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A332958 Number of labeled forests with 2n nodes consisting of n-1 isolated nodes and a labeled tree with n+1 nodes. 1
1, 12, 240, 7000, 272160, 13311144, 787218432, 54717165360, 4375800000000, 396040894180360, 40038615905992704, 4473490414613093328, 547532797546896179200, 72869747140722656250000, 10478808079059531910348800, 1619337754490833097114916960 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Given 2n vertices, we can choose n-1 of them in C(2n, n-1) ways. For each of these ways there are A000272(n+1) trees. (possibilities)
LINKS
FORMULA
a(n) = C(2*n,n-1) * (n+1)^(n-1).
a(n) = A001791(n) * A000272(n+1).
a(n) ~ exp(1) * 2^(2*n) * n^(n - 3/2) / sqrt(Pi).
EXAMPLE
a(1) = 1. The forest is the tree of 2 nodes. It can be depicted by 1--2.
a(2) = 12. Given 4 nodes we can choose 1 of them in C(4,1) = 4 ways. For each of these 4 ways there are A000272(n+1) = (n+1)^(n-1) = 3 trees to complete the forest. The 12 forests can be represented by:
1 3-2-4, 1 2-3-4, 1 2-4-3,
2 3-1-4, 2 1-3-4, 2 1-4-3,
3 2-1-4, 3 1-2-4, 3 1-4-2,
4 2-1-3, 4 1-2-3, 4 1-3-2.
MATHEMATICA
a[n_] := Binomial[2n, n-1] * (n+1)^(n-1); Array[a, 18] (* Amiram Eldar, Apr 12 2020 *)
PROG
(PARI) a(n) = binomial(2*n, n-1) * (n+1)^(n-1);
CROSSREFS
Sequence in context: A009080 A002166 A034910 * A091745 A012344 A282789
KEYWORD
nonn,easy
AUTHOR
Washington Bomfim, Apr 12 2020
STATUS
approved

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Last modified May 21 13:13 EDT 2024. Contains 372736 sequences. (Running on oeis4.)