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 A013931 #17 Mar 07 2021 03:13:59
%S A013931 1,2,3,5,6,10,13,15,19,22,26,30,31,35,38,39,41,43,51,55,59,65,66,67,
%T A013931 69,70,73,74,78,79,82,85,86,89,93,95,97,102,103,105,110,111,113,118,
%U A013931 122,123,129,130,131,133,134,137,139,142,143,145,149,154,155,157,159,163,166,167
%N A013931 a(n) is squarefree and sum of all squarefrees <= a(n) is squarefree.
%H A013931 Amiram Eldar, <a href="/A013931/b013931.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Harvey P. Dale)
%F A013931 a(n) = A005117(A013930(n)). - _Amiram Eldar_, Mar 07 2021
%t A013931 Module[{nn=200,sf},sf=Select[Range[nn],SquareFreeQ];Transpose[ Select[ Thread[ {sf, Accumulate[sf]}],SquareFreeQ[Last[#]]&]][[1]]] (* _Harvey P. Dale_, Feb 09 2013 *)
%Y A013931 Cf. A005117, A013930, A013932.
%K A013931 nonn
%O A013931 1,2
%A A013931 _Henri Lifchitz_
%E A013931 More terms from _David W. Wilson_