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

1,4

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.

Links

Table of n, a(n) for n=1..24.

Formula

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

Example

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.

Sequence in context: A138378 A250307 A363294 * A370369 A123636 A371901

Adjacent sequences: A289987 A289988 A289989 * A289991 A289992 A289993

Keyword

nonn

Author

Natan Arie Consigli, Aug 03 2017

Extensions

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

Status

approved