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 A020643 #21 Aug 19 2025 05:25:37
%S A020643 1,3,5,8,12,22,36,55,77,102,133,169,187,248,296,320,343,395,483,547,
%T A020643 652,768,809,851,893,935,1023,1065,1158,1206,1256,1359,1412,1522,1631,
%U A020643 1746,1805,1932,1992,2057,2316,2385,2454,2665,2812,2887,3043,3119,3199,3279
%N A020643 a(n)-th squarefree is sum of first k squarefrees for some k.
%H A020643 Amiram Eldar, <a href="/A020643/b020643.txt">Table of n, a(n) for n = 1..10000</a>
%F A020643 A005117(a(n)) = A013932(n). - _Amiram Eldar_, Aug 19 2025
%t A020643 With[{s = Select[Range[6000], SquareFreeQ]}, Position[s, #] & /@ Select[Accumulate[s], # <= s[[-1]] && SquareFreeQ[#] &] // Flatten] (* _Amiram Eldar_, Aug 19 2025 *)
%Y A020643 Cf. A005117, A013930, A013931, A013932.
%K A020643 nonn
%O A020643 1,2
%A A020643 _N. J. A. Sloane_, Renaud Lifchitz (100637.64(AT)CompuServe.COM), and _David W. Wilson_