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A289547
Numbers k such that k!6 - 4 is prime, where k!6 is the sextuple factorial number (A085158 ).
1
6, 7, 9, 15, 21, 27, 29, 321, 327, 333, 567, 1025, 4263, 4365, 5175, 5655, 9221, 9327, 9681, 19685, 24777, 57869, 58737
OFFSET
1,1
COMMENTS
Corresponding primes are: 2, 3, 23, 401, 8501, 229631, 623641, ...
a(24) > 10^5.
Terms > 29 correspond to probable primes.
EXAMPLE
15!6 - 4 = 15*9*3 - 4 = 401 is prime, so 15 is in the sequence.
MATHEMATICA
MultiFactorial[n_, k_] := If[n < 1, 1, n*MultiFactorial[n - k, k]];
Select[Range[4, 50000], PrimeQ[MultiFactorial[#, 6] - 4] &]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Jul 07 2017
EXTENSIONS
a(22)-a(23) from Robert Price, Aug 03 2018
STATUS
approved