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A057704 Integers n such that the n-th primorial minus 1 is prime. 17

%I

%S 2,3,5,6,13,24,66,68,167,287,310,352,564,590,620,849,1552,1849

%N Integers n such that the n-th primorial minus 1 is prime.

%C a(19) > 25000. - _Robert Price_, Nov 22 2014

%C There are two versions of "primorial": this is using the definition in A002110. - _Robert Israel_, Dec 30 2014

%C 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

%H PrimeGrid, <a href="http://primes.utm.edu/top20/page.php?id=5">The top 20: primorial primes</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Primorial.html">Primorial</a>

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/IntegerSequencePrimes.html">Integer Sequence Primes</a>

%H Wikipedia, <a href="https://en.wikipedia.org/wiki/Primorial_prime">Primorial prime</a>

%F a(n) = A000720(A006794(n)).

%e 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.

%p P:= 1:

%p p:= 1:

%p count:= 0:

%p for n from 1 to 1000 do

%p p:= nextprime(p);

%p P:= P*p;

%p if isprime(P-1) then

%p count:= count+1;

%p A[count]:= n;

%p fi

%p od:

%p seq(A[i],i=1..count); # _Robert Israel_, Dec 25 2014

%t a057704[n_] :=

%t Flatten@Position[

%t Rest[FoldList[Times, 1, Prime[Range[n]]]] - 1, _Integer?PrimeQ]; a057704[500] (* _Michael De Vlieger_, Dec 25 2014 *)

%Y Cf. A006794, A057705, A014545, A005234, A002110, A057706.

%K nonn,more,changed

%O 1,1

%A _Labos Elemer_, Oct 24 2000

%E Corrected by Holzer Werner, Nov 28 2002

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Last modified September 1 16:14 EDT 2015. Contains 261277 sequences.