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%I #21 Dec 15 2021 01:28:49
%S 2,4,7,11,17,19,30,31,40,42,60,64,79,76,94,124,130,124,156,172,139,
%T 191,194,220,212,211,260,268,276,299,295,320,335,328,357,366,378,404,
%U 427,438,437,463,476,492,505,453,490,541,563,587,542,599,638,659
%N When cototient function (A051953) is iterated with initial value A002110(n), a(n) = number of iterations required to reach the stationary value=0.
%F a(n) is smallest number such that Nest[A051953, A002110(n), a(n)]=0.
%e n=6, A002110(6)=30030; the corresponding iteration chain is {30030, 24270, 17806, 9238, 4798, 2400, 1760, 1120, 736, 384, 256, 128, 64, 32, 16, 8, 4, 2, 1, 0}. Its length is 20, so a(6) = 20-1 = 19.
%t Array[-1 + Length@ NestWhileList[# - EulerPhi@ # &, Product[Prime@ i, {i, #}], # > 0 &] &, 30] (* _Michael De Vlieger_, Nov 20 2017 *)
%Y Cf. A002110, A051953.
%K more,nonn
%O 1,1
%A _Labos Elemer_, Feb 28 2000
%E More terms from _Thomas Baruchel_, Oct 11 2003
%E More terms from _Sean A. Irvine_, Dec 15 2021
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