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 A117872 #12 Aug 22 2013 02:59:40 %S A117872 0,1,0,1,1,0,0,1,1,1,1,0,0,1,0,1,1,1,0,0,0,1,1,0,0,1,0,1,1,1,0,0,1,1, %T A117872 1,0,1,1,0,1,0,0,1,1,0,0,1,0,0,1,1,0,0,1,1,1,0,0,0,1,0,1,0,1,1,1,1,0, %U A117872 1,1,0,1,1,1,1,0,0,1,1,1,0,1,1,0,0,1,0,1,0,0,0,1,1,1 %N A117872 Parity of the iterated triangular numbers, using "2" as a seed. %C A117872 Suggested by D. R. Hofstadter. %C A117872 a(n) = A007501(n) mod 2. %H A117872 Charles R Greathouse IV, <a href="/A117872/b117872.txt">Table of n, a(n) for n = 0..10000</a> %F A117872 a(n) = mod(T^(n)(2), 2) %e A117872 a(1) = mod(T(2),2) = mod(3, 2) = 1. %e A117872 a(4) = mod(T(T(T(T(2)))),2) = mod(231,2) = 1. %o A117872 (Haskell) %o A117872 a117872 = flip mod 2 . a007501 -- _Reinhard Zumkeller_, Aug 15 2013 %o A117872 (PARI) list(n)=my(t=Mod(2,2*2^n)); concat(0, vector(n,i, t*=t+1; t=Mod(lift(t)/2, t.mod/2); lift(Mod(t,2)))) \\ _Charles R Greathouse IV_, Aug 22 2013 %Y A117872 Cf. A007501, A126768 %K A117872 easy,nonn %O A117872 0,1 %A A117872 _Greg Huber_, May 13 2006, Feb 16 2007 %E A117872 More terms from _Greg Huber_, Feb 16 2007 %E A117872 a(38) and on corrected by _Greg Huber_, Aug 21 2013