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 A219428 #29 Sep 08 2022 08:46:04
%S A219428 -1,0,0,1,0,3,0,3,2,5,0,7,0,7,6,7,0,11,0,11,8,11,0,15,4,13,8,15,0,21,
%T A219428 0,15,12,17,10,23,0,19,14,23,0,29,0,23,20,23,0,31,6,29,18,27,0,35,14,
%U A219428 31,20,29,0,43,0,31,26,31,16,45,0,35,24,45,0,47
%N A219428 a(n) = n - 1 - phi(n).
%C A219428 Apart from the first term, the same as A016035.
%C A219428 For n > 1, a(n) is also the number of numbers below n which are not coprime to n.
%C A219428 a(n) = 0 if n is prime.
%C A219428 x^(n - 1 - phi(n)) is congruent to x^(n - 1) mod n, if x is coprime to n, since x^phi(n) is congruent to 1 (mod n) if x is coprime to n.
%H A219428 Vincenzo Librandi, <a href="/A219428/b219428.txt">Table of n, a(n) for n = 1..1000</a>
%F A219428 a(n) = n - 1 - A000010(n) = A051953(n) - 1 = cototient(n) - 1. - _Omar E. Pol_, Nov 21 2012
%t A219428 Table[n - (EulerPhi[n] + 1), {n, 75}] (* _Alonso del Arte_, Nov 17 2012 *)
%o A219428 (PARI) for(n=1,100,print1(n-1-eulerphi(n)","))
%o A219428 (Magma) [(n - 1 - (EulerPhi(n))): n in [1..100]]; // _Vincenzo Librandi_, Jan 26 2013
%Y A219428 Cf. A000010, A219029.
%K A219428 sign,easy
%O A219428 1,6
%A A219428 _V. Raman_, Nov 20 2012