login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

A339281
Number of unoriented series-parallel networks with n colored elements using exactly 2 colors.
2
0, 2, 14, 84, 522, 3426, 23404, 165417, 1197934, 8847201, 66359672, 504180138, 3872043674, 30011312118, 234460670790, 1844390161675, 14597175479270, 116148990100435, 928620864502940, 7456287071153017, 60101357023288316, 486144584042042269, 3944839973878931780
OFFSET
1,2
COMMENTS
See A339282 for additional details.
FORMULA
a(n) = A339280(n) - 2*A339225(n).
EXAMPLE
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).
PROG
(PARI) \\ See A339282 for R(n, k).
seq(n) = {R(n, 2) - 2*R(n, 1)}
CROSSREFS
Column k=2 of A339282.
Sequence in context: A138126 A268881 A053141 * A361552 A036692 A075140
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Nov 30 2020
STATUS
approved