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A202426
Numbers k such that k!*k!! - 1 is prime.
1
2, 3, 4, 10, 11, 13, 17, 22, 26, 36, 79, 196, 1097, 6926
OFFSET
1,1
COMMENTS
No additional terms up to k=1000. - Harvey P. Dale, Oct 29 2016
EXAMPLE
For k = 4, 4!*4!! - 1 = 24*8 - 1 = 191 is prime.
MATHEMATICA
lst={}; Do[If[PrimeQ[n!*n!!-1], AppendTo[lst, n]], {n, 500}]; lst
Select[Range[200], PrimeQ[#!#!!-1]&] (* Harvey P. Dale, Oct 29 2016 *)
PROG
(Magma) a:=func< n | Factorial(n)*(&*[n..2 by -2])-1 >; [ n: n in [0..660] | IsPrime(a(n)) ]; // Bruno Berselli, Dec 19 2011
CROSSREFS
Cf. A000142, A006882, A202424 (associated primes).
Sequence in context: A117383 A047455 A336193 * A039002 A295393 A120023
KEYWORD
nonn,more
AUTHOR
Michel Lagneau, Dec 19 2011
EXTENSIONS
a(13) from Michael S. Branicky, Jun 17 2023
a(14) from Michael S. Branicky, Aug 13 2024
STATUS
approved