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%I #5 Nov 30 2020 21:40:03
%S 0,2,14,84,522,3426,23404,165417,1197934,8847201,66359672,504180138,
%T 3872043674,30011312118,234460670790,1844390161675,14597175479270,
%U 116148990100435,928620864502940,7456287071153017,60101357023288316,486144584042042269,3944839973878931780
%N Number of unoriented series-parallel networks with n colored elements using exactly 2 colors.
%C See A339282 for additional details.
%F a(n) = A339280(n) - 2*A339225(n).
%e In the following examples elements in series are juxtaposed and elements in parallel are separated by '|'.
%e a(2) = 2: (12), (1|2).
%e a(3) = 14: (112), (121), (122), (212), (1(1|2)), (1(2|2)), (2(1|1)), (2(1|2)), (1|12), (1|22), (2|11), (2|12), (1|1|2), (1|2|2).
%o (PARI) \\ See A339282 for R(n,k).
%o seq(n) = {R(n,2) - 2*R(n,1)}
%Y Column k=2 of A339282.
%Y Cf. A339225, A339227, A339280.
%K nonn
%O 1,2
%A _Andrew Howroyd_, Nov 30 2020