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 A175985 #13 Apr 25 2016 11:45:31 %S A175985 1,1,3,63,16777215,1329227995784915872903807060280344575 %N A175985 a(1) = 1, for n >= 2: a(n) = (a(n-1)+1)^(n-1) - 1. %C A175985 The next term (a(7)) has 217 digits. %F A175985 a(n+1) = 2^A000142(n) - 1; A000142 = factorial numbers. %e A175985 For n = 4: a(4) = (3+1)^3 - 1 = 63. %o A175985 (PARI) a(n)=if(n<=1,1,(a(n-1)+1)^(n-1)-1); /* _Joerg Arndt_, Jun 10 2011 */ %K A175985 nonn,easy %O A175985 1,3 %A A175985 _Jaroslav Krizek_, Nov 03 2010