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 A002213 M3907 N1603 #31 Apr 14 2025 10:26:09 %S A002213 1,1,5,20,84,354,1540,6704,29610,131745,591049,2669346,12131148, %T A002213 55431285,254539897,1174027598,5436826110,25269402555,117838870833, %U A002213 551192276450,2585418254532,12158383558066,57313008207960 %N A002213 Number of tree-like polyhexes rooted at a hexagon and containing n hexagons. %D A002213 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). %D A002213 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). %H A002213 T. D. Noe, <a href="/A002213/b002213.txt">Table of n, a(n) for n = 1..200</a> %H A002213 F. Harary and R. C. Read, <a href="https://doi.org/10.1017/S0013091500009135">The enumeration of tree-like polyhexes</a>, Proc. Edinburgh Math. Soc. (2) 17 (1970), 1-13. %F A002213 G.f.: x + x*U(x) + (3/2)*x*U(x)^2 + (1/2)*x*U(x^2) + (1/3)*x*U(x)^3 + (2/3)*x*U(x^3), where U(x) = (1 - 3*x - sqrt((1-x)*(1-5*x)))/(2*x). %F A002213 a(n) ~ 5^(n+1/2)/(2*sqrt(Pi)*n^(3/2)). - _Vaclav Kotesovec_, Aug 13 2013 %Y A002213 Cf. A002212, A002214, A002215, A002216. %K A002213 nonn,easy %O A002213 1,3 %A A002213 _N. J. A. Sloane_ %E A002213 Edited by _Emeric Deutsch_, Feb 18 2004