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 A220897 #10 May 23 2017 07:00:42
%S A220897 3,13,99,962,10732,131715,1741813,24537945,365779679,5744911157,
%T A220897 94786034723,1639198623818,29658034018852,560484305049943,
%U A220897 11046637024014049,226728682400563206,4839249231632997924,107262821628220974029,2465723931357450572707
%N A220897 Number of closed lambda-terms of size n with at most 3 free de Bruijn indices.
%H A220897 Katarzyna Grygiel and Pierre Lescanne, <a href="http://arxiv.org/abs/1210.2610">Counting and generating lambda-terms</a>, arXiv preprint arXiv:1210.2610, 2012
%F A220897 Grygiel and Lescanne give a recurrence.
%t A220897 T[0, m_] := m; T[n_, m_] := T[n, m] = T[n-1, m+1] + Sum[T[i, m] T[n-i-1, m], {i, 0, n-1}];
%t A220897 a[n_] := T[n, 3];
%t A220897 Table[a[n], {n, 0, 14}] (* _Jean-François Alcover_, May 23 2017 *)
%K A220897 nonn
%O A220897 0,1
%A A220897 _N. J. A. Sloane_, Dec 31 2012