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 A339227 #7 Nov 30 2020 08:56:40
%S A339227 0,3,22,146,970,6601,46012,328188,2387498,17666752,132631060,
%T A339227 1008068661,7743145556,60019505338,468911161556,3688746483355,
%U A339227 29194239490432,232297608127077,1857240493924050,14912570002666430,120202700216204324,972289121546949231
%N A339227 Number of oriented series-parallel networks with n colored elements using exactly 2 colors.
%C A339227 See A339228 for additional details.
%F A339227 a(n) = A339226(n) - 2*A003430(n).
%e A339227 In the following examples elements in series are juxtaposed and elements in parallel are separated by '|'.
%e A339227 a(2) = 3: (12), (21), (22), (1|2).
%e A339227 a(3) = 22: (112), (121), (122), (211), (212), (221), (1(1|2)), (1(2|2)), (2(1|1)), (2(1|2)), ((1|1)2), ((1|2)1), ((1|2)2), ((2|2)1), (1|12), (1|21), (1|22), (2|21), (11|2), (12|2), (1|1|2), (1|2|2).
%o A339227 (PARI) \\ See A339228 for R(n,k).
%o A339227 seq(n) = {R(n,2) - 2*R(n,1)}
%Y A339227 Column k=2 of A339228.
%Y A339227 Cf. A003430, A339226.
%K A339227 nonn
%O A339227 1,2
%A A339227 _Andrew Howroyd_, Nov 28 2020