OFFSET
1,2
COMMENTS
M. Hausman has proved (see Links) that a number belongs to this sequence if and only if it is of one of the following forms: 2^s, 2^s * 3^t, 5 * 2^t, or 7 * 2^t , where s >= 0 and t >= 1.
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..10000
M. Hausman, The solution of a special arithmetic equation, Canad. Math. Bull, 1982, 25(1), 114-117.
EXAMPLE
56 is in the sequence because phi(phi(56)) = 8 divides 56.
MATHEMATICA
Select[Range[1000], Mod[#, EulerPhi@ EulerPhi@ #] == 0 &]
PROG
(PARI) alias(e, eulerphi);
for(n = 1, 1000, if(!Mod(n, e(e(n))), print1(n, ", "))) \\ Indranil Ghosh, Mar 18 2017
(Python)
from sympy import totient as e
print([n for n in range(1, 1001) if n%e(e(n))==0]) # Indranil Ghosh, Mar 18 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Giovanni Resta, Mar 17 2017
STATUS
approved