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

0,5

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.

Links

Table of n, a(n) for n=0..24.

Example

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.

Prog

(nauty/bash) geng -c -D4 -d2 ${n} -q | multig -m1 -D4 -u

Crossrefs

Cf. A121941, A289990, A289991.

Sequence in context: A047096 A149060 A151468 * A295263 A018961 A178624

Adjacent sequences: A289986 A289987 A289988 * A289990 A289991 A289992

Keyword

nonn

Author

Natan Arie Consigli, Aug 03 2017

Extensions

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

Status

approved