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 A280006 #24 Jan 11 2020 15:57:47 %S A280006 0,0,1,6,39,399 %N A280006 Number of possible connected steric graphs with n nodes of degree less than 5, excluding the ones with all nodes of degree 4 and trees. %C A280006 Also number of hypothetical neutrally charged hydrocarbons with n carbon atoms containing a cyclic set of atoms (cycloalkanes and alkyl-cycloalkanes) satisfying the octet rule. This includes all isomers except conformers. %C A280006 Steric (three-dimensional) graphs can be represented as two-dimensional graphs provided that stereoisomers are taken into account. This means acting the group S_4 on the stereocenters (nodes with 4 different substituents) of each 2D graph and counting all of the resulting non-symmetric molecules. %C A280006 The terms of this sequence (and related ones) were found by drawing and counting the hydrocarbons in the PDFs below. If there are any missing molecules you can contact me. I'll add them in the PDF and update the data section. %C A280006 Many of those "theoretical" molecules are to be discovered and some may not exist due to their strained geometries. %H A280006 Natan Arie' Consigli, <a href="/A280006/a280006.pdf">Illustration of a(5) = 39: molecules with 5 carbons</a> %H A280006 Natan Arie' Consigli, <a href="/A280006/a280006_1.pdf">Illustration of a(6) = 399: molecules with 6 carbons</a> %F A280006 a(n) = A280012(n) - A000628(n). %Y A280006 Cf. A000628 (counts acyclic alkanes), A000639 (counts alkyl-benzenes). %K A280006 nonn,more,hard %O A280006 1,4 %A A280006 _Natan Arie Consigli_, Jan 02 2017