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%I #46 Mar 21 2020 20:10:25
%S 1,1,2,6,20,77,351,1923,12191,89343,739070,6799093,68520840,748504768,
%T 8788177682,110193653493,1468070974840,20694573733399,307578336213268,
%U 4805384561510138,78708210391001097,1348365584613851292,24108682771968553916,449043803118089980878
%N Number of unlabeled connected graphs with n nodes of degree 4 or less, excluding 4-regular graphs.
%C Also number of saturated hydrocarbons (alkanes, cycloalkanes and alkylcycloalkanes) with n carbon atoms satisfying the octet rule. Stereoisomerism is not taken into account. Some of the drawn structures may or may not exist due to their strained geometries.
%F a(n) = A121941(n) - A006820(n).
%e With 4 carbons, n-butane, i-butane, cyclobutane, bicyclobutane, methylcyclopropane and tetrahedrane are the 6 isomers satisfying the property above, so a(4)=6.
%Y Cf. A006820, A121941 (includes 4-regular graphs).
%K nonn
%O 1,3
%A _Natan Arie Consigli_, Jul 05 2017
%E a(0) removed and a(16)-a(24) from _Andrew Howroyd_, Mar 19 2020