A289990 Number of unlabeled connected simple graphs with n nodes of degree 4 or less and degree at least 2 which are not 4-regular graphs.

0, 0, 1, 3, 10, 37, 161, 878, 5605, 41912, 355558, 3359250, 34723752, 388248880, 4655392910, 59493136018, 806368182523, 11546555440892, 174096392201350, 2756178999408121, 45700016438834847, 791855740761714317, 14309459324411915139, 269188219981362999139

Offset: 1

Author

Natan Arie Consigli, Aug 03 2017

Keywords

• nonn

Comments

This is the number of pure n-cycloalkylic structures 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) = 10, because there are 10 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.
		

Crossrefs

Cf. A006820, A121941, A289989, A289991.

Formula

a(n) = A289989(n) - A006820(n).

Extensions

a(0) removed and a(15)-a(24) from Andrew Howroyd, Mar 20 2020