Let be a sequence such that , and for any :
Find the value of
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First, we have a n + 1 − a n = − a n 2 < 0 and a n > 0 ⇔ a n + 1 > 0 , Which means that ( a n ) is positively decreasing and therefore convergent and the limit is clearly 0 .
We have a n + 1 1 − a n 1 = 1 − a n 1 → 1 Then by Cesaro-Stolz theorem , we get n a n → 1 .
We have n → ∞ lim ln n n ( 1 − n a n ) = n → ∞ lim ln n a n 1 − n And we have : ln ( n + 1 ) − ln n a n + 1 1 − a n 1 − 1 = n a n ⋅ 1 − a n 1 ⋅ n ln ( 1 + n 1 ) 1 = 1 . Then (by Cesaro Stolz) the result is 1 .