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 A034021 #15 Jul 08 2025 20:02:04
%S A034021 0,1,3,7,13,19,21,31,37,39,43,57,61,67,73,79,91,93,97,103,109,111,127,
%T A034021 129,133,139,151,157,163,181,183,193,199,201,211,217,219,223,229,237,
%U A034021 241,247,259,271,273,277,283,291,301,307,309,313,327,331,337,349,367
%N A034021 Numbers that are primitively but not imprimitively represented by x^2+xy+y^2.
%C A034021 Also numbers that are squarefree and primitively represented by x^2+x*y+y^2. - _Frank M Jackson_, Nov 08 2013
%t A034021 lst = {}; Do[k=x^2+x*y+y^2; If[(SquareFreeQ[k] && GCD[x, y]==1) || k==0, AppendTo[lst, k]], {x, 0, 100}, {y, 0, x}]; Union@lst (* _Frank M Jackson_, Nov 08 2013 *)
%Y A034021 Cf. A034017, A003136, A045897.
%K A034021 nonn
%O A034021 1,3
%A A034021 _N. J. A. Sloane_
%E A034021 Extended by _Ray Chandler_, Jan 29 2009