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A051800
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Numbers k such that 1 plus twice the product of the first k primes is also a prime.
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0
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1, 2, 3, 4, 5, 11, 18, 23, 26, 30, 80, 120, 148, 220, 395, 776, 884, 977, 3535, 3927
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OFFSET
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1,2
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LINKS
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EXAMPLE
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5 is in the sequence because 2*(2*3*5*7*11) + 1 = 4621 is prime.
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MATHEMATICA
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Position[2#+1&/@FoldList[Times, Prime[Range[800]]], _?PrimeQ]//Flatten (* Harvey P. Dale, Oct 09 2018 *)
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PROG
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(PARI) isok(k) = isprime(1+2*prod(j=1, k, prime(j))); \\ Michel Marcus, May 28 2018
(Python)
from sympy import isprime, nextprime
def afind(limit):
p, primorialk = 2, 2
for k in range(1, limit+1):
if isprime(2*primorialk + 1):
print(k, end=", ")
p = nextprime(p)
primorialk *= p
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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