Thread: Please explain about logistic equation

Please explain about logistic equation

dN/dt = (a-bN)N where a and b are positive number

if W(0)=N<sub>0

N=0 and N=a/b is stable or unstable?

I try to simplify the eq. to:

N(a-bN) = Na(1-(b/a)N)

but it does not explain anything, please help</sub>

If you set the derviative equal to zero you will get the constant solutions to the equation. They are

and

This divides the plane into three parts.

If you pick an N value less than zero lets call it and test it in the ODE you get



This is negative so the derivative is always negative in this regrion

If you pick an N value



So the product is positive this gives that the derivative is always positive here

If you check the last region you will find the derivative is always negative


The horizontal line is unstable all solutions move away from it.

The horizontal line is stable all solutions approch it as time increases.

Try to sketch the above it will help.

I still don't get it....why is it divided into 3 parts and how you get (dN/dt)t
Could you please explain more?

Thank you

The critical values of the derivative are found by setting it equal to zero.

If you solve that equation you get. N=0 or N =a/b

If you draw a plot they will be horizontal lines intersecting the vertical axis.

This divides the plance into three parts. The derivative can only change from positive to negative when it crosses these horizontal lines.

What this means is, that is where the function changes from increasing to decreasing and from decreasing to increasing.

See attached.


The plot would look like this.