Problems & Puzzles: Puzzles

Puzzle 1019. Follow-up to Puzzle 625

Carlos Rivera propose the following puzzle:

Perhaps some of you noticed the very large solution found by Giorgos Kalogeropoulos to the Puzzle 625.


In my humble opinion this is a great achievement, mainly because his approach open an semi-analytical way to search for more solutions.


But I hardly can accept that there are no more smaller solutions than the gotten by Giorgos. Accordingly with this skepticism the challenge now is...


Q. Find smaller solutions to Puzzle 625 than the gotten by Giorgos.



During the week 2-9, October, 2020, contributions came from Giorgos Kalogeropoulos and Paul Cleary.


Giorgos wrote

I found one smaller (123 digits) solution:  
8218470334175459700705971960791869592104 = 
 2  2   2   23  1109123111  57766182616657495290612267717977498812931942308391 
 11788844704086155814066994795339207139099517865226893357415731 = 
 2^2 + 2^2 + 2^2 + 23^2 + 1109123111^2 + 57766182616657495290612267717977498812931942308391^2 + 11788844704086155814066994795339207139099517865226893357415731^2      

The algorithm:     
We are searching for prime tuples {p1,p2,...,pk} with p1,p2, being prime numbers 
P= p1  p2    ...   pk
S= p1^2 + p2^2 + ... + pk^2
such that  the hyperbola X^2 + Y^2 + S - P  X  Y = 0  (1)  has positive integer solutions X ,Y.   
Our goal is to find X and Y being both prime numbers.    
If {x1,y1} are integer solutions of the above hyperbola,
then this is equivalent to finding two consecutive terms, {f(n),f(n+1)} being both primes, of the following recursive function:    
f(0)=x1, f(1)=y1, f(n) = Pf(n-1) - f(n-2)    

I have found 2 methods which produce those prime tuples:   
  • Consider the prime tuple with one less factor {p(1),p(2),...,p(k-1)}
           P' = p(1)p(2)...p(k-1)
           S' = p(1)^2 + p(2)^2 + ... + p(k-1)^2      
           The key is this new hyperbola:  X^2 + Y^2 + S' + 1 - P'XY = 0   (2)
           The only valid prime numbers of our prime tuple are those for which the above hyperbola has integer solutions
           If we choose the right initial primes {p(1),p(2),...p(k-1)} (see example below), then we can find p(k):    
           p(k) can be any of the prime terms of the following recursive function based on the minimal integer solutions {x1,y1} of hyperbola (2):
           g(0)=x1, g(1)=y1, g(n)=P'g(n-1) - g(n-2)    
           If we obtain our prime tuple {p1,p2,} using this method, checking the final step on hyperbola (1), using the recursive function f, is easier because x1=1 will
           always return an integer solution for Y.  So the recursive function becomes
           f(0)=1, f(1)= (P - Sqrt(P^2 - 4(S+1))) / 2,  f(n)=Pf(n-1) - f(n-2)

           1) If we are searching for solutions with 5 prime factors {p1,p2,p3,X,Y} then k=3  
               The only primes that return integer solutions for hyperbola (2) can be easily searched using the fact that f(3) can be obtained by the above method.
                We exhaustively search p1,p2 candidates in order hyperbola (2) to have positive integer solutions and we get the results 
                {3, 5, 23, 31, 107, 157, 179, 311, 1453, 4139, 4969, 9811, 15193, 43781, 50549, 86719....}     

           2) If we are searching for solutions with 7 prime factors {p1,p2,p3,p4,p5,X,Y} then k=5
               If p1,p2,p3=2 then p4 (in order hyperbola (2) to have integer solutions) can only be one of these primes
               {3, 5, 23, 181, 307, 1559, 2417, 88327, 95783, 1179491, 5474849....}
               then we find p5 using the first method

               Using these primes as building blocks we can find a huge amount of valid prime tuples

  • The second method (which was used in order to find the 123-digits number) is the following     
           If {p(1),p(2),...p(k)} is a valid prime tuple    
           X^2 + Y^2 + S - P  X  Y = 0    with minimal positive integer solution {x1,y1}
           f(0)=x1, f(1)=y1, f(n) = Pf(n-1) - f(n-2)    
           Then ALL swaps of one prime factor of the tuple with one prime term of the recursive function f, always lead to a new valid tuple.  
           (those last tuples should always be checked in the final step considering that the integer solutions x1,y1 will always be >1) 


Paul wrote:

I haven't managed to find any smaller solutions, though using Giorgos's method I have managed to extend the range of the x and y values to 30500+ digits with no more solutions.  I used a linear recurrence where any consecutive prime solution are also the two values of x and y.




Monitor[Do[If[PrimeQ[FromDigits[pf[u]]]&&PrimeQ[FromDigits[pf[u+1]]],Print[u," , ",pf[u]," , ",pf[u+1]]],{u,1,1000}],u]







pf2[54,55] gives






On October 19, 2020, Giorgos Kalogeropoulos wrote again:

I found on more number with 163 digits and I thought I should send it to you since it qualifies as an answer to puzzle 1019     

1150821349128867534891641545518285925735804347623249366552 = 2*2*2*95783*13445403342601809061726519*180778871045555786730494902253611483680494135761943*18625156354
00191345742222416847392668657068433219626455654264772436151243547155029 = 2^2+2^2+2^2+95783^2+13445403342601809061726519^2+180778871045555786730494902253611483680494135761943^2


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