

A303831


Birooted graphs: number of unlabeled connected graphs with n nodes rooted at 2 indistinguishable roots.


7



0, 1, 3, 16, 98, 879, 11260, 230505, 7949596, 483572280, 53011686200, 10589943940654, 3880959679322754, 2623201177625659987, 3286005731275218388682, 7663042204550840483139108, 33407704152242477510352455230, 273327599183687887638526170380380
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OFFSET

1,3


LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..40
JeanFrançois Alcover, Mathematica program


FORMULA

G.f.: B(x)/G(x)  (C(x^2) + C(x)^2)/2 where B(x) is the g.f. of A303829, G(x) is the g.f. of A000088 and C(x) is the g.f. of A126100.  Andrew Howroyd, May 03 2018
a(n) = A303830(n) + A304071(n).  Brendan McKay, May 05 2018


MATHEMATICA

(* See the links section. *)


CROSSREFS

Cf. A303829 (not necessarily connected). 3rd column of A304311.
Cf. A000088 (not rooted), A126100 (connected single root), A053506 (2 roots adjacent).
Sequence in context: A295810 A233203 A074553 * A193037 A246056 A091641
Adjacent sequences: A303828 A303829 A303830 * A303832 A303833 A303834


KEYWORD

nonn


AUTHOR

Brendan McKay, May 01 2018


EXTENSIONS

a(12)a(18) from Andrew Howroyd, May 03 2018


STATUS

approved



