OFFSET
1,1
COMMENTS
2^j is in the sequence if and only if 2^{j+1}-1 is a Mersenne prime. In other words 2^j is the "even part" of a perfect number. Thus we have some generalization of perfect numbers.
Odd prime divisors of the first 19 terms of a(n) are exclusively 3, 5, 17, i.e., Fermat's primes, but 3334800 = 2^4*3*5^2*7*397.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..73
EXAMPLE
18 is in the sequence because phi(sigma(18)) = phi(39) = 24 = 2*sigma(6) = 2*sigma(phi(18)).
MAPLE
s:=n->phi(sigma(n))/sigma(phi(n));
for i to 9000000 do if s(i)=2 then print(i) fi od:
PROG
(PARI) isok(n) = (eulerphi(sigma(n)) == 2*sigma(eulerphi(n))); \\ Michel Marcus, Sep 23 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Letsko, Sep 17 2013
EXTENSIONS
Extra term 4624 and more terms from Michel Marcus, Sep 23 2013
STATUS
approved