

A007833


Number of pointlabeled reduced twographs 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 (n1)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 twographs related to trees, Elec. J. Combin., Vol. 2, #R4.


FORMULA

a(n) = Sum_{k=1..n} s(n, k) * 2^((k1) * (k2) / 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



