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A057704
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Primorial - 1 prime indices: integers m such that the m-th primorial minus 1 is prime.
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21
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2, 3, 5, 6, 13, 24, 66, 68, 167, 287, 310, 352, 564, 590, 620, 849, 1552, 1849, 67132, 85586, 234725
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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FORMULA
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EXAMPLE
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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.
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MAPLE
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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:
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MATHEMATICA
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a057704[n_] :=
Flatten@Position[
Rest[FoldList[Times, 1, Prime[Range[n]]]] - 1, _Integer?PrimeQ]; a057704[500] (* Michael De Vlieger, Dec 25 2014 *)
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PROG
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(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
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CROSSREFS
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Cf. A006794 (Primorial -1 primes: Primes p such that -1 + product of primes up to p is prime).
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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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)
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STATUS
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approved
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