A289989 Number of unlabeled connected simple graphs with n nodes of degree 4 or less and degree at least 2.

1, 0, 0, 1, 3, 11, 38, 163, 884, 5621, 41971, 355823, 3360794, 34734530, 388337048, 4656198401, 59501173436, 806454404157, 11547541311414, 174108338688997, 2756331807471302, 45702073130849321, 791884792034547926, 14309888992592592578, 269194860146567854175

Offset: 0

Author

Natan Arie Consigli, Aug 03 2017

Keywords

• nonn

Comments

Also counts number of n-cycloalkanes and single bonded allotropes with n carbon atoms.

Stereoisomerism is not taken into account. Some of the drawn structures may or may not exist due to their strained geometries.

Examples

			a(4) = 3, because here are 3 molecules with 4 carbons following the above rule: cyclobutane, bicyclobutane, tetrahedrane.
a(5) = 11, because there are 11 molecules with 5 carbons following above rule: cyclopentane, housane, spiropentane, pyramidane, bicyclopentane, propellane, tricyclo[2.1.0.0^(1,3)]pentane, tricyclo[2.1.0.0^(2,5)]pentane tetracyclopentane, pentacyclopentane, hexacyclopentane.
		

Crossrefs

Cf. A121941, A289990, A289991.

Programs

nauty

geng -c -D4 -d2 ${n} -q | multig -m1 -D4 -u

Extensions

a(15)-a(24) from Andrew Howroyd, Mar 20 2020