Problems & Puzzles: Puzzles

 Puzzle 685 Follow up to puzzle 683 Emmanuel Vantieghem sent the following follow up to Puzzle 683: A possible follow up of puzzle 683 is to ask for an arrangement of all the numbers from 1 to N in a row such that the sum of two adjacent members is NOT prime.  So, this is our first question : Q1.  Prove that it is possible to arrange the numbers from 1  to  N  in a row such that the sum of two adjacent members is NOT prime Another possible follow up is to asks for an arrangement of all the numbers from 1 to N in a row such that the sum of two adjacent members is ODD composite. This situation is quite different since not all  N  allow a solution. For instance: N = 2 ; possible rows  (1,2) and (2,1). Clearly no one is a solution N = 3 ; possible rows  (1,2,3), (1,3,2),(2,1,3),(2,3,1), (3,1,2), (3,2,1). Also none is a solution.     ... But there is a solution for  N = 20 : (20,19,14,1,8,7,2,13,12,3,6,9,16,5,4,11,10,15,18,17). So, this are our next questions : Q2.  Find the smallest  N0  for which there is a solution (it is not 20). Q3.  Is there a solution for  N > N0 ?

Contributions came from W. E. Clark, Antoine Verroken, Hakan Summakoglu & Emmauel Vantieghem

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W. E. Clark wrote:

This puzzle may be restated in graph theoretic terms as follows

Let G(n) = graph whose vertices are 1,...,n and whose edges are pairs {i,j}
where  i+j is composite.

Let H(n) = graph whose vertices are 1,...,n and whose edges are pairs {i,j}
where i+j is odd and composite.

Then the question becomes which of these graphs has a Hamiltonian path?
And, we may add, if so, then does it have a Hamiltonian cycle?

With the help of Maple 17's  GraphTheory package I find that:

1. G(n) has no Hamiltonian path for n = 1,2,3,4. For n = 5,..., 200, G(n) has a
Hamiltonian path. For n from 6 to 200 G(n) has a Hamiltonian cycle.

For example here is a  Hamiltonian cycle for n = 50:
1, 3, 5, 4, 2, 6, 8, 7, 9, 11, 10, 12, 13, 14, 16, 17, 15, 18, 20, 19, 21, 23, 22, 24,
25, 26, 28, 27, 29, 31, 32, 30, 33, 35, 34, 36, 38, 37, 39, 41, 40, 42, 43, 44, 46,
45, 47, 48, 50, 49, 1

2.  H(n) has no Hamiltonian path for n = 1,2, . . . , 13.  For n = 14,..., 200, H(n) has a
Hamiltonian path. For EVEN n from 18 to 200  H(n) has a Hamiltonian cycle.

Here is a Hamiltonian path for n = 14:
4, 5, 10, 11, 14, 1, 8, 7, 2, 13, 12, 3, 6, 9
And here is a Hamiltonian cycle for n = 50:
1, 8, 7, 2, 13, 12, 3, 6, 9, 16, 5, 4, 11, 10, 15, 18, 17, 22, 23, 26, 19, 14, 21, 24,
25, 20, 29, 28, 27, 30, 33, 32, 31, 34, 35, 40, 37, 38, 39, 36, 41, 44, 43, 42, 45,
48, 47, 46, 49, 50, 1

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Antoine wrote:

Q1.

1.      N > 4

2.      Sum of 2 odd different numbers cannot be a prime

3.      Sum of 2 even different numbers cannot be a prime

4.      In the row “ any permutation of the odd numbers from 1 to N the digit 5 excepted, 5 , 4 , any

permutation of the even numbers from 1 to N except the digit 4 “ the sum of 2 adjacent

numbers is not a prime.

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Hakan wrote:

Q1: If arranged even and odd numbers separately, all sums of two adjacent members except one will be even numbers and greater than 2.(therefore not prime)

It is possible to arrange for N>4. Because for N<5, there aren't an even and an odd number whose sums are not prime.

For example if N=20:

2,4,6,8,10,12,14,16,18,20,1,3,5,7,9,11,13,15,17,19

Q2: Smallest N0 is 14. (3,6,9,12,13,2,7,8,1,14,11,4,5,10)

Q3: Yes,we showed that there is a solution for all N> 4.

Therefore There is a solution for N>N0=14.

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Emmanuel wrote:

Q1.  If  N < 5, it is impossible to find such an arrangement.  If  N >= 5, every arrangement of the form : ( all odd numbers < 5, 5, 4, all even numbers >= 4 )  is obviously a solution.

Example  (1, 3, 7, 9, 11, 5, 4, 2, 6, 8, 10, 12).

Q2.  N = 14  is the smallest  N  for which there is a solution.

Q3.  I conjecture 'yes'.  Actually, the best strategy for me was to take one of the solutions for  N = 14, say : ( 9,6,3,12,13,2,7,8,1,14,11,4,5,10)  and expand it to the right or to the left as long as possible.  For all  N < 20000  there were solutions, except for N = 17 and  N = 39 which I solved 'by hand' :(13,2,7,14,1,8,17,16,5,4,11,10,15,6,3,12,9)  and (37,38,39,36,3,6,9,12,13,2,7,8,1,14,11,4,5,10,15,18,17,16,19,20,25,24,21,28,23,22,27,30,33,32,31,26,29,34,35)

But I cannot prove this will work for all  N.  (To save time for the bigger  N, I replaced my initial  14-solution by a bigger one).

The most surprising for me was that it was much more difficult to find a working strategy. Since there are more odd composites than primes (for  N > 121) one may expect this would have been easier than puzzle 683...

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