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A215763
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Number of undirected labeled graphs on n nodes with exactly 3 cycle graphs as connected components.
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2
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1, 6, 25, 120, 742, 5446, 45559, 427275, 4437279, 50532669, 626068404, 8382651732, 120614527728, 1855938746160, 30411098170560, 528670269132480, 9718179216603840, 188342519176964160, 3838144417230700800, 82046606738914540800, 1835772107381126208000
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listen;
history;
text;
internal format)
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OFFSET
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3,2
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LINKS
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EXAMPLE
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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.
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MAPLE
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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);
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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