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 A121941 #33 Jun 02 2025 15:57:45
%S A121941 1,1,1,2,6,21,78,353,1929,12207,89402,739335,6800637,68531618,
%T A121941 748592936,8788983173,110201690911,1468157196474,20695559603921,
%U A121941 307590282700915,4805537369573319,78710267083015571,1348394635886684901,24109112440149231355,449050443283294835914
%N A121941 Number of unlabeled connected simple graphs with n nodes of degree 4 or less.
%C A121941 Number of graphs of hydrogen bonded water clusters.
%C A121941 This counts the connected graphs where each vertex has degree 4 or less. - _Charles R Greathouse IV_, Jul 07 2017
%C A121941 Also number of saturated hydrocarbons and allotropes with n carbon, or valence 4, atoms (excluding stereoisomers) following the octet rule. - _Natan Arie Consigli_, Jul 07 2017
%H A121941 G. Brinkmann, <a href="https://doi.org/10.1007/s10910-008-9496-y">Generating water clusters and other directed graphs</a>, J. Math. Chem. 46 (4) (2009) 1112-1121, Table 1.
%H A121941 T. Miyake and M. Aida, <a href="https://doi.org/10.1016/S0009-2614(02)01150-8">Enumeration of topology-distinct structures of hydrogen bonded water clusters</a>, Chem. Phys. Lett., vol. 363 (2002) pp. 106-110. See Table 1 column 2 on page 109.
%e A121941 With 4 carbons, n-butane, i-butane, cyclobutane, bicyclobutane, methylcyclopropane and tetrahedrane are the 6 isomers satisfying the property above, so a(4)=6. - _Natan Arie Consigli_, Jul 07 2017
%e A121941 If n=5 then the number of graphs of hydrogen bonded water clusters is 21.
%o A121941 (nauty) geng -c -D4 ${n} -q | multig -m1 -D4 -u
%Y A121941 Cf. A121942, A243393 (degree 3 or less), A287424 (excludes allotropes), A289157, A289158, A303032 (degree 5 or less).
%K A121941 nonn,nice
%O A121941 0,4
%A A121941 _Parthasarathy Nambi_, Sep 03 2006
%E A121941 More terms sent by _Natan Arie Consigli_, Jul 07 2017
%E A121941 Renamed by _Andrew Howroyd_, Mar 19 2020 based on comment by _Charles R Greathouse IV_.
%E A121941 a(16)-a(24) from _Andrew Howroyd_, Mar 19 2020