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 A057704 Primorial - 1 prime indices: integers n such that the n-th primorial minus 1 is prime. 18
 2, 3, 5, 6, 13, 24, 66, 68, 167, 287, 310, 352, 564, 590, 620, 849, 1552, 1849, 67132, 85586 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS There are two versions of "primorial": this is using the definition in A002110. - Robert Israel, Dec 30 2014 As of 28 February 2012, the largest known primorial prime is A002110(85586) - 1 with 476311 digits, found by the PrimeGrid project (see link). - Dmitry Kamenetsky, Aug 11 2015 LINKS Chris K. Caldwell, Prime Pages: Database Search Chris K. Caldwell, The top 20: primorial primes Eric Weisstein's World of Mathematics, Integer Sequence Primes Eric Weisstein's World of Mathematics, Primorial Prime Wikipedia, Primorial prime FORMULA a(n) = A000720(A006794(n)). a(n) = primepi(A006794(n)). EXAMPLE The 6th primorial is A002110(6) = 2*3*5*7*11*13 = 30030, and 30030 - 1 = 30029 is a prime, so 6 is in the sequence. MAPLE P:= 1: p:= 1: count:= 0: for n from 1 to 1000 do   p:= nextprime(p);   P:= P*p;   if isprime(P-1) then     count:= count+1;     A[count]:= n;   fi od: seq(A[i], i=1..count); # Robert Israel, Dec 25 2014 MATHEMATICA a057704[n_] := Flatten@Position[ Rest[FoldList[Times, 1, Prime[Range[n]]]] - 1, _Integer?PrimeQ]; a057704[500] (* Michael De Vlieger, Dec 25 2014 *) PROG (PARI) lista(nn) = {s = 1; for(k=1, nn, s *= prime(k); if(ispseudoprime(s - 1), print1(k, ", ")); ); } \\ Altug Alkan, Dec 08 2015 (PARI) is(n) = ispseudoprime(prod(k=1, n, prime(k)) - 1); \\ Altug Alkan, Dec 08 2015 CROSSREFS Cf. A006794 (Primorial -1 primes: Primes p such that -1 + product of primes up to p is prime). Cf. A057705, A014545, A005234, A002110, A057706. Sequence in context: A098833 A075371 A218948 * A281382 A078203 A285937 Adjacent sequences:  A057701 A057702 A057703 * A057705 A057706 A057707 KEYWORD nonn,more AUTHOR Labos Elemer, Oct 24 2000 EXTENSIONS Corrected by Holzer Werner, Nov 28 2002 a(19)-a(20) from Eric W. Weisstein, Dec 08 2015 (Mark Rodenkirch confirms based on saved log files that all p < 700,000 have been tested) STATUS approved

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Last modified December 16 19:57 EST 2018. Contains 318188 sequences. (Running on oeis4.)