This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A066672 #21 Aug 16 2025 03:17:39
%S A066672 1,1,1,2,1,1,2,1,1,2,2,2,2,2,3,2,3,3,2,2,3,1,1,2,3,2,3,3,2,1,4,2,2,3,
%T A066672 3,3,3,3,1,3,3,3,3,3,2,1,5,3,4,4,2,1,3,4,4,3,3,1,5,4,4,3,3,2,4,2,4,4,
%U A066672 2,3,5,4,3,4,4,1,1,4,2,3,4,2,3,1,4,3,5,2,6,5,5,4,5,5,4,3,1,4,6,5,4,1,3,4,3
%N A066672 Exponent of the largest power of 2 that divides phi(A066669(n)).
%H A066672 Amiram Eldar, <a href="/A066672/b066672.txt">Table of n, a(n) for n = 1..10000</a>
%F A066672 From _Amiram Eldar_, Jul 18 2024: (Start)
%F A066672 a(n) = A053574(A066669(n)).
%F A066672 a(n) = log_2(A066671(n)) = A007814(A000010(A066669(n))). (End)
%e A066672 A066669(263) = 769 and phi(769) = 768 = 3*256, so a(263) = log_2(256) = log_2(phi(769)/3) = log_2(phi(A066669(263))/A066670(263)) = 8.
%t A066672 f[n_] := Module[{phi = EulerPhi[n], e}, e = IntegerExponent[phi, 2]; If[PrimeQ[phi/2^e], e, Nothing]]; Array[f, 300] (* _Amiram Eldar_, Jul 18 2024 *)
%Y A066672 Cf. A000010, A007814, A053574, A066669, A066670, A066671.
%K A066672 nonn
%O A066672 1,4
%A A066672 _Labos Elemer_, Dec 18 2001
%E A066672 Name corrected by _Amiram Eldar_, Jul 18 2024