But I went not interested in
the question about the quantity of distinct solutions for each
N. Instead of that, I asked my
self if it could exist a non trial & error method for getting
at least one solution for each and any N.
What I wanted to discover was a method
for completing the assignation of the N values in exactly N safe
steps, without having to go back, erase and start again
when the remaining values to assign exhibit no one available option.
What is that of "go back, erase and
start again"? Proceeding without any safe strategy, that is to say
making the assignments purely at random, is possible to reach to a
dead end. For example, in the following partial successful assignation
for N=10,
8, 9,
2, 1, 10,
7, 6,
5, ???
you can not go ahead with the remaining
3 &
4 in
any way. The only you can do in this moment is to go back, erase
and start again...
During several weeks a tried several
strategies, unsuccessfully. When I was prone to abandon, unexpectedly
I discovered that such safe & lineal method exist and is extremely
simple:
Starting from the N value, each new number
in the sequence will be the first largest value not used before, of
opposite parity to the last value assigned, such that the sum of
this number and the last value assigned is a prime number.
I have tested the above method for N
values as large as 2<N<10000 and it works! So I conjecture
that this algorithm always works.
Examples:
For N=10 the sequence is:
10, 9,
8, 5, 6,
7, 4,
3, 2,
1
For N=100 the sequence is:
100
99 98
95 96
97 94 87
92 89 90
91 88
93 86
81 82
85 78
79 84
83 80
77 74
75 76
73 66
71 68
69 70
67 72
65 62
51 58
55 54
59 50
63 64
49 60
53 56
57 52
61 48
41 42
47 36
43 46
37 34
45 44
39 40
33 38
35 32
29 30
31 28
25 22
21 26
27 20
23 24
19 18
13 16
15 14
17 12
11 8
9 10
7 6
5 2
3 4
1
Questions:
1. Can you "solve" the
given conjecture? That is to say: or to find a counterexample, or to
demonstrate why this method always should produce a solution as the
desired one.
2. Exist other safes
procedures, distinct to the described by this conjecture?
3. If you know that
the Rivera's algorithm
was posed previously by somebody, please send to me the reference to
give that person the proper credits.
____
(*) As a matter of fact the problem of
finding the quantity of such solutions for each N is discussed
in the R. K. Guy's known book, p. 105
Solution
The 27/05/2000 Jud McCranie wrote:
"I tested your algorithm for conjecture 20 out to 25,000
and it still works. Your algorithm is a form of the greedy
algorithm. For some problems the greedy algorithm always works,
for others  not always. In this case it seems to always work,
but I don't know for sure".
***
Imran Ghory sent (24/Oct/2000) the following very
interesting interpretation of my algorithm:
"I've found a partial solution to question 2. If N=(A+B)/2,
where A and B are both primes, and B=A+2 (That is that they are
twin primes). Then you can make a solution for that value of N
such that the sum of every consecutive numbers is equal to A or
B.
I think the best way to demonstrate how it works is by
example, if we set A=11, B=13 and N=12. We first express B as
the sum of every number below it,
B=13: (12+1), (11+2), (10+3), (9+4), (8+5), (7+6).
This gives us a set of combinations of every number from 1
to N, such that their sum equals B. However these combinations
do not connect to each other, so we have to work out how to put
them together.
We can do this by expressing A as the sum of every number
below it,
A=11: (10+1), (9+2), (8+3), (7+4), (6+5)
This gives us a set of combinations of every number from 1
to N2, such that their sum equals A.
The combinations that equal A can now be used to combine
the
combinations that equal B to get,
(12+1), (1+10), (10+3), (3+8), (8+5), (5+6), (6+7), (7+4),
(4+9), (9+2), (2+11), alternating B, A, B, A, B, etc..
Now removing duplication and converting it into the form
that you used for the previous algorithm we get:
12,1,10,3,8,5,6,7,4,9,2,11
This technique works for all twin primes
While producing this algorithm I also noticed that the
numbers produced by your algorithm seemed to add up twin primes
quite often, doing some work along these lines I think that your
algorithm will also work if we change it to:
"Starting from the N value, each new number in the sequence
will be the first largest value not used before, of opposite
parity to the last value assigned, such that the sum of this
number and the last value assigned is a prime number which has
a twin prime."
This alternate version of your algorithm would move it
closer to the algorithm which I demonstrated earlier and it
might be possible to somehow link the two algorithms".
***
Let's take a second look of the Imran
algorithm using the same example solved by him:
12,
1,
10, 3,
8, 5,
6, 7,
4, 9,
2, 11
At the end, his solution can be described this
other way:
The solution is formed by all the even
numbers from N to 2 in descending order (in the odd positions)
and the odd numbers from 1 to N1 in ascending order (in the
even positions)
I hope Imran likes this form of looking
his algorithm... (C.R.)
Isn't it a beauty solution the Imran's
one? No matter that it only works or when N is even such that N+1
& N1 are primes or when N is odd but N & N+2 are primes...
***
Sooner than later, Imran cleverly has solved completely
this conjecture from an unexpected way that involves the twin
primes. Enjoy his collaboration:
"I've managed to improve upon it so that it can be used
for any value of N. But it only works if we assume following
statement is true:
"For every value of x, where x>3, between x and 2x there
exists a twin prime. Both of the primes do not have to be
between x and 2x, only the higher one"
My new algorithm works like this:
B=N
A+B=P
A<B
Where P is the larger value from two twin primes. We then
apply the algorithm I showed earlier, but instead of using 1 in
the first even position we use A. For example, with N=28, B=28,
A=15, P=43 (with the other twin prime being 41), we get:
28,15,26,17,24,19,22,21,20,23,18,25,16,27,14
This gives us a list containing every number from B to
A1.
Now we let B=A1, then we find a new value of A, which
again has to agree with,
A+B=P
A<B
Where P is the larger value from two twin primes.(This
prime is different from the earlier prime).
We know A exists, because of the statement we assumed at
the begining which tells us that a twin prime exists between B
and 2B. Now with these new values of A, B, and P we can repeat
our algorithm. Carrying on from the earlier example B=14 A=5,
and P=19, we now get:
14,5,12,7,10,9,8,11,6,13,4
We now repeat the last part of the algorithm, which gives
us B=4 A=3, we now get:
4,3,2
Now we have these lines,
28,15,26,17,24,19,22,21,20,23,18,25,16,27,14
14,5,12,7,10,9,8,11,6,13,4
4,3,2
Joining them together and removing the replication at the
end of lines we get,
28,15,26,17,24,19,22,21,20,23,18,25,16,27,14,5,12,7,10,9,8,11,6,13,4,3,2
Now we just have to add 1 to the end and we have our
complete sequence.
Assuming the statement at the beginning is true this
technique will work for all values of N.
***
The only thing I can add after
a sincere Congratulations! to Imran is that his
hypothetical statement seems to be true according to the
following: The known approximate counting twin primes function (
p(x)
= K.x/ln(x)^{2} , K=1.32 ) can provide an idea of when
will exist at least one twin prime in the interval (x, 2.x) for
each x:
p(2.x)
 p(x)
> 1, for x>22
Empirically we may verify that
it happens that for each 22=>x=>3 also exist at least one twin
prime in the interval (x, 2.x)
Then the Imran's
hypothetical statement seems to be totally true.
***
Why do we say that is
absolutely true the Imran's postulate? Well, paraphrasing a
Chris Nash's answer to another similar question "...It
is not a proof because it only depends on
[twin] primes density. There are many sequences
that have density just like the [twin]
primes but..." are not twin primes at all "...
So to turn [Imran's]
work into a proof, something else is needed more than just prime
density  but I do not know what that 'something else' is!..."
BTW, Chris Nash has
named the Imran's postulate as the "Imran's
'Bertrand's Postulate', twin prime conjecture". For sure
everyone knows why...
***
I (C.R.) have found a
different solution for the example N=28 than the Imran's
one, using the same and his method. This is my solution:
28
3 26
5 24
7 22
9 20
11 18
13 16
15
14
17
12
19 10
21 8
23 6
25 4
27 2
1
The only change I made was to
use the least A<N value possible, while it seems that
Imran was using the largest A<N value possible.
Proceeding the way I did the
number of cycles needed to halt the algorithm are minimized. This
is the code I made in Ubasic of the Iram's algorithm seeking for
the least A, etcetera:
10 input N:'Imran01.ub about
conjecture 20
20 NN=N:print N;
30 gosub *A:print A;:AA=A
40 N=N2:print N;
50 if N=AA1 then goto 70
60 A=A+2:print A;:goto 40
70 if N<=2 then stop:N=NN+1:goto 20
80 print ""; goto 30
90 *A:A1=1:if odd(N)=1 then A1=2
100 for A=A1 to N1 step 2
110 if and{(A+N)=prmdiv(A+N),(A+N2)=prmdiv(A+N2)} then cancel
for:goto 140
120 next A
130 print "Algorithm fails!":end
140 return
*** 
