- #1

Math100

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- 219

- Homework Statement
- Prove the assertion below:

The only prime of the form n^3-1 is 7.

[Hint: Write n^3-1 as (n-1)(n^2+n+1).]

- Relevant Equations
- None.

Proof: Suppose p is a prime such that p=n^3-1.

Then we have p=n^3-1=(n-1)(n^2+n+1).

Note that prime number is a number that has only two factors,

1 and the number itself.

Since n^2+n+1>1 for ##\forall n\in\mathbb{N}##,

it follows that n-1=1, and so n=1+1=2.

Thus n=2, and so p=n^3-1=2^3-1=8-1=7.

Therefore, the only prime of the form n^3-1 is 7.

Above is my proof for this assertion. Can anyone please verify/review it and see if it's correct?

Then we have p=n^3-1=(n-1)(n^2+n+1).

Note that prime number is a number that has only two factors,

1 and the number itself.

Since n^2+n+1>1 for ##\forall n\in\mathbb{N}##,

it follows that n-1=1, and so n=1+1=2.

Thus n=2, and so p=n^3-1=2^3-1=8-1=7.

Therefore, the only prime of the form n^3-1 is 7.

Above is my proof for this assertion. Can anyone please verify/review it and see if it's correct?