Induction

**weasley74** · April 18th 2008, 10:20 AM

Assume that x+(1/x) is an integer, how do I, by using induction, show that x^2 + (1/x^2) , x^3 + (1/x^3), .... , x^n + (1/x^n) are also integers?

**flyingsquirrel** · April 18th 2008, 11:15 AM

Hello

First, show that $x^2+\frac{1}{x^2}$ lies in $\mathbb{N}$ by developping $\left(x+\frac{1}{x}\right)\left(x+\frac{1}{x}\righ t)$ .

Then, assuming there exists an integer $n$ such that $x^n+\frac{1}{x^n}$ and $x^{n-1}+\frac{1}{x^{n-1}}$ lie in $\mathbb{N}$ , try to develop $\left(x^{n}+\frac{1}{x^{n}}\right)\left(x+\frac{1} {x}\right)$ .

**weasley74** · April 18th 2008, 01:01 PM

So by assuming that there exists an integer

such that

and

lie in

, what we then do is to show that this is also true for n+1 ?

**flyingsquirrel** · April 18th 2008, 01:03 PM

Yes. I should have written it, sorry.

**weasley74** · April 18th 2008, 01:05 PM

would it make any difference if we assumed that it's true for n+1, and for n+2, and by using this proving that it's also true for n?

**flyingsquirrel** · April 18th 2008, 01:27 PM

Yes, it would make a difference : it would be false.

Let's call $P(n)$ the relation " $x^n+\frac{1}{x^n}$ is an integer"
If one shows that if $P(n-1)$ and $P(n)$ are true then $P(n+1)$ is true too, as $P(1)$ and $P(2)$ are true, we get that $P(3)$ is true. Then, as $P(2)$ and $P(3)$ are true, we get that $P(4)$ is true... so we'll reach any integer.

Instead, if you show that $P(n+1)$ and $P(n+2)$ true imply $P(n)$ true, you go decreasing and won't reach all the integers...

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