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 A006081 M2593 #33 Feb 18 2019 13:10:23
%S A006081 1,1,3,6,17,44,133,404,1319,4392,15114,52796,187410,671848,2430509,
%T A006081 8852584,32438603,119463056,441926206,1641246060,6117011262,
%U A006081 22871513128,85766691938,322479555096,1215493792902,4591852445104,17383416640308,65937045017304
%N A006081 Number of line-rooted projective plane trees with n nodes.
%D A006081 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%D A006081 P. K. Stockmeyer, The charm bracelet problem and its applications, pp. 339-349 of Graphs and Combinatorics (Washington, Jun 1973), Ed. by R. A. Bari and F. Harary. Lect. Notes Math., Vol. 406. Springer-Verlag, 1974.
%H A006081 Alois P. Heinz, <a href="/A006081/b006081.txt">Table of n, a(n) for n = 2..1000</a>
%H A006081 P. J. Stockmeyer, <a href="/A006078/a006078.pdf">The charm bracelet problem and its applications</a>, pp. 339-349 of Graphs and Combinatorics (Washington, Jun 1973), Ed. by R. A. Bari and F. Harary. Lect. Notes Math., Vol. 406. Springer-Verlag, 1974. [Scanned annotated and corrected copy]
%H A006081 <a href="/index/Tra#trees">Index entries for sequences related to trees</a>
%F A006081 Stockmeyer gives g.f.
%F A006081 a(n) ~ 2^(2*n-4) / (sqrt(Pi) * n^(3/2)). - _Vaclav Kotesovec_, Sep 12 2014
%p A006081 b:= proc(n) option remember; binomial(n, floor(n/2)) end: c:= proc(n) option remember; binomial(2*n, n) /(n+1) end: a:= proc(m) local n; n:=m-2; (add(c(k)*c(n-k) +b(k)*b(n-k), k=0..n)) /4 +`if`(irem(n,2)=0, c(n/2)/2, 0) end: seq(a(n), n=2..30); # _Alois P. Heinz_, Feb 03 2009
%t A006081 b[n_] := Binomial[n, Floor[n/2]]; c[n_] := Binomial[2n, n]/(n+1); a[m_] := (n = m-2; Sum[c[k]*c[n-k] + b[k]*b[n-k], {k, 0, n}]/4 + If[Mod[n, 2] == 0, c[n/2]/2, 0]); Table[a[n], {n,2,30}] (* _Jean-François Alcover_, Jul 20 2011, after _Alois P. Heinz_ *)
%Y A006081 Cf. A000108, A001405. - _Alois P. Heinz_, Feb 03 2009
%K A006081 nonn,easy,nice
%O A006081 2,3
%A A006081 _N. J. A. Sloane_
%E A006081 More terms from _Alois P. Heinz_, Feb 03 2009