Sequence limit! Think carefully..

Calculus Level pending

Let γ \gamma be a irrational number

Consider the behaviour of sin ( 2 π γ n ) 1 n n |\sin(2\pi\gamma n)|^{\frac{1}{n^{n}}} when n n goes to infinity along positive integer values.

Which statement is true?

I had also posted a related problem :Find the limit.

for any γ \gamma the limit exists and equals to 1 there exists a γ \gamma such that the sequence has limit 0 for some γ \gamma the limit doesn't exsist for any γ \gamma the limit exists but the value depends on γ \gamma

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1 solution

Cuize Han
Jan 3, 2015

At first, one may think that it always has limit one since sin ( 2 π γ n ) \sin(2\pi\gamma n) is never zero for irrational γ \gamma and 1 n n \frac{1}{n^{n}} approach zero very fast. But actually we can find some γ \gamma such that there is a subsequence of { sin ( 2 π γ n ) } \{\sin(2\pi\gamma n)\} approach zero so fast that the corresponding subsequence of { sin ( 2 π γ n ) 1 n n } \{|\sin(2\pi\gamma n)|^{\frac{1}{n^{n}}}\} also goes to zero. Following is a way to construct: Let γ = 0.10..010....010... \gamma=0.10..010....010... where the γ \gamma is consists of digits 0 , 1 0,1 and the number of consecutive 0 0 after the n n th 1 1 is given by 1 0 ( k n + 1 ) 1 0 k n 10^{(k_{n}+1)10^{k_{n}}} , where k n k_{n} is the position where we encounter n n th 1 1 in the decimal expression of γ \gamma . For example, after the first 1 1 appears, there are 1 0 20 10^{20} zeroes after it. k 1 = 1 , k n + 1 = k n + 1 0 ( k n + 1 ) 1 0 k n + 1 k_{1}=1, k_{n+1}=k_{n}+10^{(k_{n}+1)10^{k_{n}}}+1 which increase to infinity extremely fast. Now because sin ( 2 π γ n ) = sin ( 2 π { γ n } ) \sin(2\pi\gamma n)=\sin(2\pi\{\gamma n\}) where { x } = x [ x ] \{x\}=x-[x] and [ x ] [x] is the integer part of x x , sin ( 2 π { γ 1 0 k n } ) 2 π { γ 1 0 k n } 2 π 2 1 0 ( k n + 1 k n ) 2 π 1 0 1 0 ( k n + 1 ) 1 0 k n |\sin(2\pi\{\gamma 10^{k_{n}}\})|\leq 2\pi\{\gamma 10^{k_{n}}\}\leq 2\pi*2*10^{-(k_{n+1}-k_{n})}\leq 2\pi*10^{-10^{(k_{n}+1)10^{k_{n}}}} . Then the 1 0 k n 10^{k_{n}} th term of the sequence is less then ( 2 π 1 0 1 0 ( k n + 1 ) 1 0 k n ) 1 1 0 k n 1 0 k n = ( 2 π ) 1 1 0 k n 1 0 k n 1 0 1 0 1 0 k n (2\pi*10^{-10^{(k_{n}+1)10^{k_{n}}}})^{\frac{1}{10^{k_{n}10^{k_{n}}}}}=(2\pi)^{\frac{1}{10^{k_{n}10^{k_{n}}}}}10^{-10^{10^{k_{n}}}} which goes to zero!

And also for any value between -1, 1 , there is a subsequence of sin ( 2 π γ n ) \sin(2\pi\gamma n) has the limit of that value (intuitively true but may not that easy to prove ). If the value is not zero, then the corresponding subsequence of { sin ( 2 π γ n ) 1 n n } \{|\sin(2\pi\gamma n)|^{\frac{1}{n^{n}}}\} goes to 1. So the sequence doesn't converge.

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