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 A109957 #3 Mar 30 2012 18:59:08
%S A109957 1,-1,4,-16,74,-368,1926,-10455,58333,-332489,1927761,-11334128,
%T A109957 67416570,-404956315,2453012916,-14967587873,91910798556,
%U A109957 -567566697627,3522334450512,-21957276973619,137425078658354,-863229921626723,5440215989173113,-34388244134246836
%N A109957 Diagonal sums of number triangle A109956.
%F A109957 G.f. : g/(1-f) where f/(1-f)^3=x and g=1-f; a(n)=(-1)^n*sum{k=0..floor(n/2), ((3k+1)/(2n-k+1))*binomial(3n-3k, n-2k)}.
%K A109957 easy,sign
%O A109957 0,3
%A A109957 _Paul Barry_, Jul 06 2005