Logistic Equation: Finding the Limit of Population Growth

  • Thread starter Thread starter nietzsche
  • Start date Start date
  • Tags Tags
    Limit
Click For Summary

Homework Help Overview

The discussion revolves around a logistic equation modeling population growth, specifically examining the limit of the population as time approaches infinity. Participants are analyzing the implications of initial conditions on the limit of the population function.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss finding equilibrium solutions and the behavior of the population over time. Questions are raised about the validity of limits when initial population values are zero versus non-zero. There is exploration of the dependency of the solution on initial conditions and the interpretation of constants in the solution.

Discussion Status

The discussion is active, with participants offering different perspectives on the implications of initial conditions and the nature of the solutions to the differential equation. Some guidance has been provided regarding the behavior of the solution as time approaches infinity, but there is still confusion about the initial population's role in determining the limit.

Contextual Notes

Participants are navigating the constraints of the logistic model, particularly the assumption that a population cannot grow from an initial value of zero. There is an ongoing debate about the interpretation of constants in the solution and their impact on the limit.

nietzsche
Messages
185
Reaction score
0

Homework Statement



A population develops according to logistic equation:
[tex]\frac{dy}{dt} = y(0.5-0.01y)[/tex]

Determine the following:
[tex]\lim_{t\rightarrow\infty} y(t)[/tex]


The Attempt at a Solution



By finding an equilibrium solution to the differential equation, we see that dy/dt = 0 when y = 0 or y = 50.

But does the limit exist? What if y = 0 initially?
 
Physics news on Phys.org
You should try to solve the differential equation first.
 
But do you have to do the differential to determine the limit? Because we can see that if y=0 and t goes to infinity, y will still equal 0. and if y does not equal 0 initially, then y will go to 50.
 
You have picked two fixed values for y which indeed satisfy the differential equation, however these values aren't the only solutions. The more interesting solutions are the ones that still depend on t.
 
I'm sorry, I don't understand.

I solved the differential equation:

y(t) = 50 / (1+Ae^(-0.5t)

where A = (50-y(0)) / y(0)

so isn't it still dependent on the value of y(0)?
 
Last edited:
OH. so now if we take the limit, as t approaches infinity, then A will go to 0, no matter what. is that correct?
 
Yea, more correctly, Ae^(-0.5t) goes to zero
 
ah okay, thanks.

i still don't get the concept though. how can the limit of the equation be 50 if the initial population is 0?
 
Since the equation is modeling the development of a population, that would imply that y(0) cannot be zero; otherwise you have no population to begin with.
 
  • #10
haha, i suppose. thanks very much cyosis and fightfish.
 
  • #11
nietzsche said:
OH. so now if we take the limit, as t approaches infinity, then A will go to 0, no matter what. is that correct?
No, it's not. As fightfish said, Ae-.5t goes to 0 as t approaches infinity. This, however, does not imply that A is 0 or is approaching zero. A is a constant in the problem.
 
  • #12
sorry, i'll be more specific next time...
 
  • #13
>> how can the limit of the equation be 50 if the initial population is 0?

You need to object to nietsche, who said the solution is:
y(t) = 50 / (1+Ae^(-0.5t)
that is NOT the solution if y(0)=0...
 

Similar threads

  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
Replies
3
Views
2K
Replies
2
Views
2K
Replies
3
Views
2K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
5
Views
2K
  • · Replies 16 ·
Replies
16
Views
3K
  • · Replies 7 ·
Replies
7
Views
2K