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 A109972 #8 Jan 26 2019 11:09:10
%S A109972 1,0,1,2,8,34,162,822,4365,23956,134814,773746,4511693,26652346,
%T A109972 159170385,959412290,5829083420,35661048886,219491344362,
%U A109972 1358204062536,8444658457530,52729475008690,330518562341537,2078987880103170
%N A109972 Diagonal sums of number triangle A109970.
%H A109972 Paul Barry, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL19/Barry/barry321.html">Jacobsthal Decompositions of Pascal's Triangle, Ternary Trees, and Alternating Sign Matrices</a>, Journal of Integer Sequences, 19, 2016, #16.3.5.
%F A109972 a(0) = 1, a(n) = Sum_{k=0..n-1} (k/(n-k))*binomial(3n-4k-1, n-2k), n>0. [corrected by _Michel Marcus_, Jan 25 2019]
%o A109972 (PARI) a(n) = if (n==0, 1, sum(k=0, n-1, (k/(n-k))*binomial(3*n-4*k-1, n-2*k))); \\ _Michel Marcus_, Jan 25 2019
%K A109972 easy,nonn
%O A109972 0,4
%A A109972 _Paul Barry_, Jul 06 2005