

A275166


Number of nnode graphs that have 2 nonisomorphic components.


10



0, 1, 1, 3, 8, 29, 140, 998, 12139, 273400, 11991356, 1018707920, 165078860603, 50500999728875, 29053989521339474, 31426435300576595334, 64000986599534312444935, 245935832697890955733422940, 1787577661113111145804012075034, 24637809007125076355873926288686728
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OFFSET

0,4


COMMENTS

"Component" means there are no edges from a node of one component to any node of the other component.
Each of the 2 components may be the empty graph with 0 nodes. That means the graph has only one "visible" component in these cases.
Each of the 2 components must be a connected graph (see A001349). (The empty graph has all properties and is a connected graph.)
The graphs of the 2 components must not be the same (not be isomorphic).


LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..75


FORMULA

G.f.: [A(x)^2  A(x^2)]/2 where A(x) is the o.g.f. for A001349.
a(n) = A275165(n) if n odd.


EXAMPLE

a(4)=8 = 1*6 + 1*2 where 1*6=A001349(0)*A001349(4) counts graphs with an empty component and a component with 4 nodes, where 1*2 = A001349(1)*A001349(3) counts graphs with a component of 1 node and a component of 3 nodes. There is no contribution from a component of 2 nodes and another component of 2 nodes (both components were isomorphic in that case).


CROSSREFS

Cf. A216785, A001349, A275165.
Sequence in context: A063839 A192744 A130470 * A182117 A207828 A208678
Adjacent sequences: A275163 A275164 A275165 * A275167 A275168 A275169


KEYWORD

nonn


AUTHOR

R. J. Mathar, Jul 18 2016


STATUS

approved



