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 A072073 #17 Jun 03 2024 03:48:52
%S A072073 1,2,3,3,4,4,5,5,5,5,5,5,6,6,6,6,7,7,8,8,8,8,8,8,8,8,8,8,8,8,9,9,9,9,
%T A072073 9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,10
%N A072073 Number of solutions to cototient(x) = A051953(x) = 2^n.
%C A072073 a(n) increases at A000043(n).
%C A072073 Since A051953(p) = 1 for p prime, and given that there are an infinite number of primes, we disregard a(0) = oo. - _Michael De Vlieger_, Mar 25 2020
%F A072073 a(n) = A063740(A000079(n)). - _Ridouane Oudra_, Jun 02 2024
%e A072073 InvCototient(2^0) has an infinite number of entries, so 2^0=1 is left out.
%e A072073 n=14: 2^14=16384, InvCototient(16384) = {24576,28672,31744,32512,32764,32768}, so a(14)=6;
%t A072073 Length /@ Most@ Split@ DeleteCases[Select[Array[# - EulerPhi[#] &, 10^6], IntegerQ@ Log2@ # &], 1] (* _Michael De Vlieger_, Mar 25 2020 *)
%Y A072073 Cf. A051953, A058764, A000043, A063740, A000079.
%K A072073 nonn
%O A072073 1,2
%A A072073 _Labos Elemer_, Jun 13 2002