A251758 -id:A251758 - cp's OEIS frontend

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.

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, 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, 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

Offset: 1

Antti Karttunen & Michel Lagneau, Dec 09 2014

nonn

Antti Karttunen, Table of n, a(n) for n = 1..10000

Cf. also A005171 (characteristic function for nonprimes), A020639 (the smallest prime divisor of n).

Differs from A251758 for the first time at n = 2431 = 11*13*17, where a(2431) = 10, while A251758(2431) = 9.

Join[{0},Table[If[PrimeQ[n],n,FactorInteger[n][[1,1]]-1],{n,2,100}]] (* Harvey P. Dale, Apr 14 2023 *)

(define (A250480 n) (if (prime? n) n (- (A020639 n) 1)))

a(n) = n for all prime n, and a(n) = A020639(n) - 1 for all composite n.

a(n) = A020639(n) - A005171(n).

cp's OEIS Frontend

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.

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