears in Figure 3.5.
To investigate some of the general properties of solutions to the logistic equation, recall that the equilibrium solutions are
A quick inspection of the ODE shows that > 0 when 0 < < and < 0 when > . The long-term behavior of the solutions can be determined from the explicit solution to the IVP by evaluating the following limit.
The limit exists as a finite number only if both and are nonnegative. With these physically reasonable assumptions, the limit is
Because all solutions with decrease to and all solutions with 0 < < increase to , the parameter is known as the carrying capacity of the logistic equation.
The equilibrium solution is called a stable equilibrium because, as increases, nearby solutions, those close and on either side of it, tend towards the equilibrium. This is shown very well in Figure 3.5.
The equilibrium solution is called an unstable equilibrium because, as increases, nearby solutions, those close and on either side of it, move away from the equilibrium. Figure 3.6, exhibiting solutions starting both above and below the equilibrium solution , shows that this equilibrium is unstable.
An equilibrium solution can also be called semi-stable if, as increases, nearby solutions on one side approach the equilibrium, but nearby solutions on the other move away from the equilibrium. Adifferential equation with a semi-stable equilibrium would be the equation
The equilibrium solutions are and , with the latter being the semi-stable equilibrium, as shown in Figure 3.7.
Solutions starting above move away from this equilibrium, but solutions starting below move towards it. Solutions on one side of the equilibrium are attracted to the equilibrium, but solutions on the other side are repelled.
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