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 A117251 #3 Mar 30 2012 18:36:55 %S A117251 1,1,-1,4,-40,896,-43008,4325376,-899678208,383862702080, %T A117251 -334114095890432,590956713604022272,-2117988861556815822848, %U A117251 15348576513041885002989568,-224527747847927003472304537600,6621592717332784844802427579269120 %N A117251 Column 0 of triangle A117250. %F A117251 a(n) = (-1)^(n-1) * C(2*n,n)/(2*n-1) * 2^((n-1)*(n-2)/2-1). %o A117251 (PARI) a(n)=(-1)^(n-1)*2^((n-1)*(n-2)/2-1)*binomial(2*n,n)/(2*n-1) %K A117251 sign %O A117251 0,4 %A A117251 _Paul D. Hanna_, Mar 14 2006