

A051800


Numbers k such that 1 plus twice the product of the first k primes is also a prime.


0



1, 2, 3, 4, 5, 11, 18, 23, 26, 30, 80, 120, 148, 220, 395, 776
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OFFSET

1,2


LINKS

Table of n, a(n) for n=1..16.


EXAMPLE

5 is in the sequence because 2*(2*3*5*7*11) + 1 = 4621 is prime.


MATHEMATICA

Position[2#+1&/@FoldList[Times, Prime[Range[800]]], _?PrimeQ]//Flatten (* Harvey P. Dale, Oct 09 2018 *)


PROG

(PARI) isok(k) = isprime(1+2*prod(j=1, k, prime(j))); \\ Michel Marcus, May 28 2018


CROSSREFS

2*A002110(n)+1 is prime. Cf. A051887, A051915.
Sequence in context: A181288 A255260 A052418 * A121432 A118968 A073528
Adjacent sequences: A051797 A051798 A051799 * A051801 A051802 A051803


KEYWORD

nonn,more,changed


AUTHOR

Labos Elemer, Dec 20 1999


EXTENSIONS

More terms from Harvey P. Dale, Oct 09 2018


STATUS

approved



