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 A351520 #11 Dec 10 2024 09:02:33
%S A351520 1,2,3,4,4,5,6,8,7,7,8,10,9,10,11,14,12,14,13,15,14,15,16,20,17,17,19,
%T A351520 19,18,19,20,24,21,22,23,28,24,25,26,30,27,28,29,31,31,30,31,37,32,33,
%U A351520 32,34,33,37,34,38,35,36,37,41,38,39,41,44,40,41,42,44,43,44,45,53,46,47,49
%N A351520 Number of numbers <= n that are either squarefree, a divisor of n, or both.
%H A351520 Robert Israel, <a href="/A351520/b351520.txt">Table of n, a(n) for n = 1..10000</a>
%F A351520 a(n) = tau(n) + Sum_{k=1..n} mu(k)^2 - Sum_{d|n} mu(d)^2.
%F A351520 a(n) = A000005(n) + A013928(n+1) - A034444(n).
%e A351520 a(10) = 7; There are 7 numbers less than or equal to 10 that are either squarefree, a divisor of 10, or both. The numbers are 1,2,3,5,6,7,10.
%p A351520 f:= proc(n) nops(select(t -> n mod t = 0 or numtheory:-issqrfree(t), [$1..n])) end proc:
%p A351520 map(f, [$1..100]); # _Robert Israel_, Dec 09 2024
%t A351520 Module[{nn=80,sf},sf=Select[Range[nn],SquareFreeQ[#]&];Table[Length[Union[Select[sf,#<= n&],Divisors[n]]],{n,nn}]] (* _Harvey P. Dale_, Jul 03 2023 *)
%Y A351520 Cf. A000005 (tau), A013928, A034444.
%K A351520 nonn
%O A351520 1,2
%A A351520 _Wesley Ivan Hurt_, Feb 12 2022