# Thread: Rearranging recurrance relation into logistic form

1. ## Rearranging recurrance relation into logistic form

Hi, this is probably simple for most but I'm a little puzzled.

Say I have a recurrence system

P_0=50

P_n+1 - P_n=(0.45 + 0.005P_n)P_n (n=0,1,2,3....)

and I want to put it in logistic form

rP_n(1 - P_n/E)

where r and E are positive parameters

Both examples I have are of the form (a - bP_n)P_n which means I get the 1 -, but now my logistic form is going to be 1 +, does this matter??

Apologies if this is the wrong forum but it seemed quite general.
Also sorry if this is a boneheaded question.

2. Originally Posted by Turlock
Hi, this is probably simple for most but I'm a little puzzled.

Say I have a recurrence system

P_0=50

P_n+1 - P_n=(0.45 + 0.005P_n)P_n (n=0,1,2,3....)

and I want to put it in logistic form

rP_n(1 - P_n/E)

where r and E are positive parameters

Both examples I have are of the form (a - bP_n)P_n which means I get the 1 -, but now my logistic form is going to be 1 +, does this matter??

Apologies if this is the wrong forum but it seemed quite general.
Also sorry if this is a boneheaded question.
There is something wrong with your problem, it cannot be transformed into a logistic equation (that is if you want a positive attarctor)

CB

3. Originally Posted by CaptainBlack
There is something wrong with your problem, it cannot be transformed into a logistic equation (that is if you want a positive attarctor)

CB
Hi. It's for an assignment question so I thought it was frowned upon to use the exact values when asking for help.

My recurrance relation is (0.27 + 0.0009P_n)P_n

4. Originally Posted by Turlock
Hi. It's for an assignment question so I thought it was frowned upon to use the exact values when asking for help.

My recurrance relation is (0.27 + 0.0009P_n)P_n
It is not the exact values that are the problem but the "+" inside the brackets, when the other P_n term is brought over you have P_{n+1}=(1.27+0.0009*P_n)P_n, and so if P_n>0 we have P_{n+1}>1.27 P_n, so the sequence grows without bound.

CB

5. Ok, I think I've found my school boy error. I've got birth rate is 4.18 and death rate is 3.91+0.0009P. So obviously births minus deaths makes my P coefficient now minus. All the other times I had the P in the birth rate and it was already minus.

Thanks for looking. I'm now suitably embarrassed.

Much appreciated.