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 A199581 #12 Apr 05 2023 08:13:30 %S A199581 1,2,7,29,107,431,1619,6079,22937,87083,332393,1273541,4896103, %T A199581 18877711,72968563,282664351,1097088989,4265342057,16608401041, %U A199581 64758466127,252814859149,988089813541,3865761355523,15138431958437,59333638261529,232737382916429 %N A199581 Central terms of the triangle in A199333: a(n) = A199333(2*n,n). %C A199581 a(n) = A199582(2*n). %H A199581 Reinhard Zumkeller, <a href="/A199581/b199581.txt">Table of n, a(n) for n = 0..75</a> %o A199581 (Haskell) %o A199581 a199581 n = a199333_row (2*n) !! n %Y A199581 Cf. A000984, A199333, A199582. %K A199581 nonn %O A199581 0,2 %A A199581 _Reinhard Zumkeller_, Nov 09 2011