Convergence of Sequence a_n = a_{n+1}/sin(a_n) and its Limit Calculation

In summary, the conversation is discussing how to prove the convergence and calculate the limit of a series given certain conditions. The series is defined by a_0, where 0 < a_0 < \frac {\pi}{2}, and the relation sin(a_n)= \frac {a_n}{a_{n+1}}. The conversation mentions that at first, it was thought that the sequence converges to \frac {\pi}{2}, but it is realized that the inequality can hold for any n. Some pointers are requested in order to solve this problem. A suggestion to consider the function f(x)=x/sin(x) and show that it is bounded by pi/2 for x in (0,pi/2) is
  • #1
talolard
125
0
Hello, this is a question we had on an exam and I can't figure it out. Our professors won't publish solutions so I'd be glad for your help.

Homework Statement


Prove the following series converges and calculate its limit.

[tex] 0 < a_0 < \frac {\pi}{2} [/tex][tex]sin(a_n)= \frac {a_n}{a_{n+1}} [/tex]and so [tex] 1 > sin(a_0)= \frac {a_0}{a_{1}} [/tex] therefore [tex]a_{1}> a_0 [/tex]

At first I thought this was simple and the sequence converges to [tex] \frac {\pi}{2} [/tex]
But I realized that the inequality can hold for any n, i.e [tex] a_{n+1}> a_n [/tex] because we have no way of knowing by how much it is bigger. This one was on our exam and no one I talked to managed to overcome this little detail.
Some pointers would be greatly apreciated.
Thanks
Tal
 
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  • #2
It does converge to pi/2 and it is pretty simple. Consider the function f(x)=x/sin(x). First figure out what is the range of values of f(x) for x in (0,pi/2). I.e. look for maxs and mins of the function using critical points. Second, show f(x)>x for x in (0,pi/2). If you can show f(x) is bounded by pi/2 for x in (0,pi/2) then a_n is an increasing sequence bounded by pi/2. So a limit exists. If a limit exists then it must satisfy L=L/sin(L). What's the limit?
 

FAQ: Convergence of Sequence a_n = a_{n+1}/sin(a_n) and its Limit Calculation

What is the definition of convergence of a sequence?

Convergence of a sequence refers to the behavior of a sequence of numbers as its terms approach a specific limit value. In other words, a sequence is said to converge if its terms get closer and closer to a particular value as the sequence progresses.

How do you determine if a sequence is convergent or divergent?

A sequence is convergent if and only if its limit exists. This means that as the terms of the sequence get closer and closer to a certain value, that value is the limit. If the terms do not approach a specific value, the sequence is divergent.

What is the difference between absolute convergence and conditional convergence?

Absolute convergence refers to a sequence that converges regardless of the order of its terms, while conditional convergence refers to a sequence that only converges if the terms are arranged in a specific order. In other words, absolute convergence is more strict than conditional convergence.

Can a sequence have more than one limit?

No, a sequence can only have one limit. If a sequence has more than one limit, it is not considered convergent.

How do you prove that a sequence is convergent?

To prove that a sequence is convergent, you can use the definition of convergence and show that the terms approach a specific limit value as the sequence progresses. This can be done by using various mathematical techniques such as limits, inequalities, and theorems.

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