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 A179081 #10 Jan 07 2018 10:12:33 %S A179081 0,1,0,1,0,1,0,1,0,1,1,0,1,0,1,0,1,0,1,0,0,1,0,1,0,1,0,1,0,1,1,0,1,0, %T A179081 1,0,1,0,1,0,0,1,0,1,0,1,0,1,0,1,1,0,1,0,1,0,1,0,1,0,0,1,0,1,0,1,0,1, %U A179081 0,1,1,0,1,0,1,0,1,0,1,0,0,1,0,1,0,1,0,1,0,1,1,0,1,0,1,0,1,0,1,0,1,0,1,0,1 %N A179081 Parity of sum of digits of n. %C A179081 a(n) = A000035(A007953(n)); %C A179081 characteristic function of numbers with an odd sum of digits in their decimal representation: %C A179081 a(A054684(n)) = 1; a(A054683(n)) = 0; %C A179081 a(A179083(n)) = a(A179085(n)) = 1; %C A179081 a(A179082(n)) = a(A179084(n)) = 0. %H A179081 R. Zumkeller, <a href="/A179081/b179081.txt">Table of n, a(n) for n = 0..10000</a> %H A179081 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a> %F A179081 a(n) = if n<10 then (n mod 2) else a([n/10]) XOR (n mod 2). %e A179081 a(789) = (7+8+9) mod 2 = 0; %e A179081 a(790) = (7+9+0) mod 2 = 0; %e A179081 a(791) = (7+9+1) mod 2 = 1. %t A179081 Array[Mod[Total[IntegerDigits[#]],2]&,110,0] (* _Harvey P. Dale_, Feb 20 2013 *) %o A179081 (PARI) a(n) = vecsum(digits(n)) % 2 \\ _Jeppe Stig Nielsen_, Jan 06 2018 %K A179081 base,nonn %O A179081 0,1 %A A179081 _Reinhard Zumkeller_, Jun 28 2010