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 A339280 #5 Nov 30 2020 21:39:57
%S A339280 2,6,22,106,582,3622,24060,167803,1206852,8881775,66496238,504729590,
%T A339280 3874281594,30020527274,234498941338,1844550287865,14597849688004,
%U A339280 116151844649673,928633009522942,7456338969251761,60101579662366508,486145542528043029,3944844113529346468
%N A339280 Number of unoriented series-parallel networks with n elements of 2 colors.
%C A339280 See A339282 for additional details.
%e A339280 In the following examples, elements in series are juxtaposed and elements in parallel are separated by '|'.
%e A339280 a(1) = 2: (1), (2).
%e A339280 a(2) = 6: (11), (12), (22), (1|1), (1|2), (2|2).
%o A339280 (PARI) \\ See A339282 for R(n,k).
%o A339280 seq(n) = {R(n,2)}
%Y A339280 Cf. A339225, A339226, A339281, A339282.
%K A339280 nonn
%O A339280 1,1
%A A339280 _Andrew Howroyd_, Nov 30 2020