Prove True/False that n^3-n is Always Divisible By 6

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SUMMARY

The statement that \( n^3 - n \) is always divisible by 6 for all natural numbers \( n \) is true. The expression can be factorized as \( n(n-1)(n+1) \), which represents the product of three consecutive integers. Since at least one of these integers is even, \( n^3 - n \) is divisible by 2. Additionally, among any three consecutive integers, one must be divisible by 3, ensuring that \( n^3 - n \) is also divisible by 3. Therefore, \( n^3 - n \) is divisible by both 2 and 3, confirming that it is divisible by 6.

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  • Understanding of polynomial factorization
  • Familiarity with divisibility rules, particularly for 2 and 3
  • Knowledge of mathematical induction
  • Basic concepts of number theory regarding consecutive integers
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CheesyPeeps
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Homework Statement


For all natural numbers n, prove whether the following is true or false:
n3-n is always divisible by 6.

From SQA Advanced Higher Mathematics 2006 Exam Paper

Homework Equations


I can choose from the following types of proof:
Direct proof
Proof by contradiction
Proof by contrapositive
Proof by induction

The Attempt at a Solution


I know the statement is true, but proving it has been more difficult than I thought it would be!
I tried proof by induction, but got stuck with trying to prove true for n=k+1. I then tried proving the statement true for n=2k (even number) and n=2m+1 (odd number), but again, I didn't seem to be getting anywhere.
Am I along the right lines, or should I be trying something different?
 
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Try to factor ##n^3-n## and see what you can say about the factors.
 
CheesyPeeps said:
Am I along the right lines
A simple direct proof is possible. Start by factorising ##n^3-n## into three integer factors.

EDIT: Now I am jinxed and will have to wait for somebody to say my name before I can speak aloud again.
 
fresh_42 said:
Try to factor ##n^3-n## and see what you can say about the factors.

Okay, so I've factorised it and found that ##n^3-n## is always even and therefore always divisible by 2. In order for it to be divisible by 6, it must be divisible by 2 and 3, but I'm not sure how to go about proving that it's divisible by 3.

EDIT: I looked up the divisibility rule for 3, and found it expressed as ##n(n+1)(n-1)## which is exactly what I have from factorising ##n^3-n##!
 
Last edited:
What kind of numbers are ##n-1\, , \,n\, , \,n+1##?
 

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