A071008 - OEIS

%I #18 Jun 13 2017 21:43:39

%S 2,4,16,256,364,1456,3276,13104,21600,23296,65536,209664,249984,

%T 367200,1285632,3110400,5963776,6596304,9749376,23046144,27216000,

%U 33022080,52876800,53673984,76639680,94370400,105540864,119992320,245765520,285405120,426037248

%N Numbers n such that uphi(uphi(n)) = n/2.

%C If n = Product p_i^r_i then uphi(n) = Product (p_i^r_i-1); for example uphi(12) = (4-1)*(3-1) = 6.

%C If 2^n+1 is a Fermat prime then 2^(2*n) is a solution of the equation.

%C 3110400 and 4294967296 are also in the sequence.

%H Donovan Johnson, <a href="/A071008/b071008.txt">Table of n, a(n) for n = 1..41</a> (terms < 2^35)

%F {n: 2*A047994(A047994(n)) = n}.

%o (PARI) forstep(n=2,1e9,2,A047994(A047994(n))*2-n || print1(n", ")) \\ _M. F. Hasler_, Nov 20 2010

%Y Cf. A030163, A047994.

%K nonn

%O 1,1

%A _Yasutoshi Kohmoto_

%E More terms from _R. J. Mathar_, _Alois P. Heinz_ and _M. F. Hasler_, Nov 20 2010