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 A368677 #11 Jan 03 2024 05:34:28
%S A368677 0,0,1,1,3,2,4,4,5,5,6,5,8,7,8,9,11,10,12,10,12,12,14,13,16,15,16,14,
%T A368677 17,16,18,18,18,19,20,19,23,22,23,22,26,23,27,25,27,28,29,28,30,31,30,
%U A368677 29,32,31,33,32,33,33,35,32,37,36,37,38,39,37,40,38,40,40
%N A368677 Number of numbers k less than n and not dividing n such that n-k is squarefree.
%F A368677 a(n) = Sum_{k=1..n} mu(n-k)^2 * (ceiling(n/k) - floor(n/k)).
%e A368677 a(12) = 5. The numbers less than 12 that do not divide 12 are: {5,7,8,9,10,11} with values of n-k: {7,5,4,3,2,1} (exactly 5 of which are squarefree).
%t A368677 Table[Sum[MoebiusMu[n - k]^2 (Ceiling[n/k] - Floor[n/k]), {k, n}], {n, 100}]
%o A368677 (PARI) a(n) = sum(k=1, n-1, (n % k) && issquarefree(n-k)); \\ _Michel Marcus_, Jan 03 2024
%Y A368677 Cf. A008683 (mu), A368673, A368679, A368680.
%K A368677 nonn,easy
%O A368677 1,5
%A A368677 _Wesley Ivan Hurt_, Jan 02 2024