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 A368572 #13 Jan 06 2024 22:35:49
%S A368572 0,0,1,0,1,0,1,0,2,2,3,2,4,3,4,2,4,2,5,4,5,4,5,2,6,5,5,5,6,5,6,4,6,5,
%T A368572 6,2,7,6,7,5,8,7,8,7,7,7,8,4,8,7,8,7,8,5,8,6,9,8,9,7,10,9,9,8,10,9,10,
%U A368572 9,10,9,10,3,11,10,11,10,11,10,11,8,11,11,12,10
%N A368572 Number of refactorable numbers less than n that do not divide n.
%F A368572 a(n) = Sum_{k=1..n} c(k) * (ceiling(n/k) - floor(n/k)), where c = A336040.
%e A368572 a(15) = 4 since there are 4 refactorable numbers that are less than 15 and do not divide 15, namely: 2, 8, 9, 12.
%t A368572 Table[Sum[(1 - Ceiling[k/DivisorSigma[0, k]] + Floor[k/DivisorSigma[0, k]]) (Ceiling[n/k] - Floor[n/k]), {k, n}], {n, 100}]
%Y A368572 Cf. A033950, A336040, A368817.
%K A368572 nonn,easy
%O A368572 1,9
%A A368572 _Wesley Ivan Hurt_, Jan 06 2024