A287424 Number of unlabeled connected graphs with n nodes of degree 4 or less, excluding 4-regular graphs.

1, 1, 2, 6, 20, 77, 351, 1923, 12191, 89343, 739070, 6799093, 68520840, 748504768, 8788177682, 110193653493, 1468070974840, 20694573733399, 307578336213268, 4805384561510138, 78708210391001097, 1348365584613851292, 24108682771968553916, 449043803118089980878

Offset: 1

Author

Natan Arie Consigli, Jul 05 2017

Keywords

• nonn

Comments

Also number of saturated hydrocarbons (alkanes, cycloalkanes and alkylcycloalkanes) with n carbon atoms satisfying the octet rule. Stereoisomerism is not taken into account. Some of the drawn structures may or may not exist due to their strained geometries.

Examples

			With 4 carbons, n-butane, i-butane, cyclobutane, bicyclobutane, methylcyclopropane and tetrahedrane are the 6 isomers satisfying the property above, so a(4)=6.
		

Crossrefs

Cf. A006820, A121941 (includes 4-regular graphs).

Formula

a(n) = A121941(n) - A006820(n).

Extensions

a(0) removed and a(16)-a(24) from Andrew Howroyd, Mar 19 2020