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 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS 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

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Last modified October 17 14:25 EDT 2018. Contains 316281 sequences. (Running on oeis4.)