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 A139491 #22 May 09 2021 07:51:43
%S A139491 3,8,9,12,15,16,21,24,40,45,48,60,72,120,168,240,840,1848
%N A139491 Numbers arising in A139490.
%C A139491 _M. F. Hasler_, Apr 24 2008, observed that the numbers in this sequence are differences of two squares. For example: 3=2^2-1^2, 8=3^2-1^2, 9=5^2-4^2, 15=4^2-1^2, 16=5^2-3^2, 21=5^2-2^2, 24=5^2-1^2, 40=7^2-3^2, 45=7^2-2^2, 48=7^2-1^2, 60=8^2-2^2.
%C A139491 This sequence is a subsequence of A024352.
%C A139491 These numbers appear to be a subset of the idoneal numbers A000926. If so, then the sequence is probably complete. - _T. D. Noe_, Apr 27 2009
%H A139491 N. J. A. Sloane et al., <a href="https://oeis.org/wiki/Binary_Quadratic_Forms_and_OEIS">Binary Quadratic Forms and OEIS</a> (Index to related sequences, programs, references)
%t A139491 f = 200; g = 300; h = 30; j = 100; b = {}; Do[a = {}; Do[Do[If[PrimeQ[x^2 + n y^2], AppendTo[a, x^2 + n y^2]], {x, 0, g}], {y, 1, g}]; AppendTo[b, Take[Union[a], h]], {n, 1, f}]; Print[b]; c = {}; Do[a = {}; Do[Do[If[PrimeQ[n^2 + w*n*m + m^2], AppendTo[a, n^2 + w*n*m + m^2]], {n, m, g}], {m, 1, g}]; AppendTo[c, Take[Union[a], h]], {w, 1, j}]; Print[c]; bb = {}; cc = {}; Do[Do[If[b[[p]] == c[[q]], AppendTo[bb, p]; AppendTo[cc, q]], {p, 1, f}], {q, 1, j}]; Union[bb]
%Y A139491 Cf. A000926, A024352.
%K A139491 nonn
%O A139491 1,1
%A A139491 _Artur Jasinski_, Apr 24 2008, Apr 26 2008
%E A139491 Extended by _T. D. Noe_, Apr 27 2009