

A057704


Primorial  1 prime indices: integers n such that the nth 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
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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

Table of n, a(n) for n=1..20.
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(P1) 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



