Problems & Puzzles: Puzzles

 Puzzle 875.Vector of primes that generates distinct primes Dmitry Kamenetsky very recently, published a paper related to "a variety of prime-generating constructions that are based on sums of primes". One of these constructions is this one: A prime vector of order n is an array of distinct primes P = ( p0, p1, . . . , pn−1), such that every sum of an odd number of consecutive elements is also prime. His best example is this one, with 23 elements: (239, 131, 109, 181, 83, 43, 41, 223, 53, 233, 271, 103, 269, 71, 19, 47, 241, 23, 277, 199, 281, 29, 37) which produces 121 distinct primes. Q1.Would you produce a better vector (with more than 23 elements) then the one from Kamenetsky.

Contributions came from Emmanuel Vantieghem, Jan van Delden, J. K. Andersen, Dmitry Kamenetsky and Michael Hürter.

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Emmanuel wrote:

This is a 26-prime vector :

(211507, 239, 131, 109, 181, 83, 43, 41, 223, 53, 233, 271, 103, 269, 71, 19, 47, 241, 23, 277,
199, 281, 29, 37, 473513, 559201)

I came to this by joining primes to the left/right of Dmitri's solution.
If there is an extension of my solution by this method (which is maybe not impossible), one should
search among primes > 22*10^9.

***

Jan wrote:

My contribution

The vector [991,89,43,67,101,83,283,53,277,113,151,193,1487,109,401,2029,5807,827,2803,68473,
1048877,1170277,277169, 3000301,37234667,150879853,99485549,504200261,238748743]

has 29 elements. The number of different prime sums, including the primes in the vector itself, is 225, which is the maximum number of primes possible for a vector of this size.

However it is probably not optimal in the sense of Dmitry, I did not try to minimize it’s weight.

I used a routine where the first prime number is a random prime in [3..1000] and for the other elements a random starting prime is used in [3..100]. Better procedures will probably exist.

***

Andersen wrote:

Kamenetsky does not require distinct sums. His 121 primes only include
105 distinct primes. For example, 181+83+43 = 43+41+223 = 19+47+241 = 307.

If we start with an empty vector and extend it one element at the time
with the smallest possible odd prime then the first 34 elements are:

(3, 5, 11, 7, 41, 19, 23, 61, 29, 151, 137, 79, 1013, 14347, 43151, 7873,
82469, 444187, 63680783, 80158627, 531845381, 13726723, 2948038229,
341461831, 5391683657, 4759989589, 45033191681, 3342118271593,
57517957292507, 25358009530039, 2584135512217541, 616856808553033,
21225241347141287, 10855325323825603)

They produce 272 primes, 270 distinct. 67 and 83 are produced twice.

Kamenetsky defines the weight of a prime vector as the sum of the n elements.
He then defines an optimal vector as having the lowest possible weight.
He lists optimal vectors for n = 1 to 14. My computation agrees.
He lists upper bounds on the weight of the optimal vectors for n = 15 to 23.
My computation says his upper bounds for n = 15 to 17 are optimal.
n=15, weight 443: (23, 11, 19, 31, 53, 67, 29, 17, 7, 47, 13, 43, 41, 5, 37)
n=16, weight 522: (23, 67, 59, 37, 5, 61, 31, 47, 29, 13, 11, 7, 53, 19, 17,
43)
n=17, weight 641: (11, 59, 67, 73, 71, 13, 53, 31, 23, 29, 37, 7, 17, 19, 47,
41, 43)

Some of his other upper bounds are not optimal.

***

Dmitry wrote:

Regarding the example in my paper, a vector of 23 primes, "There are 144 sums (including single numbers) with 128 of them distinct. There are 144-23=121 sums if you don't include the single numbers, where 128-23=105 are distinct. I hope that helps."

...

And now I have found prime vectors of length 28!

(104829763 188927 880703 15319 32251 239 131 109 181 83 43 41 223 53 233 271 103
269 71 19 47 241 23 277 199 281 2843 43122199)

(229082171 64512013 211507 479 181 83 43 41 223 53 233 271 103 269 71 19 47 241
23 277 199 281 29 37 324403 8751593 78840191 4538953)

(484102337 201847 3157949 62851 653 4483 197 43 41 223 53 233 271 103 269 71 19
47 241 23 277 199 281 29 37 30103 1448021 484846553

***

Michael wrote:

I found a vector with 27 elements:

( 168548962471, 473513, 37, 29, 281, 199, 277, 23, 241, 47, 19, 71, 269, 103, 271, 233, 53, 223, 41, 43, 83, 181, 109, 131, 239, 211507, 1223550247 )

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