A339227 Number of oriented series-parallel networks with n colored elements using exactly 2 colors.

0, 3, 22, 146, 970, 6601, 46012, 328188, 2387498, 17666752, 132631060, 1008068661, 7743145556, 60019505338, 468911161556, 3688746483355, 29194239490432, 232297608127077, 1857240493924050, 14912570002666430, 120202700216204324, 972289121546949231

Offset: 1

Author

Andrew Howroyd, Nov 28 2020

Keywords

• nonn

Comments

See A339228 for additional details.

Examples

			In the following examples elements in series are juxtaposed and elements in parallel are separated by '|'.
a(2) = 3: (12), (21), (22), (1|2).
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).
		

Crossrefs

Column k=2 of A339228.

Cf. A003430, A339226.

Programs

PARI

\\ See A339228 for R(n,k).
seq(n) = {R(n,2) - 2*R(n,1)}

Formula

a(n) = A339226(n) - 2*A003430(n).