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 A287424 #46 Mar 21 2020 20:10:25 %S A287424 1,1,2,6,20,77,351,1923,12191,89343,739070,6799093,68520840,748504768, %T A287424 8788177682,110193653493,1468070974840,20694573733399,307578336213268, %U A287424 4805384561510138,78708210391001097,1348365584613851292,24108682771968553916,449043803118089980878 %N A287424 Number of unlabeled connected graphs with n nodes of degree 4 or less, excluding 4-regular graphs. %C A287424 Also number of saturated hydrocarbons (alkanes, cycloalkanes and alkylcycloalkanes) with n carbon atoms satisfying the octet rule. Stereoisomerism is not taken into account. Some of the drawn structures may or may not exist due to their strained geometries. %F A287424 a(n) = A121941(n) - A006820(n). %e A287424 With 4 carbons, n-butane, i-butane, cyclobutane, bicyclobutane, methylcyclopropane and tetrahedrane are the 6 isomers satisfying the property above, so a(4)=6. %Y A287424 Cf. A006820, A121941 (includes 4-regular graphs). %K A287424 nonn %O A287424 1,3 %A A287424 _Natan Arie Consigli_, Jul 05 2017 %E A287424 a(0) removed and a(16)-a(24) from _Andrew Howroyd_, Mar 19 2020