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Recursive sequence question

by happyxiong531
Tags: recursive, sequence
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happyxiong531
#1
Sep3-14, 01:34 PM
P: 3
I want to prove that
if the sequence [itex]a_n [/itex] satisfy that
[itex]a_{n+1}=a_n\left(1-c\frac{a_n}{1+a_n}\right)[/itex]
then [itex]a_n*c*n\rightarrow 1[/itex] for all positive [itex]c[/itex].

Like when [itex]c=1[/itex], then [itex]a_n*n\rightarrow 1[/itex],
but if [itex]c\neq 1[/itex], it's difficult to prove.
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mathman
#2
Sep3-14, 03:10 PM
Sci Advisor
P: 6,104
What makes you believe it is true? Your question implies [itex]a_n*n ->\frac{1}{c}[/itex]. Doesn't look right, especially for large c.
happyxiong531
#3
Sep3-14, 03:24 PM
P: 3
Quote Quote by mathman View Post
What makes you believe it is true? Your question implies a_n*n -> 1/c. Doesn't look right, especially for large c.
Thank you for you reply.
I think it's correct.
First, I can have [itex]a_n*n\rightarrow 1[/itex] when [itex]c=1[/itex], from
[itex]a_{n+1}=\frac{a_n}{1+a_n}=\frac{a_{n-1}}{1+2a_{n-1}}=\cdots=\frac{a_1}{1+(n+1)a_1}[/itex]

Then, let [itex]ca_n=b_n [/itex] if [itex]c\neq 1[/itex], [itex]c[/itex] is some constant. we can have [itex]b_{n+1}=b_n\left(1-\frac{b_n}{1+b_n/c}\right)[/itex].

Actually, it's easy to prove [itex]a_n[/itex] and [itex]b_n[/itex] will go to zero,
so, [itex]\frac{b_n}{1+b_n/c}\sim\frac{b_n}{1+b_n}[/itex], then[itex] b_n*n\rightarrow 1[/itex].

I have made a plot, it's correct no matter [itex]c[/itex] is larger or less than 1.
But I think my proof is not strict.
Thanks for your concern.

happyxiong531
#4
Sep3-14, 03:37 PM
P: 3
Recursive sequence question

Quote Quote by mathman View Post
What makes you believe it is true? Your question implies [itex]a_n*n ->\frac{1}{c}[/itex]. Doesn't look right, especially for large c.
Oh, I forget there is a condition that
the sequence should satisfy that[itex]1-c\frac{a_1}{1+a_1}[/itex]>0,
so that all the elements in this sequence should be positive, and c cannot be too large.
I have made some plots like [itex]c=0.5, c=2[/itex], the conclusion is correct.
Thanks
mathman
#5
Sep4-14, 04:41 PM
Sci Advisor
P: 6,104
Write out your complete proof.


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