Tuesday, July 12, 2016

calculus - Limit of the sequence nn/n!, is this sequence bounded, convergent and eventually monotonic?



I am trying to check whether or not the sequence an={nnn!}n=1

is bounded, convergent and ultimately monotonic (there exists an N such that for all nN the sequence is monotonically increasing or decreasing). However, I'm having a lot of trouble finding a solution that sufficiently satisfies me.



My best argument so far is as follows,




an=nnnnn(n1)(n2)(n3)(2)(1)=nnnn1n2n



so liman since $n1$. Since the sequence is divergent, it follows that the function must be ultimately monotonic.



This feels a little dubious to me, I feel like I can form a much better argument than that, or at the very least a more elegant one. I've tried to assume {an} approaches some limit L so there exists some N such that



|anL|<ϵ whenever n>N and derive a contradiction, but this approach got me nowhere.



Finally, I've also tried to use the fact that an+1ane to help me, but I couldn't find an argument where that fact would be useful.


Answer




HINT for the last part: Note that



an+1an=(n+1)n+1(n+1)!nnn!=(n+1)n+1n!nn(n+1)!=(n+1)n+1nn(n+1)=(n+1n)n.


No comments:

Post a Comment

analysis - Injection, making bijection

I have injection f:AB and I want to get bijection. Can I just resting codomain to f(A)? I know that every function i...