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 A120581 #2 Mar 30 2012 18:59:15
%S A120581 1,4,-64,-28672,-17825792,-24696061952,-4398046511104,
%T A120581 6413125869375586304,1279761316857673852911616,
%U A120581 565738597953418785628899770368,372547299599873882347063111621541888,-5264848781000759906705331926184317767647232
%N A120581 Hankel transform of sum{k=0..n, C(2k,k)*2^k}.
%F A120581 a(n)=4^(n+C(n,2))(-1)^n*sum{k=0..n, C(n+k,2k)(-2)^(n-k)}; a(n)=8^n*4^C(n,2)*sum{k=0..n, C(n+k,2k)(-2)^(-k)}; a(n)=4^n*4^C(n,2)(-1)^n*A087168(n);
%Y A120581 Cf. A120580.
%K A120581 easy,sign
%O A120581 0,2
%A A120581 _Paul Barry_, Jun 15 2006