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 A055590 #3 Mar 30 2012 18:51:26 %S A055590 0,1,3,81,35917545547686059365808220080151141317043 %N A055590 a(n) = 3^(sum of a(i) where i<n). %F A055590 a(n) = 3^A055591(n-1) = A055591(n)-A055591(n-1) %e A055590 a(3) = 3^(0+1+3) = 3^4 = 81 %K A055590 easy,nonn %O A055590 0,3 %A A055590 _Henry Bottomley_, May 26 2000