login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A006794 Primorial -1 primes: primes p such that -1 + product of primes up to p is prime.
(Formerly M2474)
20
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

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

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.

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-Fran├žois 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

N. J. A. Sloane

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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified August 24 05:50 EDT 2016. Contains 275767 sequences.