OFFSET
1,1
COMMENTS
The search for multifactorial primes started by Ray Ballinger is now continued by a team of volunteers on the website of Ken Davis (see link).
LINKS
Ken Davis, Status of Search for Multifactorial Primes.
Ken Davis, Results for n!3-1.
ProthSearch.net (on web.archive.org), n3minus.txt.
EXAMPLE
a(4) = 8 since 8!!! - 1 = 8*5*2 - 1 = 79 is the 4th prime of that form.
26!!! - 1 = 2504902399 is prime.
MATHEMATICA
multiFactorial[n_, k_] := If[n < 1, 1, n * multiFactorial[n - k, k]];
Select[Range[0, 1000], PrimeQ[multiFactorial[#, 3] - 1] & ] (* Robert Price, Apr 19 2019 *)
Select[Range[650], PrimeQ[Times @@ Range[#, 1, -3] - 1] &] (* The program generates the first 17 terms of the sequence. To generate more, change the Range constant but the program may take a long time to run. *) (* Harvey P. Dale, May 22 2021 *)
PROG
(PARI) A007661(n) = prod(i=1, (n-1)\3, n-=3, n+!n)
for(n=1, 999, if(isprime(A007661(n)-1), print1(n", "))) \\ M. F. Hasler, Nov 26 2007
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Hugo Pfoertner, Jun 25 2003
EXTENSIONS
Missing 26 inserted by M. F. Hasler, Nov 26 2007
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008
Edited by N. J. A. Sloane, Feb 11 2009 at the suggestion of M. F. Hasler
STATUS
approved