%I #12 May 30 2018 03:35:37
%S 2,2,8,16,128,512,8192,16384,262144,2097152,16777216,67108864,
%T 2147483648,8589934592,274877906944,4398046511104,70368744177664,
%U 1125899906842624,18014398509481984,288230376151711744
%N When the Euler phi function is iterated with initial value A002110(n)=primorial, a(n)= first (maximal) power of 2 emerging during iteration.
%C Analogous to A053047 (the same with a similar large initial value, n!).
%e n=6, A002110(6)=30030; the corresponding iteration chain is {30030, 5760, 1536, 512, 256, 128, 64, 32, 16, 8, 4, 2, 1}. The first power of 2 is the 4th item after 3 iterations. It is 512, therefore a(6)=512.
%t a[n_] := 2^ Max@ IntegerExponent[ FixedPointList[ EulerPhi, Times @@ Prime[Range[n]]], 2]; Array[a, 20] (* _Giovanni Resta_, May 30 2018 *)
%Y Cf. A000010, A002110.
%K nonn
%O 1,1
%A _Labos Elemer_, Feb 28 2000