OFFSET
1,2
COMMENTS
a(n) == 0 (mod 3) for any n > 1.
k cannot be divisible by 5 for k = 1, 3, 63, 231, 3003, 3927, 4389, 4641, 5187, 5313, 9009, 11781, 13167, 13923, 15561, 15939, 21021, 27027, 27489, 30723, ... - Jinyuan Wang, Apr 05 2020
PROG
(PARI) is(k) = (k%6==3 && eulerphi(k!)%eulerphi(k)!==0) || k==1; \\ Jinyuan Wang, Apr 05 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Jan 29 2002; corrected Jun 03 2003
EXTENSIONS
More terms from Jinyuan Wang, Apr 05 2020
STATUS
approved