# Thread: -1^n = 1 or -1

1. ## (-1)^n = 1 or -1

Okay, this might be a little bit stupid question...
I'm studying ross elementary analysis.
And I'm trying to get some skills in writing formal proofs, instead of intuitive ideas.

I want to proof that the sequence (-1)^n = 1 or -1.
But how do I give a real proof for this ?

2. ## Re: -1^n = 1 or -1

Originally Posted by kasper90
Okay, this might be a little bit stupid question...
I'm studying ross elementary analysis.
And I'm trying to get some skills in writing formal proofs, instead of intuitive ideas.

I want to proof that the sequence -1^n = 1 or -1.
But how do I give a real proof for this ?
-1^n = -1, since you need to do the exponentiation before the negation.

3. ## Re: (-1)^n = 1 or -1

I actually meant to say (-1)^n.
sorry for that

4. ## Re: -1^n = 1 or -1

(-1)^n = 1 if n is even, and (-1)^n = -1 if n is odd.

5. ## Re: -1^n = 1 or -1

Okay, but how do I prove that ?

6. ## Re: -1^n = 1 or -1

Originally Posted by kasper90
Okay, but how do I prove that ?
If $\displaystyle n$ is even, $\displaystyle n=2k$ then $\displaystyle (-1)^n=(-1)^{2k}=[(-1)^{2}]^k=[1]^k=1$.

If $\displaystyle n$ is odd, $\displaystyle n=2k+1$ then (WHAT?)

7. ## Re: -1^n = 1 or -1

Okay, I'll try:
If n is even,
than ∃k∈N : n = 2k,
than (-1)^n = (-1)^2k=[(-1)^2]^k = 1^k

If n is uneven,
than ∃k∈N : n=2k-1,
than (-1)^n = (-1)^[2k-1] = -1^k,

Induction (prove that 1^k =1)
1^1
if 1^k = 1, than 1^(k+1)=1^k * 1 = 1^k = 1
By induction, 1^k =1 for all natural numbers.

8. ## Re: -1^n = 1 or -1

Plato posted his post just before I did,
but I think I got the idea, right ?

Is this a valid proof ?

9. ## Re: -1^n = 1 or -1

Originally Posted by kasper90
Plato posted his post just before I did,
but I think I got the idea, right ? Is this a valid proof ?
You have proved that $\displaystyle (-1)^{2k}=1$ so use it.
$\displaystyle (-1)^{2k+1}=(-1)^{2k}(-1)^{1}=(1)(-1)=-1$.