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A007833 Number of point-labeled reduced two-graphs with n nodes. 3
1, 0, 1, 1, 28, 448, 18788, 1419852, 207249896, 58206408344, 31725488477648, 33830818147141904, 71068681534173472576, 295648155633330113713344, 2444510010072634827916776064, 40269686339597630128483872278656, 1323732128140903183968664175047409152 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,5

COMMENTS

Also number of (n-1)-node labeled mating graphs without isolated nodes, cf. A006024. - Vladeta Jovovic, Mar 23 2004

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..83

P. J. Cameron, Counting two-graphs related to trees, Elec. J. Combin., Vol. 2, #R4.

FORMULA

a(n) = Sum_{k=1..n} s(n, k) * 2^((k-1) * (k-2) / 2) where s(n, k) are the Stirling numbers of the first kind. - Sean A. Irvine, Feb 03 2018

MATHEMATICA

Array[Sum[StirlingS1[#, k] 2^((k - 1) (k - 2)/2), {k, #}] &, 15] (* Michael De Vlieger, Feb 03 2018 *)

CROSSREFS

Cf. A092430 (connected).

Sequence in context: A125485 A054337 A009685 * A080315 A022752 A000771

Adjacent sequences:  A007830 A007831 A007832 * A007834 A007835 A007836

KEYWORD

nonn

AUTHOR

Peter J. Cameron

STATUS

approved

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Last modified December 7 09:33 EST 2019. Contains 329843 sequences. (Running on oeis4.)