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 A025569 #10 Sep 13 2019 22:10:37 %S A025569 1,4,22,120,665,3732,21153,120835,694590,4013088,23284424,135580865, %T A025569 791862354,4637009300,27215510265,160053426720,942939231825, %U A025569 5564024727096,32878367902380,194530139772844,1152303896900338,6832908809693304,40556775927067299,240938382783554100 %N A025569 T(2n-1,n), where T is the array defined in A025564. %o A025569 (PARI) T(n, k) = if( k<0 || k>2*n, 0, if(n==0, 1, polcoeff( (1 + x + x^2)^n, k)+ polcoeff( (1 + x + x^2)^(n-1), k-1))); %o A025569 a(n) = T(2*n, n+1); \\ _Michel Marcus_, Sep 08 2019 %Y A025569 Cf. A025564. %K A025569 nonn %O A025569 1,2 %A A025569 _Clark Kimberling_ %E A025569 a(19) corrected and more terms from _Sean A. Irvine_, Sep 07 2019