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 A001137 #34 Feb 16 2025 08:32:22
%S A001137 1,2,2,4,8,16,33,56,90,164,330,688,1440,3008,6291,13168,27604,57896,
%T A001137 120730,248312,501464,995664,1954582,3821328,7495996,14848472,
%U A001137 29815976,60741680,125363472,261452256,549461078,1160693056,2459679936,5221717888
%N A001137 Number of black-rooted red-black trees with n internal nodes.
%H A001137 Alois P. Heinz, <a href="/A001137/b001137.txt">Table of n, a(n) for n = 1..900</a>
%H A001137 F. Ruskey, <a href="https://webhome.cs.uvic.ca/~cos/inf/tree/RedBlackTree.html">Information on Red-Black Trees</a>
%H A001137 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Red-BlackTree.html">Red-Black Tree.</a>
%H A001137 <a href="/index/Ro#rooted">Index entries for sequences related to rooted trees</a>
%F A001137 G.f. satisfies: A(x) = x*(1+x)^2*(1 + A((x+x^2)^2)). - _Paul D. Hanna_, Jun 14 2012
%t A001137 m = 35; A[_] = 0;
%t A001137 Do[A[x_] = x*(1 + x)^2*(1 + A[(x + x^2)^2]) + O[x]^m // Normal, {m}];
%t A001137 CoefficientList[A[x]/x, x] (* _Jean-François Alcover_, Oct 19 2019 *)
%o A001137 (PARI) {a(n)=local(A=x);for(i=1,n,A=x*(1+x)^2*(1 + subst(A,x,(x+x^2)^2+x*O(x^n))));polcoeff(A,n)} \\ _Paul D. Hanna_, Jun 14 2012
%Y A001137 Cf. A001131.
%K A001137 nonn
%O A001137 1,2
%A A001137 _Frank Ruskey_
%E A001137 More terms from _Sean A. Irvine_, Mar 08 2012