From: riderofgiraffes on
This isn't new, this isn't deep, and it isn't a mystery,
but I sometimes use this to get kids interested in proof.

For prime p>3, p^2-1 is a multiple of 24.

It's not hard to prove, but it's a good one to get kids
thinking about how we can know something is always true
without just checking every number.

Just an idle thought. I'll now return you to your usual
program of deep maths and troll baiting.
From: José Carlos Santos on
On 10-06-2010 20:56, riderofgiraffes wrote:

> This isn't new,

It was for me.

> this isn't deep, and it isn't a mystery,
> but I sometimes use this to get kids interested in proof.
>
> For prime p>3, p^2-1 is a multiple of 24.
>
> It's not hard to prove, but it's a good one to get kids
> thinking about how we can know something is always true
> without just checking every number.

Very good example.

Best regards,

Jose Carlos Santos
From: TCL on
On Jun 10, 3:56 pm, riderofgiraffes <mathforum.org...(a)solipsys.co.uk>
wrote:
> This isn't new, this isn't deep, and it isn't a mystery,
> but I sometimes use this to get kids interested in proof.
>
> For prime p>3, p^2-1 is a multiple of 24.
>
> It's not hard to prove, but it's a good one to get kids
> thinking about how we can know something is always true
> without just checking every number.
>
> Just an idle thought.  I'll now return you to your usual
> program of deep maths and troll baiting.

Here is an improvement:

For any odd number n not divisible by 3, n^2-1 is a multiple of 24.

-TCL
From: Liviu on
"TCL" <tlim1(a)cox.net> wrote...
> On Jun 10, 3:56 pm, riderofgiraffes wrote:
>>
>> This isn't new, this isn't deep, and it isn't a mystery,
>> but I sometimes use this to get kids interested in proof.
>>
>> For prime p>3, p^2-1 is a multiple of 24.
>>
>> It's not hard to prove, but it's a good one to get kids
>> thinking about how we can know something is always true
>> without just checking every number.
>
> Here is an improvement:
>
> For any odd number n not divisible by 3, n^2-1 is a multiple of 24.

Right, of course. However, whether that's an improvement or not
(pedagogically speaking) largely depends on the target audience and
intended point. The original statement leaves room for "relevancy of
the hypothesis" followups like "now that you've proved it, where was
the primeness of p used in the proof? ...or, if it wasn't, then what
could be a more general condition on p for the conclusion to hold".

Liviu




From: alainverghote on
On 11 juin, 06:43, "Liviu" <lab...(a)gmail.c0m> wrote:
> "TCL" <tl...(a)cox.net> wrote...
> > On Jun 10, 3:56 pm, riderofgiraffes wrote:
>
> >> This isn't new, this isn't deep, and it isn't a mystery,
> >> but I sometimes use this to get kids interested in proof.
>
> >> For prime p>3, p^2-1 is a multiple of 24.
>
> >> It's not hard to prove, but it's a good one to get kids
> >> thinking about how we can know something is always true
> >> without just checking every number.
>
> > Here is an improvement:
>
> > For any odd number n not divisible by 3, n^2-1 is a multiple of 24.
>
> Right, of course. However, whether that's an improvement or not
> (pedagogically speaking) largely depends on the target audience and
> intended point. The original statement leaves room for "relevancy of
> the hypothesis" followups like "now that you've proved it, where was
> the primeness of p used in the proof? ...or, if it wasn't, then what
> could be a more general condition on p for the conclusion to hold".
>
> Liviu

Good morning,

We may also consider the product
P = (n-1)n(n+1)(n+2) always a multiple of 24,
more,since (n-1)(n+2)= n^2+n-2, n(n+1)= n^2+n
P+1 is a square (n^2+n-1)^2

Alain