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 A378642 #15 Dec 10 2024 08:57:58
%S A378642 0,0,1,0,3,1,5,1,7,5,9,1,11,9,13,5,15,9,17,10,19,17,21,5,23,21,25,19,
%T A378642 27,19,29,16,31,29,33,16,35,33,37,22,39,33,41,34,43,41,45,16,47,43,49,
%U A378642 43,51,41,53,41,55,53,57,34,59,57,61,42,63,55,65,59,67,63
%N A378642 Number of numbers m <= n such that phi(m) does not divide n, where phi is the Euler totient function (A000010).
%H A378642 Paolo Xausa, <a href="/A378642/b378642.txt">Table of n, a(n) for n = 1..10000</a>
%H A378642 Max Alekseyev, <a href="https://oeis.org/wiki/User:Max_Alekseyev/gpscripts">PARI/GP Scripts for Miscellaneous Math Problems</a> (invphi.gp).
%F A378642 a(n) = n - A069932(n).
%t A378642 Table[Count[Divisible[n, #[[;;n]]], False], {n, Length[#]}] & [EulerPhi[Range[100]]]
%o A378642 (PARI) a(n) = n - sumdiv(n, d, #select(x -> x<=n, invphi(d))); \\ _Amiram Eldar_, Dec 10 2024, using _Max Alekseyev_'s invphi.gp
%Y A378642 Cf. A000010, A069932, A378637, A378641.
%K A378642 nonn,easy
%O A378642 1,5
%A A378642 _Paolo Xausa_, Dec 05 2024