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 A228620 #25 Sep 08 2022 08:46:05
%S A228620 1,0,0,2,0,5,0,4,3,7,0,8,0,9,8,8,0,12,0,12,10,13,0,16,5,15,9,16,0,21,
%T A228620 0,16,14,19,12,24,0,21,16,24,0,29,0,24,21,25,0,32,7,30,20,28,0,36,16,
%U A228620 32,22,31,0,44,0,33,27,32,18,45,0,36,26,45,0,48,0
%N A228620 a(n) = n - phi(n) + mu(n).
%C A228620 Sum of the cototient and the Moebius function. If n is prime, then a(n) = 0. Proof: p - phi(p) + mu(p) = p - (p-1) + (-1) = 0. If n is semiprime, then a(n) is equal to the sum of the distinct prime factors of n.
%H A228620 Vincenzo Librandi, <a href="/A228620/b228620.txt">Table of n, a(n) for n = 1..2000</a>
%F A228620 a(n) = n - A000010(n) + A008683(n) = A051953(n) + A008683(n) = A076369(n) - A000010(n) = A062830(n) + A008683(n) - 1. a(A001358(n)) = sopf(A001358(n)).
%F A228620 Dirichlet g.f.: (zeta(s-1)*(zeta(s) - 1) + 1)/zeta(s). - _Ilya Gutkovskiy_, Dec 06 2016
%e A228620 a(4) = 2, Since 4 - phi(4) + mu(4) = 4 - 2 + 0 = 2.
%e A228620 a(6) = 5, since 6 - phi(6) + mu(6) = 6 - 2 + 1 = 5. Note that the sum of the distinct prime factors of 4 is 2 and the sum of the distinct prime factors of 6 is 5.
%p A228620 with(numtheory); seq(k - phi(k) + mobius(k), k=1..70);
%t A228620 Table[n - EulerPhi[n] + MoebiusMu[n], {n, 100}]
%o A228620 (PARI) a(n) = n - eulerphi(n) + moebius(n); \\ _Michel Marcus_, Dec 06 2016
%o A228620 (Magma) [n-EulerPhi(n)+MoebiusMu(n): n in [1..80]]; // _Vincenzo Librandi_, Jul 30 2017
%Y A228620 Cf. A000010, A008683, A051953, A062830, A076369.
%K A228620 nonn,easy
%O A228620 1,4
%A A228620 _Wesley Ivan Hurt_, Aug 27 2013