%I #7 Mar 05 2015 14:44:38
%S 9,225,729,18225,65025,140625,531441,5267025,11390625,13286025,
%T 18792225,40640625,87890625,1522170225,2197265625,3291890625,
%U 3839661225,5430953025,7119140625,8303765625,11745140625,25400390625
%N Odd solutions of phi(sigma(n)) = sigma(phi(n)).
%C sigma is the multiplicative sum-of-divisors function.
%C phi is Euler's totient.
%C Complete through 25558816403.
%C All given here are products of powers of consecutive Fermat primes based on generalized repunit primes; see links.
%C It is conjectured (see links) that all odd solutions are of this form, for which at least 10130 solutions are known.
%D Richard Guy, "Unsolved Problems in Number Theory", section B42
%D Oystein Ore, "Number Theory and Its History", 1948, reprinted 1988, Dover, ISBN-10: 0486656209, pp. 88 et seq., 109 et seq.
%H Walter Nissen, <a href="http://upforthecount.com/math/sigmaphi.html"> phi(sigma(n)) = sigma(phi(n))</a>
%e sigma(9) = 13, phi(9) = 6, sigma(6) = phi(13) = 12, so 9 is in the sequence.
%o (PARI) isok(n) = (n % 2) && (eulerphi(sigma(n)) == sigma(eulerphi(n))) \\ _Michel Marcus_, Jul 23 2013
%Y Cf. A000203, A000010, A033632, A019434.
%K nonn
%O 1,1
%A _Walter Nissen_, Apr 26 2009
%E Edited by _Charles R Greathouse IV_, Oct 28 2009