How to calculate the limit of a recursively defined sequence?

814 Views Asked by At

enter image description here

My attempt:

(i) For r=1, $x_n$ = n+1 $\iff$ $\frac{n+1}{1+1/n}$ = $x_{n-1}$ $\iff$ $\frac{n(n+1)}{n+1}$ = $x_{n-1}$ $\iff$ n = $x_{n-1}$

But I don't know how to use what I have shown so far to show that $x_n$ = n+1

(ii) $$\lim_{n\to\infty} x_n = (r+0) \ell = r\ell $$

I know this is the wrong answer, but I'm not sure what else to try.

1

There are 1 best solutions below

0
On BEST ANSWER

We have the recursive relationship $x_n=\left(r+\frac1n\right)x_{n-1}$, with initial condition $x_0=1$. For $r=1$, we have $x_n=\left(1+\frac1n\right)x_{n-1}=\left(\frac{n+1}{n}\right)x_{n-1}$.

Thus, we can generate the solution recursively as

$$x_n=\frac{n+1}{n} \frac{n}{n-1} \frac{n-1}{n-2}\cdots \frac{4}{3}\frac32 \frac21=n+1$$

as expected!


Alternatively, we can use induction. We see that $x_1=2$ and $x_2=3$. So, we assume that for some $k$, $x_k=k+1$. Then,

$x_{k+1}=\frac{k+2}{k+1}\times x_k=k+2$

which completes the proof.


For part $(2)$, we assume that $\lim_{n\to \infty}x_n=L$. Then,

$$x_n=\left(r+\frac1n\right)x_{n-1}\implies L=rL\implies r=1\,\,\text{or}\,\,L=0$$

Since we already showed that $x_n$ has no limit if $r=1$, then we conclude that $L=0$.