Convergence or divergence

Calculus Level 3

Consider the recursive relation, x n = x n 1 + 1 x n 1 ! , x_n = x_{n-1} + \dfrac1{\lfloor x_{n-1} \rfloor ! } , where x 1 = 1 x_1 = 1 .

Is it true that lim n x n = 2 e \displaystyle\lim_{n\to\infty} x_n = 2e ?

Yes No

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1 solution

Brian Moehring
Oct 26, 2018

Assume the limit exists and is finite. Then 0 = lim n x n + 1 lim n x n = lim n x n + 1 x n = lim n 1 ( x n ) ! \begin{aligned} 0 &= \lim_{n\to\infty} x_{n+1} - \lim_{n\to\infty} x_n \\ &= \lim_{n\to\infty} x_{n+1} - x_n \\ &= \lim_{n\to\infty} \frac{1}{(x_n)!} \end{aligned} which in particular implies lim n ( x n ) ! = \lim_{n\to\infty} \Big|(x_n)!\Big| = \infty

However, since obviously x n x 1 = 1 , x_n \geq x_1 = 1, the only singularity it could be approaching is at infinity, giving a limit of lim n x n = , \lim_{n\to\infty} x_n = \infty, a contradiction to the assumption the limit is finite, making the statement in the problem to be no . \boxed{\text{no}}.


Note : If the limit had come back as 2 e , 2e, then we'd have to actually prove the limit exists to make our conclusion. However, since we show the limit is infinite, the assumption that the limit exists and is finite is harmless to conclude the statement is false.

Great job on the solution!

Garv Khurana - 2 years, 7 months ago

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