Saturday, 20 September 2014

convergence divergence - Prove that the given sequence converges

Prove that the given sequence {an} converges:



a1>0,a2>0




an+1=2an+an1 for n2



As I observed, this sequence does not seem to be monotonic and that it could be bounded since the values of a1 and a2 are arbitrary positive numbers.



If the limit of the sequence existed, it would be equal to 1 by letting the limit of an be x as n goes to infinity, and solving the equation x = 2x+x => x = 1 or -1, from which we choose x = 1 since x must be positive.



The only idea that came to my mind is bounding the sequence using two other sequences that could be shown to converge to 1 (Let these sequences be bn and cn):



bn<=an<=cn




If we could find such sequences,and prove that they converge to 1, the problem would be solved. So, I tried to bound the sequence from both sides, and try to show that the limits are equal to 1, but failed to find such sequences. I found that it is a little difficult to analyze sequences of the form presented in the problem since the sequence fluctuates a lot.



I am not sure how to start off, any ideas or tricks for such problems would be appreciated.

No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find limh0sin(ha)h without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...