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

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Homework Help Overview

The problem involves determining whether the expression n3 - n is always divisible by 6 for all natural numbers n. This falls under the subject area of number theory and divisibility.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster attempts various proof techniques, including induction and case analysis for even and odd n, but expresses difficulty in progressing. Some participants suggest factoring the expression and exploring its properties, while others question the nature of the factors involved.

Discussion Status

The discussion is ongoing, with participants exploring different methods of proof and sharing insights about the factors of the expression. There is no explicit consensus yet, but some productive lines of reasoning have been introduced regarding the divisibility by 2 and 3.

Contextual Notes

Participants are considering the implications of the expression's factors and the rules of divisibility, particularly in relation to natural numbers. The original poster mentions constraints related to the types of proofs available for use.

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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