Thursday, September 7, 2017

elementary number theory - Proof by induction that $n^3 - n$ is divisible by $6$


Show using induction that $n^3-n$ is divisible by 6 $\forall n\ge1, \quad n \in \mathbb{N}$




First off i show that the basis step: $1^3-1=0, \quad \frac{0}{6}=0$



Now I factorised it and set it equal to a multiple of 6: $\mathbf{n(n+1)}(n-1)=6A$



Assuming the result is true for k terms, and trying for $k+1$ terms:




$\mathbf{k(k+1)}(k+2)=6B$



I'm stuck here, I realise that the bold terms are the same, but $k+2$ and $n-1$ are not. Could someone show me what do to next to solve this.



Also is it possible to prove this using modular arithmetic?



Thanks,

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