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A289989
Number of unlabeled connected simple graphs with n nodes of degree 4 or less and degree at least 2.
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.
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
KEYWORD
nonn
AUTHOR
Natan Arie Consigli, Aug 03 2017
EXTENSIONS
a(15)-a(24) from Andrew Howroyd, Mar 20 2020
STATUS
approved