

A084438


Positive integers n such that n!!!  1 = A007661(n)  1 is prime.


59



3, 4, 6, 8, 20, 26, 36, 50, 60, 114, 135, 138, 248, 315, 351, 429, 642, 5505, 8793, 12086, 13580, 23109, 34626, 34706, 56282, 57675, 58298
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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



EXAMPLE

a(4)=8 since 8!!!1 = 8*5*21 = 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, (n1)\3, n=3, n+!n) /* newline */ for(n=1, 999, if(isprime(A007661(n)1), print1(n", ")))  M. F. Hasler, Nov 26 2007


CROSSREFS



KEYWORD

hard,nonn


AUTHOR



EXTENSIONS

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 03 2008


STATUS

approved



