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Topic: x_(n+1) = sin(x_n) (Read 15485 times) |
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Aryabhatta
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x_(n+1) = sin(x_n)
« on: Jul 27th, 2009, 4:22pm » |
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Consider the sequence
xn+1 = sin(xn)
x1 = 1.
Show that for any t, such that 0 < t < 1/2
the sequence
nt xn converges to 0 as n -> infinity.
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Aryabhatta
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Re: x_(n+1) = sin(x_n)
« Reply #1 on: Jul 27th, 2009, 6:25pm » |
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Also show that:
n0.5 xn converges to sqrt(3).
(i think this is so, i hope my working is right)
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| « Last Edit: Jul 27th, 2009, 6:55pm by Aryabhatta » |
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Eigenray
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Re: x_(n+1) = sin(x_n)
« Reply #2 on: Jul 27th, 2009, 8:17pm » |
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Many moons ago, I showed the following on probability problem set. You might find it interesting.
Background: In a branching process, we have a fixed probability distribution p, and in each generation, every individual independently has k offspring with probability p(k), k = 0,1,2,.... So if Xn is the number of individuals alive in generation n, then Xn+1 is the sum of Xn-many independent, identically distributed random variables.
Let's assume that X0 = 1, p(0) > 0, and  =  k p(k) = E(X1)  1.
(a) If = 1 and  2 <  , then there exist constants 0 < c1 < c2 < such that
c1/n < P( Xn  0 ) < c2/n
(b) If > 1, then there exist c,b > 0 such that P( extinction | Xn 0 )  ce-bn.
If we let
1 - ( sin  {1-x} )2 = pk xk,
then part (a) applies to show
c1/n < xn < c2/n
for some c1,c2.
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Eigenray
wu::riddles Moderator
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Re: x_(n+1) = sin(x_n)
« Reply #3 on: Jul 27th, 2009, 8:54pm » |
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I would guess that under the conditions of (a) above, we have generally that
n * P(Xn 0)  2/ ''(1),
where (x) = p(k) xk.
If so this would imply n1/2 xn 3.
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Aryabhatta
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Re: x_(n+1) = sin(x_n)
« Reply #4 on: Jul 28th, 2009, 10:28am » |
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That is an interesting approach Eigenray. Do you have a proof of a) & b) written down somewhere?
The approach I had in mind:
I think we can show that
sqrt(3)/(sqrt(n) + 1) <= xn <= sqrt(3/n)
using
x - x3/6 < sin(x) < x - x3/6 + x5/120
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| « Last Edit: Jul 28th, 2009, 10:30am by Aryabhatta » |
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Aryabhatta
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Re: x_(n+1) = sin(x_n)
« Reply #5 on: Jul 28th, 2009, 2:09pm » |
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I think I discovered a mistake in the 'proof' I had.
Back to the board.
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Eigenray
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Background : If an = P( Xn = 0 ), then an+1 = (an)  a, where the probability of extinction, a, is the smallest positive root of (a) = a. We have a < 1 iff = '(1) > 1.
It's a little awkward because it's phrased in terms of iterating instead of what would be more natural, f(t) = 1 - (1-t).
Suppose f : [0,1] [0,1] is sufficiently smooth, with f(0) = 0, f'(0) = 1, f''(x) < 0. If xn+1 = f(xn), x0=1, then we should have n xn -2/f''(0). Heuristically, we can think of f(x) ~ x(1-rx), and xn ~ 1/(r n).
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Aryabhatta
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Re: x_(n+1) = sin(x_n)
« Reply #7 on: Jul 28th, 2009, 8:48pm » |
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Actually, I reworked the whole thing and now it looks correct to me!
i.e. for all n > 0,
sqrt(3)/(sqrt(n) +1) < xn < sqrt(3/n)
Proved using (very tedious) induction on n, and the identities
x - x3/6 < sin x < x - x3 + x5/120.
The LHS is pretty straightforward, but the RHS yields a cubic
f(x) = 960x3 -520x2 + 111x - 9
which needs to be > 0 for x >= 1.
This has only one real root < 1, and so f(x) > 0 for x >= 1.
Perhaps we can simplify it.
I did find the result(inspite of the boring proof) to be interesting enough to post it here.
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