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