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A215763
Number of undirected labeled graphs on n nodes with exactly 3 cycle graphs as connected components.
2
1, 6, 25, 120, 742, 5446, 45559, 427275, 4437279, 50532669, 626068404, 8382651732, 120614527728, 1855938746160, 30411098170560, 528670269132480, 9718179216603840, 188342519176964160, 3838144417230700800, 82046606738914540800, 1835772107381126208000
OFFSET
3,2
LINKS
EXAMPLE
a(4) = 6: .1.2o .1-2. o1.2. o1.2o o1.2. .1.2o
. .|... ..... ...|. ..... ../.. ..\..
. .3.4o o3.4o o3.4. .3-4. .3.4o o3.4.
MAPLE
T:= proc(n, k) option remember; `if`(k<0 or k>n, 0, `if`(n=0, 1,
add(binomial(n-1, i)*T(n-1-i, k-1)*ceil(i!/2), i=0..n-k)))
end:
a:= n-> T(n, 3):
seq (a(n), n=3..25);
CROSSREFS
Column k=3 of A215771.
Sequence in context: A227914 A179603 A298700 * A153481 A099359 A073967
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Aug 23 2012
STATUS
approved