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 A151880 #12 Dec 26 2023 13:09:09 %S A151880 1,1,1,1,1,2,3,1,3,9,12,1,4,18,52,60,1,5,30,136,360,360 %N A151880 Triangle: R*(n,k) (n>=2, k from 2 to n-1 or to 2 if n = 2), where R*(n,k) = number of trees with n nodes and k unlabeled end-nodes. %C A151880 All nodes are labeled except for the end-nodes. %H A151880 F. Harary, A. Mowshowitz and J. Riordan, <a href="https://doi.org/10.1016/S0021-9800(69)80106-7">Labeled trees with unlabeled end-points</a>, J. Combin. Theory, 6 (1969), 60-64. %F A151880 There is an explicit formula in terms of Stirling numbers of the second kind. %e A151880 Triangle (in fact the columns in the original have been reversed and the triangle transposed): %e A151880 (n=2) 1 %e A151880 (n=3) 1 %e A151880 (n=4) 1 1 %e A151880 (n=5) 1 2 3 %e A151880 (n=6) 1 3 9 12 %e A151880 (n=7) 1 4 18 52 60 %e A151880 (n=8) 1 5 30 136 360 360 %Y A151880 See A213262 for a better version with more terms and a program. %Y A151880 Row sums give A001258. %K A151880 nonn,tabf %O A151880 0,6 %A A151880 _N. J. A. Sloane_, Jul 21 2009