Let be a irrational number
Consider the behaviour of when goes to infinity along positive integer values.
Which statement is true?
I had also posted a related problem :Find the limit.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
At first, one may think that it always has limit one since sin ( 2 π γ n ) is never zero for irrational γ and n n 1 approach zero very fast. But actually we can find some γ such that there is a subsequence of { sin ( 2 π γ n ) } approach zero so fast that the corresponding subsequence of { ∣ sin ( 2 π γ n ) ∣ n n 1 } also goes to zero. Following is a way to construct: Let γ = 0 . 1 0 . . 0 1 0 . . . . 0 1 0 . . . where the γ is consists of digits 0 , 1 and the number of consecutive 0 after the n th 1 is given by 1 0 ( k n + 1 ) 1 0 k n , where k n is the position where we encounter n th 1 in the decimal expression of γ . For example, after the first 1 appears, there are 1 0 2 0 zeroes after it. k 1 = 1 , k n + 1 = k n + 1 0 ( k n + 1 ) 1 0 k n + 1 which increase to infinity extremely fast. Now because sin ( 2 π γ n ) = sin ( 2 π { γ n } ) where { x } = x − [ x ] and [ x ] is the integer part of 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 . Then the 1 0 k n th term of the sequence is less then ( 2 π ∗ 1 0 − 1 0 ( k n + 1 ) 1 0 k n ) 1 0 k n 1 0 k n 1 = ( 2 π ) 1 0 k n 1 0 k n 1 1 0 − 1 0 1 0 k n which goes to zero!
And also for any value between -1, 1 , there is a subsequence of sin ( 2 π γ 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 ) ∣ n n 1 } goes to 1. So the sequence doesn't converge.