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A119833
Primes p such that 2*p#-1 is prime.
2
2, 3, 5, 7, 17, 19, 37, 71, 79, 113, 857, 863, 16361, 62989
OFFSET
1,1
EXAMPLE
2*2 - 1 = 3, 3 prime so a(1)=2;
2*2*3 - 1 = 11, 11 prime so a(2)=3;
2*2*3*5 - 1 = 59, 59 prime so a(3)=5.
MATHEMATICA
Module[{nn=900, pr, pl}, pr=Prime[Range[nn]]; pl=FoldList[Times, pr]; Select[ Thread[{pr, pl}], PrimeQ[2*#[[2]]-1]&][[All, 1]]] (* Harvey P. Dale, May 02 2018 *)
PROG
(Magma) [p:p in PrimesUpTo(4000)|IsPrime(2*&*PrimesUpTo(p)-1)]; // Marius A. Burtea, Mar 25 2019
(Python)
from sympy import isprime, nextprime
def afind(limit):
p = 2
twoprimorialp = 4
while p <= limit:
if isprime(twoprimorialp - 1):
print(p, end=", ")
p = nextprime(p)
twoprimorialp *= p
afind(1000) # Michael S. Branicky, Jan 08 2022
CROSSREFS
Sequence in context: A059498 A247147 A158085 * A127049 A142885 A108547
KEYWORD
nonn,more
AUTHOR
Pierre CAMI, May 25 2006
EXTENSIONS
a(13) from Michael S. Branicky, Jan 08 2022
a(14) from Michael S. Branicky, May 14 2023
STATUS
approved