February 28th, 2016, 03:29 AM  #1 
Banned Camp Joined: Dec 2013 Posts: 1,117 Thanks: 41  Prime numbers
Hi, Here is a sequence of prime numbers: 3,5,11,17,47,257,510767,.... The algorithm used to build such sequence is : We define primorial p noted #p=2*3*5*7*....*p Start from U(0)=3 the first odd prime Compute a(1)=int(sqrt(U(0)))+1=2 U(1)=U(0)+a(1) where a(1) is equal to primorial #2 = 2 U(1)=5 Compute a(2)=int(sqrt(U(1))+1=3 U(2)=U(1)+a(2)=5+#3=5+(2*3)=11 Compute a(3)=int(sqrt(11))+1=4 (4 is not prime so we use #3=2*3 U(3)=11+(2*3)=17 and so on Are all the numbers of the sequence prime numbers? The sequence is growing faster it is not easy to know. Thank you for any help. Last edited by skipjack; February 28th, 2016 at 05:01 AM. 
February 28th, 2016, 04:58 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,627 Thanks: 2077 
Can you post some further numbers in the sequence?

February 28th, 2016, 06:35 AM  #3 
Banned Camp Joined: Dec 2013 Posts: 1,117 Thanks: 41 
The next number is 300 digits size. So the sequence is exploding. I can not even estimate the digit size of the further numbers. 
February 28th, 2016, 07:11 AM  #4 
Senior Member Joined: Jul 2014 From: भारत Posts: 1,178 Thanks: 230 
It is not so useful because some prime numbers are missing in the sequence. Maybe tweaking the algorithm a little bit accordingly can give all prime numbers.

February 28th, 2016, 08:25 AM  #5 
Member Joined: Oct 2013 Posts: 57 Thanks: 5 
I'm sorry, but I can't confirm that U(7) is a prime number: Code: (18:18) gp > U6=510767 %40 = 510767 (18:18) gp > A7=sqrtint(U6)+1 %41 = 715 (18:18) gp > V7=primes(primepi(A7)) %42 = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149,151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709] (18:19) gp > U7=U6+factorback(V7) %43 = 13802651106711802536344050306133362992649963656229914863058580142142610482430817949922104531639351381921564573865712490763569228376295661814770390189505137031692781919527713285374540164408571278055683171593020170233128086464775974520546386806644011091046992108509661969860784773011026549130272637 (18:20) gp > isprime(U7) %44 = 0 
February 28th, 2016, 09:26 AM  #6 
Banned Camp Joined: Dec 2013 Posts: 1,117 Thanks: 41 
U(7) is composite but what about the other computable terms. @Prakhar It is not easy to find an infinite sequence of only prime numbers. I hope that my sequence up to U(10000) contains 90% of prime numbers I will be then very happy 

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