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 A289990 #21 Mar 20 2020 21:07:23 %S A289990 0,0,1,3,10,37,161,878,5605,41912,355558,3359250,34723752,388248880, %T A289990 4655392910,59493136018,806368182523,11546555440892,174096392201350, %U A289990 2756178999408121,45700016438834847,791855740761714317,14309459324411915139,269188219981362999139 %N A289990 Number of unlabeled connected simple graphs with n nodes of degree 4 or less and degree at least 2 which are not 4-regular graphs. %C A289990 This is the number of pure n-cycloalkylic structures with n carbon atoms. %C A289990 Stereoisomerism is not taken into account. Some of the drawn structures may or may not exist due to their strained geometries. %F A289990 a(n) = A289989(n) - A006820(n). %e A289990 a(4) = 3, because here are 3 molecules with 4 carbons following the above rule: cyclobutane, bicyclobutane, tetrahedrane. %e A289990 a(5) = 10, because there are 10 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. %Y A289990 Cf. A006820, A121941, A289989, A289991. %K A289990 nonn %O A289990 1,4 %A A289990 _Natan Arie Consigli_, Aug 03 2017 %E A289990 a(0) removed and a(15)-a(24) from _Andrew Howroyd_, Mar 20 2020