A question regarding Logistic population model

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Discussion Overview

The discussion revolves around the Logistic population model, specifically the implications of the equation $$\frac{dp}{dt} = kp\left(1- \frac{p}{N}\right)$$ and the conditions under which the derivative $$\frac{dp}{dt}$$ equals zero when the initial population $$p(0)$$ is set to zero. Participants seek clarification on the reasoning behind the model's behavior and its mathematical implications.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the logic presented in the course regarding the condition that if $$p(0) = 0$$, then $$\frac{dp}{dt} = 0$$ for all $$t$$, seeking further explanation.
  • Another participant suggests that if there are no individuals to reproduce, it raises a question about the feasibility of reproduction.
  • A participant proposes a hypothetical scenario regarding the minimum number of individuals required to start reproducing, indicating a practical application of the model.
  • There is a query about whether the condition $$p(0) = 0$$ would still lead to $$\frac{dp}{dt} = 0$$ if $$\frac{dp}{dt}$$ were a function of $$t$$.
  • A later reply discusses the derivative of a constant function, emphasizing that the derivative of a constant is zero, but does not resolve the initial question about the logistic model.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the implications of the initial condition $$p(0) = 0$$ and its effects on the derivative $$\frac{dp}{dt}$$. There is no consensus on the interpretation of the model's behavior under these conditions.

Contextual Notes

The discussion includes assumptions about the nature of the population model and the mathematical properties of derivatives, which may not be fully articulated by all participants. The implications of having $$\frac{dp}{dt}$$ as a function of $$t$$ remain unresolved.

issacnewton
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Hi
I am going through Edx course on Introduction to Differential Equations by Paul Blanchard (BUx: Math226.1x). At one point, he is explaining the Logistic population model.$$\frac{dp}{dt} = kp\left(1- \frac{p}{N}\right) $$ After this, he says that since the right hand side does not involve t, if p(0) = 0 then \frac{dp}{dt} = 0 for all t. I don't quite get his logic here. Can anyone explain this please ?

Thanks
 
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If you can answer a question like the one below you get the gist of what is going on.
Example: How many people does it take to start reproducing more people? Or mammals or bacteria?
 
Jim, it will take 2 persons or in case of bacterias, it will just take one.
 
Can anybody give more hints ?
 
If there are none to start with, how can you reproduce?
 
Ok that makes sense. But if we have \frac{dp}{dt} function of t, then does p(0) = 0 still lead to \frac{dp}{dt} = 0 for all t ?
 
IssacNewton said:
After this, he says that since the right hand side does not involve t, if p(0) = 0 then \frac{dp}{dt} = 0 for all t. I don't quite get his logic here.

The derivative of a constant function is zero -meaning the derivative of a constant function is the "zero function". For example, if ##f(x) = 3x + 15## when you compute ##\frac{df}{dx} ## as the derivative of ##3x## plus the derivative of ##15## what do you get for the answer when you differentiate the ##15##?

Note that ##15## must be taken to denote the constant function ##g(x) = 15## in order to differentiate it because we differentiate functions, not single numbers.
 

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