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A250480 a(1) = 0, and for n > 1: if n is a prime, a(n) = n, otherwise a(n) = A020639(n) - 1, where A020639(n) gives the least prime dividing n.

%I A250480 #17 Apr 14 2023 12:06:38
%S A250480 0,2,3,1,5,1,7,1,2,1,11,1,13,1,2,1,17,1,19,1,2,1,23,1,4,1,2,1,29,1,31,
%T A250480 1,2,1,4,1,37,1,2,1,41,1,43,1,2,1,47,1,6,1,2,1,53,1,4,1,2,1,59,1,61,1,
%U A250480 2,1,4,1,67,1,2,1,71,1,73,1,2,1,6,1,79,1,2,1,83,1,4,1,2,1,89
%N A250480 a(1) = 0, and for n > 1: if n is a prime, a(n) = n, otherwise a(n) = A020639(n) - 1, where A020639(n) gives the least prime dividing n.
%H A250480 Antti Karttunen, <a href="/A250480/b250480.txt">Table of n, a(n) for n = 1..10000</a>
%F A250480 a(n) = n for all prime n, and a(n) = A020639(n) - 1 for all composite n.
%F A250480 a(n) = A020639(n) - A005171(n).
%t A250480 Join[{0},Table[If[PrimeQ[n],n,FactorInteger[n][[1,1]]-1],{n,2,100}]] (* _Harvey P. Dale_, Apr 14 2023 *)
%o A250480 (Scheme) (define (A250480 n) (if (prime? n) n (- (A020639 n) 1)))
%Y A250480 Cf. also A005171 (characteristic function for nonprimes), A020639 (the smallest prime divisor of n).
%Y A250480 Differs from A251758 for the first time at n = 2431 = 11*13*17, where a(2431) = 10, while A251758(2431) = 9.
%K A250480 nonn
%O A250480 1,2
%A A250480 _Antti Karttunen_ & _Michel Lagneau_, Dec 09 2014