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Consider the sequence ${x_n}$ defined by $x_n = [nx]/ n$ for $x\in\mathbb R$ where $[·]$ denotes the integer part. Then ${x_n}$
(a) converges to $x$.
(b) converges but not to $x$.
(c) does not converge
(d) oscillates.

I think (a) is correct as $\lim_{n \to \infty}[n]/n=1$. Am I right?

  • 0
    answer should depend on x.2012-11-23

4 Answers 4

0

$x_n=\frac{\lfloor nx \rfloor}{n}=\frac{nx-\{nx\}}{n}=x-\frac{\{nx\}}{n}$

$0\leq\{nx\}<1\implies\lim_{n\to\infty}x_n=x$

9

The answer is (a), but your reasoning is not correct: you cannot factor out $x$ like that. What you can do is observe that

$0\le x-\frac{\lfloor nx\rfloor}n=\frac{nx-\lfloor nx\rfloor}n<\frac1n$

and take limits as $n\to\infty$.

6

You are right since for $x=1$ you get $\displaystyle{\lim_{n \to \infty}\frac{[xn]}{n}=\lim_{n \to \infty}\frac{[n]}{n}=1=x}$.
Therefore you can eliminate (b),(c) and (d) and the correct answer is (a).
For the proof use that $nx-1<[nx]\leq nx\Rightarrow x-\frac{1}{n}<\frac{[nx]}{n}\leq x, \ \forall n \in \mathbb{N}.$

  • 1
    :What if x=1.7?2016-08-19
6

We have $|\,\lfloor nx\rfloor -nx\,|<1$, which implies $ \left|\frac{\lfloor nx\rfloor}n - x\right|<\frac1n. $ So $x_n$ always converges to $x$, for any $x\in\mathbb R$.