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 A291821 #5 Sep 01 2017 17:17:19
%S A291821 1,2,38,1245,55566,3062271,197983540,14567100211,1196032764639,
%T A291821 108105913233804,10651377348228706,1135394214204504346,
%U A291821 130161542660895451934,15969338705916554563833,2088140238440781625249392,289961586742613652475061397,42623636996226195002733858964,6613842247421357561153219091827,1080515428870235624428208631160409,185421585676394321869412169323524031
%N A291821 Central terms of triangle A291820.
%C A291821 The g.f. F(x,y) of A291820 satisfies: F(x - x*y*F(x,y), y) = x + x*(1-y)*F(x,y).
%H A291821 Paul D. Hanna, <a href="/A291821/b291821.txt">Table of n, a(n) for n = 1..50</a>
%F A291821 a(n) = A291820(2*n-1, n-1) for n>=1.
%o A291821 (PARI) /* As central terms of triangle A291820 */
%o A291821 {A291820(n, k) = my(A=x); for(i=1, n, A = x + subst(x*A, x, y*A + (1-y)*x +x*O(x^n)) ); polcoeff(polcoeff(A, n, x), k, y)}
%o A291821 for(n=1, 20, print1(A291820(2*n-1, n-1), ", "));
%Y A291821 Cf. A291820.
%K A291821 nonn
%O A291821 1,2
%A A291821 _Paul D. Hanna_, Sep 01 2017