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 A063689 #8 May 07 2023 23:24:53 %S A063689 1,1,2,6,21,83,356,1599,7434,35381,171508,843419,4197179,21094355, %T A063689 106915928,545859112,2804656069,14491370996,75248398034,392476363133, %U A063689 2055245992376,10801442696736,56953957110855,301207378815752,1597342159296786,8492297139795170 %N A063689 Number of 2-trees rooted at a triangle with 3 similar edges. %D A063689 F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 76, Eq. (3.5.17). %F A063689 a(n) = A058870(n) + A063687(n) with A058870(0)=0. - _Sean A. Irvine_, May 07 2023 %e A063689 Sequence really begins 1, 0, 0, 1, 0, 0, 2, 0, 0, 6, 0, 0, 21, 0, 0, 83, 0, 0, 356, ... but only nonzero trisection is shown. %Y A063689 Cf. A005750, A058870, A058866, A054581. %K A063689 nonn %O A063689 1,3 %A A063689 _Vladeta Jovovic_, Aug 22 2001 %E A063689 More terms from _Sean A. Irvine_, May 07 2023