## [PrimeNumbers] Re: all primes as 1a^2 + 2b^2 +3c^2

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• ... If we give up the restriction to *odd* integers, it is interesting to find that some (even) numbers can t be represented. This is fully in accord with the
Message 1 of 7 , Apr 8, 2006
"Mark Underwood" <mark.underwood@...> wrote:

>>>It looks like all primes can also be written as a^2 + b^2 + 2c^2
>>
>> Legendre proved that all positive odd integers can be represented
>>in that form - see e.g. page 3 of:
>> www.math.wisc.edu/~ono/reprints/025.pdf
>>
>> Since all even primes can also be so represented, your conjecture
>>has indeed been proved (several centuries ago).
>>
>
>Yes, a guru has just presented me with the proof. He has also
>conjectured that 1a^2 + 2b^2 + 3c^2 itself can represent all odd
>positive integers.
>

If we give up the restriction to *odd* integers, it is interesting to find that some (even) numbers can't be represented.

This is fully in accord with the remark on p.6 of the following excellent paper:-
http://www.pubmedcentral.gov/articlerender.fcgi?artid=1076292
namely: "Universal forms - A positive form representing all positive integers .. is said to be universal. Methods of the previous section show that no positive ternary quadratic form is universal."

So there were *bound* to be integers not representable.

Up to 200, the list is just the following:-
10=2*5 = 2 mod 8
26=2*13 = 2 mod 8
40=2^3*5 = 0 mod 8
42=2*3*7 = 2 mod 8
58=2*29 = 2 mod 8
74=2*37 = 2 mod 8
90=2*3^2*5 = 0 mod 8
104=2^3*13 = 0 mod 8
106=2*53 = 2 mod 8
122=2*61 = 2 mod 8
138=2*3*23 = 2 mod 8
154=2*7*11 = 2 mod 8
160=2^5*5 = 0 mod 8
168=2^3*3*7 = 0 mod 8
170=2*7*17 = 2 mod 8
186=2*3*31 = 2 mod 8

Note the remarkable restriction to 0 or 2 mod 8 !
Other than that, there doesn't appear to be any obvious pattern.

(NB Even Ramanujan was unable to come up with patterns for the exceptions to the ternary forms he studied, so that strongly indicates there aren't any:-)

(Note that all this is departing more and more from the theme of our list: prime numbers...)

-Mike Oakes
• ... That was for the form a^2 + 2*b^2 + 3*c^2. It s interesting to explore also the exceptions to the form a^2 + b^2 + 2*c^2 (*) In this case there *does*
Message 2 of 7 , Apr 10, 2006
I wrote:

>"Mark Underwood" <mark.underwood@...> wrote:
>
>>>>It looks like all primes can also be written as a^2 + b^2 + 2c^2
>>>
>>> Legendre proved that all positive odd integers can be represented
>>>in that form - see e.g. page 3 of:
>>> www.math.wisc.edu/~ono/reprints/025.pdf
>>>
>>
>>Yes, a guru has [...] also
>>conjectured that 1a^2 + 2b^2 + 3c^2 itself can represent all odd
>>positive integers.
>>
>
>If we give up the restriction to *odd* integers, it is interesting to find that some (even) numbers can't be represented.
>
>This is fully in accord with the remark on p.6 of the following excellent paper:-
>http://www.pubmedcentral.gov/articlerender.fcgi?artid=1076292
>namely: "Universal forms - A positive form representing all positive integers .. is said to be universal. Methods of the previous section show that no positive ternary quadratic form is universal."
>
>So there were *bound* to be integers not representable.
>
>Up to 200, the list is just the following:-
>10=2*5 = 2 mod 8
>26=2*13 = 2 mod 8
>40=2^3*5 = 0 mod 8
>42=2*3*7 = 2 mod 8
>58=2*29 = 2 mod 8
>74=2*37 = 2 mod 8
>90=2*3^2*5 = 0 mod 8
>104=2^3*13 = 0 mod 8
>106=2*53 = 2 mod 8
>122=2*61 = 2 mod 8
>138=2*3*23 = 2 mod 8
>154=2*7*11 = 2 mod 8
>160=2^5*5 = 0 mod 8
>168=2^3*3*7 = 0 mod 8
>170=2*7*17 = 2 mod 8
>186=2*3*31 = 2 mod 8
>
>Note the remarkable restriction to 0 or 2 mod 8 !
>Other than that, there doesn't appear to be any obvious pattern.

That was for the form a^2 + 2*b^2 + 3*c^2.

It's interesting to explore also the exceptions to the form
a^2 + b^2 + 2*c^2 (*)

In this case there *does* seem to be a regular pattern, and moreover one which is understandable.

Consider the form (*) modulo 16.

Any odd integer has its square = 1 or 9 mod 16,
any even integer has square = 0 or 4 mod 16.

So (a^2 + b^2) = {0, 1, 2, 4, 5, 8, 9, 10, 13} mod 16,
and (2*c^2) = {0, 2, 8} mod 16.

So (*) <> 14 mod 16.

Clearly, multiplying a, b and c by 2,
we have also that
(*) <> 56 mod 64,
(*) <> 224 mod 256,
and so on.

I have checked up to 760, and there are no other exceptions.

Would anyone like to check up to some high bound that this is the *complete* set of unrepresetable integers?

Can anyone *prove* same? [This is likely to be *very* hard!]

Does anyone know a web page that gives Legendre's proof that all *odd* integers are representable?
(What a really clever guy he was!)

-Mike Oakes
• ... From: To: Sent: Monday, April 10, 2006 6:52 PM Subject: [PrimeNumbers] Re: all primes as 1a^2 + 2b^2
Message 3 of 7 , Apr 10, 2006
----- Original Message -----
From: <mikeoakes2@...>
Sent: Monday, April 10, 2006 6:52 PM
Subject: [PrimeNumbers] Re: all primes as 1a^2 + 2b^2 +3c^2

> It's interesting to explore also the exceptions to the form
> a^2 + b^2 + 2*c^2 (*)
>
> (*) <> 56 mod 64,
> (*) <> 224 mod 256,
> and so on.
>
> I have checked up to 760, and there are no other exceptions.
>
> Would anyone like to check up to some high bound that this is the
> *complete* set of unrepresetable integers?
>

I have checked up to 1000000 (10^6) : no other exceptions

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