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 A143731 #15 Aug 05 2021 01:31:06
%S A143731 0,0,0,0,0,1,0,0,0,1,0,1,0,1,1,0,0,1,0,1,1,1,0,1,0,1,0,1,0,1,0,0,1,1,
%T A143731 1,1,0,1,1,1,0,1,0,1,1,1,0,1,0,1,1,1,0,1,1,1,1,1,0,1,0,1,1,0,1,1,0,1,
%U A143731 1,1,0,1,0,1,1,1,1,1,0,1,0,1,0,1,1,1,1
%N A143731 Characteristic function of numbers with at least two distinct prime factors (A024619).
%C A143731 a(n) = 1 iff n is divisible by at least two distinct primes; otherwise 0.
%H A143731 Reikku Kulon, <a href="/A143731/b143731.txt">Table of n, a(n) for n = 1..10000</a>
%H A143731 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>
%F A143731 a(n) = abs(A079872(n)) = A057427(A079275(n)).
%F A143731 For n>1: a(n) = 1 - floor(1/A001221(n)). - _Enrique Pérez Herrero_, Aug 13 2012
%t A143731 Table[Boole[PrimeNu[n]>1],{n,1,200}] (* _Enrique Pérez Herrero_, Aug 13 2012 *)
%Y A143731 Cf. A024619, A079275, A079872, A000961, A001222, A091304, A073093, A001221.
%Y A143731 Cf. A215480.
%K A143731 easy,nonn
%O A143731 1,1
%A A143731 _Reikku Kulon_, Aug 30 2008