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• Replied to djbroadhurst, but fergitted that replies in this group don t go to group. .... o}o:-:o{o ... From: Insall [mailto:montez@rollanet.org] Sent:
Message 1 of 4 , Jul 2, 2002
Replied to djbroadhurst, but fergitted that replies in this group don't go
to group. .... o}o:-:o{o

-----Original Message-----
From: Insall [mailto:montez@...]
Sent: Monday, July 01, 2002 6:11 AM

Proposition: Let n be a positive integer such that for every pair a, b in
Z_n with ab=-1, we have a+b=0. Then for every pair a,b in Z_n such that
ab=-1, we have a^2=b^2=1.

Proof:
Let a, b in Z_n with ab=-1. Then

a^2=a^2-1+1=a^2+ab+1=a(a+b)+1=a(0)+1=1.

A similar argument works for b. (In fact, this observation is quite
trivial, but all such things include a bit of fun. :-} )

qed

Example: If a, b are in z_12 and ab=-1, then {a,b} is either {1,11} or
{5,7}. Since in Z_12, 1^2=1, 11^2=(-1)^2=1, 5^2=25=1 and 7^2=49=1, this is
an example of the proposition.

What more can be said about those positive integers n for which the set of
pairs a, b in Z_n such that ab=-1 is a subset of the set of pairs of the
form (a,b)=(a,-a)? (In particular, does this hold for all n?)

Well, the answer is not immediate, as the following proposition suggests.

Proposition: There exists a finite nontrivial ring R in which there are
elements a, b such that ab=-1 and a+b=1. The ring R can be taken to be a
field of any finite (prime) characteristic, and in this field, one can prove
that if ab=-1 and a+b=1, then both a and b are zeros of the polynomial
x^2-x-1. Moreover, if p is the characterisic of this field R, then R need
not have more than p^2 elements.

Proof:
This is a simple exercise in finite field theory. I shall do some and
leave the rest for the restless. Let p be any prime number whatsoever, and
let R be the splitting field for the polynomial x^2-x-1 over Z_p. Let a and
b be the roots (counting multiplicities) in R of the equation x^2-x-1=0.
Then

x^2-x-1=(x-a)(x-b)=x^2-(a+b)x+ab.

It follows that ab=-1 and a+b=1. Note that R is an at most degree two
extension field of Z_p, so R has at most p^2 elements. It is easy to show
that in any ring, if a and b are such that ab=-1 and a+b=1, then a^2-a-1=0
and similarly b^2-b-1=0.

qed

But this shows that one must work a bit harder to determine the rings in
which ab=-1 implies a+b=0. (But it may not be very much harder. I am just
thinking while I wile away some time here...)

Well now, there is a root of the equation x^2-x-1=0 in Z_19. (I know, I
know, it's a prime field, but I work slowly...) In particular, 5 works in
Z_19. The other root in z_19 is 15, and we have 5(15)=5(-4)=-20=-1 in Z_19.
Son o ma gun, she works out! In Z_19, Markus' theorem holds.

Is there a composite n for which there are a, b in Z_n with ab=-1 but
a+b<>0? I think so...

Let n=55 and a=8 and b=48. Then ab=-1, and a+b=1. Hence the Markus result
integers. Let us call them ``Markus' positive integers''. Which primes are
``Markus' primes''?

Proposition: Let p be a Markus' prime. Then the only elements a of Z_p for
which there is an element b with both ab=-1 and a+b=0, are 1 and -1.

Proof:
This is because a^2=1.

qed

Matt

-----Original Message-----
Sent: Sunday, June 30, 2002 10:53 PM

Markus observed:
If a*b=-1 mod 12 then a+b=0 mod 12.
Proof is easy: The only residue pairs
for {a,b} mod 12 are {1,11} or {5,7}.
It's true for both of these pairs,
so we are done.
David

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• Here are the first few primes, note that 55 is made up of the first 2 primes in our series. The only composites that can be generated have to be divisible
Message 2 of 4 , Jul 2, 2002
Here are the first few primes, note that 55 is made up of the first 2
primes in our series. The only composites that can be generated have to
be divisible by primes generated previously in the series. 11*5 =
55, the next smallest composite is 5*19 = 95 (43,53). The smallest 3
prime composite possible is 5*11*19 .

5 (3,3)
11 (4,8)
19 (5,15)
29 (6,24)
31 (13,19)
41 (7,35)
55 (8,48)

Many of these numbers are simple to generate because all you need to do is
subtract n from the closest square and then subtract one. Which is for the
most part the same as saying x^2-x-1=0
n^2-x-1=0 (49 -41-1) = 7

>Well now, there is a root of the equation x^2-x-1=0 in Z_19. (I know, I
>know, it's a prime field, but I work slowly...) In particular, 5 works in
>Z_19. The other root in z_19 is 15, and we have 5(15)=5(-4)=-20=-1 in Z_19.
>Son o ma gun, she works out! In Z_19, Markus' theorem holds.
>
>Is there a composite n for which there are a, b in Z_n with ab=-1 but
>a+b<>0? I think so...
>
>Let n=55 and a=8 and b=48. Then ab=-1, and a+b=1. Hence the Markus result
>integers. Let us call them ``Markus' positive integers''. Which primes are
>``Markus' primes''?
>
>Proposition: Let p be a Markus' prime. Then the only elements a of Z_p for
>which there is an element b with both ab=-1 and a+b=0, are 1 and -1.
>
>Proof:
> This is because a^2=1.
>
>qed
>
>
>
>
>
>Matt
>
>
>
>-----Original Message-----
>Sent: Sunday, June 30, 2002 10:53 PM
>
>
>Markus observed:
>If a*b=-1 mod 12 then a+b=0 mod 12.
>Proof is easy: The only residue pairs
>for {a,b} mod 12 are {1,11} or {5,7}.
>It's true for both of these pairs,
>so we are done.
>David
>
>
>
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>The Prime Pages : http://www.primepages.org
>
>
>
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>
>
>
>
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>
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• I wanted to add one more thing, can anyone explain why the following pattern has emerged? 11 = 5+(3*2), 19 = 11 + (4*2), 29 = 19 + (5*2)... to fininity? 5
Message 3 of 4 , Jul 2, 2002
I wanted to add one more thing, can anyone explain why the following
pattern has emerged? 11 = 5+(3*2), 19 = 11 + (4*2), 29 = 19 + (5*2)...
to fininity?

5 3, 3
11 4, 8
19 5, 15
29 6, 24
41 7
55 8
71 9
89 10
109 11
131 12
155 13
181 14
209 15
239 16
271 17
305 18
341 19
379 20
419 21
461 22
505 23
551 24
599 25
649 26
701 27
755 28
811 29
869 30
• To explain this pattern, you also have to explain why, 31 with A and B 13 and 19 divides elements 13 (155), and 19 (341) of the pattern, same with 59 with
Message 4 of 4 , Jul 2, 2002
To explain this pattern, you also have to explain why, 31 with A and B 13
and 19 divides elements 13 (155), and 19 (341) of the pattern, same with
59 with a,b 26 and 34 divide elements 26 and 24 of the main pattern
below... All primes/composites that have a*b=-1 and a+b=1 pattern follow
this pattern.

At 07:37 PM 7/2/2002 -0700, Markus Frind wrote:
>I wanted to add one more thing, can anyone explain why the following
>pattern has emerged? 11 = 5+(3*2), 19 = 11 + (4*2), 29 = 19 + (5*2)...
>to fininity?
>
>5 3, 3
>11 4, 8
>19 5, 15
>29 6, 24
>41 7
>55 8
>71 9
>89 10
>109 11
>131 12
>155 13
>181 14
>209 15
>239 16
>271 17
>305 18
>341 19
>379 20
>419 21
>461 22
>505 23
>551 24
>599 25
>649 26
>701 27
>755 28
>811 29
>869 30
>
>
>Unsubscribe by an email to: primenumbers-unsubscribe@egroups.com
>The Prime Pages : http://www.primepages.org
>
>
>
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