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 A289158 #25 Jun 02 2025 17:26:56
%S A289158 1,1,2,7,28,112,590,3419,23453,178599,1516692,14083855,142029043,
%T A289158 1542152723,17925912574,221938298129,2914638247016,40455853460661,
%U A289158 591654481313077,9091698010380468,146433114305147508,2466517505722469501,43361349681960337334
%N A289158 Number of unlabeled connected loopless multigraphs with n nodes of degree 4 or less and with at most double edges.
%C A289158 In chemical terms this counts the following molecules (excluding stereoisomers) without triple bonds, given n carbon atoms:
%C A289158 - carbon allotropes;
%C A289158 - aliphatic hydrocarbons;
%C A289158 - resonance structures of graphically non-equivalent anti-aromatic and aromatic hydrocarbons;
%C A289158 Some molecules are theoretical and may or may not exist.
%H A289158 Daniel R. Herber, <a href="https://www.engr.colostate.edu/~drherber/files/Herber2020b.pdf">Enhancements to the perfect matching approach for graph enumeration-based engineering challenges</a>, Proceedings of the ASME 2020 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference (IDETC/CIE 2020).
%o A289158 (nauty) geng -c -D4 ${n} -q | multig -m2 -D4 -u
%Y A289158 Cf. A121941, A289157 (allows more than two edges), A303030, A303031.
%K A289158 nonn
%O A289158 0,3
%A A289158 _Natan Arie Consigli_, Jul 04 2017
%E A289158 a(15)-a(22) from _Andrew Howroyd_, Mar 20 2020