A339281 Number of unoriented series-parallel networks with n colored elements using exactly 2 colors.

0, 2, 14, 84, 522, 3426, 23404, 165417, 1197934, 8847201, 66359672, 504180138, 3872043674, 30011312118, 234460670790, 1844390161675, 14597175479270, 116148990100435, 928620864502940, 7456287071153017, 60101357023288316, 486144584042042269, 3944839973878931780

Offset: 1

Author

Andrew Howroyd, Nov 30 2020

Keywords

• nonn

Comments

See A339282 for additional details.

Examples

			In the following examples elements in series are juxtaposed and elements in parallel are separated by '|'.
a(2) = 2: (12), (1|2).
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).
		

Crossrefs

Column k=2 of A339282.

Cf. A339225, A339227, A339280.

Programs

PARI

\\ See A339282 for R(n,k).
seq(n) = {R(n,2) - 2*R(n,1)}

Formula

a(n) = A339280(n) - 2*A339225(n).