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%I #10 Jun 25 2018 02:56:23
%S 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 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 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 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 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 Harry J. Smith, <a href="/A063918/b063918.txt">Table of n, a(n) for n = 1..1000</a>
%o (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 Cf. A055396, A020639.
%K nonn
%O 1,4
%A _Reinhard Zumkeller_, Sep 04 2001
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