Sequential limit.

Calculus Level 5

Let { a n } \{a_n\} be a sequence such that a 1 ( 0 , 1 ) a_1\in (0,1) , and for any n 1 n\geq 1 : a n + 1 = a n ( 1 a n ) . a_{n+1}= a_n(1-a_n).

Find the value of

lim n + n ( 1 n a n ) ln n . \lim_{n\to +\infty} \frac{n(1-na_n)}{\ln n}.


The answer is 1.00.

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1 solution

Haroun Meghaichi
Jul 11, 2014

First, we have a n + 1 a n = a n 2 < 0 a_{n+1} -a_n=-a_n^2<0 and a n > 0 a n + 1 > 0 a_n>0 \Leftrightarrow a_{n+1}>0 , Which means that ( a n ) (a_n) is positively decreasing and therefore convergent and the limit is clearly 0 0 .

We have 1 a n + 1 1 a n = 1 1 a n 1 \frac{1}{a_{n+1}} - \frac{1} {a_{n}} = \frac{1}{1-a_n} \to 1 Then by Cesaro-Stolz theorem , we get n a n 1 na_n \to 1 .

We have lim n n ( 1 n a n ) ln n = lim n 1 a n n ln n \lim_{n\to \infty}\frac{n(1-n a_n)}{\ln n} = \lim_{n\to \infty} \frac{\frac{1}{a_n}-n}{\ln n} And we have : 1 a n + 1 1 a n 1 ln ( n + 1 ) ln n = n a n 1 1 a n 1 n ln ( 1 + 1 n ) = 1. \frac{\frac{1}{a_{n+1}} - \frac{1}{a_n} -1}{\ln(n+1) -\ln n } = n a_n \cdot \frac{1}{1-a_n} \cdot \frac{1}{n\ln\left(1+\frac{1}{n} \right)} = 1. Then (by Cesaro Stolz) the result is 1 \boxed{1} .

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