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 A063918 #10 Jun 25 2018 02:56:23
%S A063918 1,0,0,2,0,2,0,2,3,2,0,2,0,2,3,2,0,2,0,2,3,2,0,2,5,2,3,2,0,2,0,2,3,2,
%T A063918 5,2,0,2,3,2,0,2,0,2,3,2,0,2,7,2,3,2,0,2,5,2,3,2,0,2,0,2,3,2,5,2,0,2,
%U A063918 3,2,0,2,0,2,3,2,7,2,0,2,3,2,0,2,5,2,3,2,0,2,7,2,3,2,5,2,0,2,3,2,0,2,0,2,3
%N A063918 a(1) = 1 and - applying the sieve of Eratosthenes - for n > 1: a(n) = if n is prime then 0 else the first prime p which marks n as composite.
%C A063918 k > 1: a(k*2) = 2, as all even numbers > 2 are marked by 2; for all primes p: a(p^k) = p and a(i) < p for i < p^2.
%H A063918 Harry J. Smith, <a href="/A063918/b063918.txt">Table of n, a(n) for n = 1..1000</a>
%o A063918 (PARI) { for (n=1, 1000, if (n==1, p=1, if (isprime(n), p=0, p=1; until (n%p == 0, p=nextprime(p + 1)))); write("b063918.txt", n, " ", p) ) } \\ _Harry J. Smith_, Sep 02 2009
%Y A063918 Cf. A055396, A020639.
%K A063918 nonn
%O A063918 1,4
%A A063918 _Reinhard Zumkeller_, Sep 04 2001