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URL: https://oeis.org/A339281

⇱ A339281 - OEIS

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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