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 A006794 Primorial -1 primes: primes p such that -1 + product of primes up to p is prime. (Formerly M2474) 23
 3, 5, 11, 13, 41, 89, 317, 337, 991, 1873, 2053, 2377, 4093, 4297, 4583, 6569, 13033, 15877, 843301, 1098133 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Or, p such that primorial(p) - 1 is prime. Conjecture: if p# - 1 is a prime number, then the previous prime is greater than p# - exp(1)*p. - Arkadiusz Wesolowski, Jun 19 2016 REFERENCES H. Dubner, Factorial and primorial primes, J. Rec. Math., 19 (No. 3, 1987), 197-203. R. K. Guy, Unsolved Problems in Number Theory, Section A2. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS C. K. Caldwell, Prime Pages: Database Search C. K. Caldwell, Primorial Primes C. K. Caldwell, On the primality of n! +- 1 and 2*3*5*...*p +- 1, Math. Comput. 64, 889-890, 1995. C. K. Caldwell and Y. Gallot, On the primality of n!+-1 and 2*3*5*...*p+-1, Math. Comp., 71 (2001), 441-448. H. Dubner, Factorial and primorial primes, J. Rec. Math., 19 (No. 3, 1987), 197-203. (Annotated scanned copy) Des MacHale, Infinitely many proofs that there are infinitely many primes, Math. Gazette, 97 (No. 540, 2013), 495-498. R. Mestrovic, Euclid's theorem on the infinitude of primes: a historical survey of its proofs (300 BC--2012) and another new proof, arXiv preprint arXiv:1202.3670 [math.HO], 2012. - N. J. A. Sloane, Jun 13 2012 PrimeGrid.  Primorial Prime Search - Primes by User Eric Weisstein's World of Mathematics, Primorial Prime FORMULA a(n) = A000040(A057704(n)). a(n) = prime(A057704(n)). MATHEMATICA primorial[p_] := Product[Prime[k], {k, 1, PrimePi[p]}]; Select[Prime[Range[1900]], PrimeQ[primorial[#] - 1] &] (* Jean-François Alcover, Mar 16 2011 *) Transpose[With[{pr=Prime[Range[2000]]}, Select[Thread[{Rest[FoldList[ Times, 1, pr]], pr}], PrimeQ[ First[#]-1]&]]][[2]] (* Harvey P. Dale, Jun 21 2011 *) With[{p = Prime[Range[200]]}, p[[Flatten[Position[Rest[FoldList[Times, 1, p]] - 1, _?PrimeQ]]]]] (* Eric W. Weisstein, Nov 03 2015 *) PROG (PARI) is(n)=isprime(n) && ispseudoprime(prod(i=1, primepi(n), prime(i))-1) \\ Charles R Greathouse IV, Apr 29 2015 CROSSREFS Cf. A057704 (Primorial - 1 prime indices: integers n such that the n-th primorial minus 1 is prime). Cf. A057705, A002110, A005234, A014545, A018239. Sequence in context: A105071 A089251 A147568 * A032457 A122564 A162876 Adjacent sequences:  A006791 A006792 A006793 * A006795 A006796 A006797 KEYWORD nonn,hard,more,nice AUTHOR EXTENSIONS Stated incorrectly in CRC Standard Mathematical Tables and Formulae, 30th ed., 1996, p. 101; corrected in 2nd printing. Corrected by Arlin Anderson (starship1(AT)gmail.com), who reports that he and Don Robinson have checked this sequence through about 63000 digits without finding another term (Jul 04 2000). 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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