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Newsgroups: sci.math
From:
William Elliot <ma... @panix.com>
Date: Sat, 27 Oct 2012 19:25:45 -0700
Local: Sat, Oct 27 2012 10:25 pm
Subject: Perfect Square
Here's an unsolved problem from Ask An Algebraist.
> Let a and b be two positive integers such that ab+1 divides a^2 + b^2.
> Show that a^2 + b^2 divided by ab + 1 is a perfect square.
Two examples of a and b with p = ab + 1 | a^2 + b^2
are (1,1) and (2,8), for which (a^2 + b^2)/p = 1 and 4.
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Newsgroups: sci.math
From:
William Elliot <ma... @panix.com>
Date: Sat, 27 Oct 2012 20:27:21 -0700
Local: Sat, Oct 27 2012 11:27 pm
Subject: Re: Perfect Square
On Sat, 27 Oct 2012, William Elliot wrote:
> Here's an unsolved problem from Ask An Algebraist.
> > Let a and b be two positive integers such that ab+1 divides a^2 + b^2.
> > Show that a^2 + b^2 divided by ab + 1 is a perfect square.
> Two examples of a and b with p = ab + 1 | a^2 + b^2
> are (1,1) and (2,8), for which (a^2 + b^2)/p = 1 and 4.
Infinitely many examples. Let a be any positive integer and b = a^3.
Then p = a^4 + 1 divides a^2 + b^2 = a^2 + a^6 = a^2 (a^4 + 1).
Are there other examples for a and b for which p | a^2 + b^2 ?
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Newsgroups: sci.math
From:
Helmut Richter <hh... @web.de>
Date: Sun, 28 Oct 2012 22:07:51 +0100
Local: Sun, Oct 28 2012 5:07 pm
Subject: Re: Perfect Square
On Sat, 27 Oct 2012, William Elliot wrote:
> On Sat, 27 Oct 2012, William Elliot wrote:
> > Here's an unsolved problem from Ask An Algebraist.
> > > Let a and b be two positive integers such that ab+1 divides a^2 + b^2.
> > > Show that a^2 + b^2 divided by ab + 1 is a perfect square.
> > Two examples of a and b with p = ab + 1 | a^2 + b^2
> > are (1,1) and (2,8), for which (a^2 + b^2)/p = 1 and 4.
> Infinitely many examples. Let a be any positive integer and b = a^3.
> Then p = a^4 + 1 divides a^2 + b^2 = a^2 + a^6 = a^2 (a^4 + 1).
> Are there other examples for a and b for which p | a^2 + b^2 ?
Here are some (a, b, p)
30 8 241
112 30 3361
240 27 6481
418 112 46817
1020 64 65281
1560 418 652081
2133 240 511921
3120 125 390001
5822 1560 9082321
7770 216 1678321
16256 1020 16581121
16800 343 5762401
18957 2133 40435282
21728 5822 126500417
32760 512 16773121
59040 729 43040161
77875 3120 242970001
81090 21728 1761923521
But the quotiont is a perfect square also in all these examples.
I have some ideas how one could try to prove it but none of them makes for a complete proof.
-- Helmut Richter
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Newsgroups: sci.math
From:
Robin Chapman <zen165... @zen.co.uk>
Date: Mon, 29 Oct 2012 08:12:42 +0000
Local: Mon, Oct 29 2012 4:12 am
Subject: Re: Perfect Square
On 28/10/2012 02:25, William Elliot wrote:
> Here's an unsolved problem from Ask An Algebraist.
>> Let a and b be two positive integers such that ab+1 divides a^2 + b^2.
>> Show that a^2 + b^2 divided by ab + 1 is a perfect square.
> Two examples of a and b with p = ab + 1 | a^2 + b^2
> are (1,1) and (2,8), for which (a^2 + b^2)/p = 1 and 4.
http://en.wikipedia.org/wiki/Vieta_jumping
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Newsgroups: sci.math
From:
William Elliot <ma... @panix.com>
Date: Mon, 29 Oct 2012 01:23:57 -0700
Local: Mon, Oct 29 2012 4:23 am
Subject: Re: Perfect Square
On Sun, 28 Oct 2012, Helmut Richter wrote:
> On Sat, 27 Oct 2012, William Elliot wrote:
> > > > Let a and b be two positive integers such that ab + 1 > > > > divides a^2 + b^2.
> > > > Show that a^2 + b^2 divided by ab + 1 is a perfect square.
All variables are positive integers.
If p = ab + 1 | a^2 + b^2 and b = na, then n = a^2.
In addtion, if b = na and n = a^2, then p | a^2 + b^2 = pa^2.
> > Are there other examples for a and b for which p | a^2 + b^2 ?
> Here are some (a, b, p)
> 30 8 241
> 112 30 3361
...
> 81090 21728 1761923521
Computer generated?
> But the quotiont is a perfect square also in all these examples.
> I have some ideas how one could try to prove it but none of them makes for > a complete proof.
What have you tried? I tried working with a and k = b - a > 0.
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Newsgroups: sci.math
From:
William Elliot <ma... @panix.com>
Date: Tue, 30 Oct 2012 02:59:07 -0700
Local: Tues, Oct 30 2012 5:59 am
Subject: Re: Perfect Square
On Mon, 29 Oct 2012, Robin Chapman wrote:
> On 28/10/2012 02:25, William Elliot wrote:
> > Here's an unsolved problem from Ask An Algebraist.
> > > Let a and b be two positive integers such that ab+1 divides a^2 + b^2.
> > > Show that a^2 + b^2 divided by ab + 1 is a perfect square.
> http://en.wikipedia.org/wiki/Vieta_jumping
Hurrah. What a mess.
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