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 A289991 #28 Mar 20 2020 23:33:18 %S A289991 0,0,0,1,7,35,181,1027,6551,47356,383353,3439508,33796286,360254030, %T A289991 4132780425,50700507116,661702767423,9148018231984,133481943863634, %U A289991 2049205561735698,33008193951255524,556509843849858317,9799223447550907197,179855583136712491494 %N A289991 Number of unlabeled connected simple graphs with n nodes of degree 4 or less and with at least a cyclic structure and a node of degree one. %C A289991 Number of alkyl-n-cycloalkanes with n carbon atoms (the pure n-cycloalkanes aren't counted here). %C A289991 Stereoisomerism is not taken into account. Some of the drawn structures may or may not exist due to their strained geometries. %F A289991 a(n) = A289159(n) - A289990(n). %F A289991 a(n) = A121941(n) - A006820(n) - A289989(n). - _Andrew Howroyd_, Mar 20 2020 %e A289991 a(5) = 7 because there are 7 structures with 5 carbons following the rule above: 1,1-dimethylcyclopropane, 1,2-dimethylcyclopropane, ethylcyclopropane, methylcyclobutane, 1-methylbicyclobutane, 2-methylbicyclobutane, methyltetrahedrane. %Y A289991 Cf. A006820, A121941, A289159, A289989, A289990. %K A289991 nonn %O A289991 1,5 %A A289991 _Natan Arie Consigli_, Aug 03 2017 %E A289991 a(0) removed and a(15)-a(24) from _Andrew Howroyd_, Mar 20 2020