This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A289989 #24 Jun 02 2025 17:27:00
%S A289989 1,0,0,1,3,11,38,163,884,5621,41971,355823,3360794,34734530,388337048,
%T A289989 4656198401,59501173436,806454404157,11547541311414,174108338688997,
%U A289989 2756331807471302,45702073130849321,791884792034547926,14309888992592592578,269194860146567854175
%N A289989 Number of unlabeled connected simple graphs with n nodes of degree 4 or less and degree at least 2.
%C A289989 Also counts number of n-cycloalkanes and single bonded allotropes with n carbon atoms.
%C A289989 Stereoisomerism is not taken into account. Some of the drawn structures may or may not exist due to their strained geometries.
%e A289989 a(4) = 3, because here are 3 molecules with 4 carbons following the above rule: cyclobutane, bicyclobutane, tetrahedrane.
%e A289989 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.
%o A289989 (nauty) geng -c -D4 -d2 ${n} -q | multig -m1 -D4 -u
%Y A289989 Cf. A121941, A289990, A289991.
%K A289989 nonn
%O A289989 0,5
%A A289989 _Natan Arie Consigli_, Aug 03 2017
%E A289989 a(15)-a(24) from _Andrew Howroyd_, Mar 20 2020