%I
%S 1,3,9,21,27,63,81,147,171,189,243,441,513,567,657,729,903,1029,1197,
%T 1323,1539,1701,1971,2187,2667,2709,3087,3249,3591,3969,4599,4617,
%U 5103,5913,6321,6561,7077,7203,8001,8127,8379,9261,9747,10773,11907,12483
%N Numbers n such that n divides 4^n  1.
%C This sequence is closed under multiplication.  _Charles R Greathouse IV_, Nov 03 2016
%F a(n) = A014741(n+1)/2.
%t Select[Range[12500],Divisible[4^#1,#]&] (* _Harvey P. Dale_, Mar 23 2011 *)
%o (PARI) is(n)=Mod(4,n)^n==1 \\ _Charles R Greathouse IV_, Nov 03 2016
%K nonn
%O 1,2
%A _Olivier GĂ©rard_
%E More terms and better description from _Benoit Cloitre_, Mar 05 2002
