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

1,2

Links

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

Formula

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

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

Mathematica

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

Prog

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

Crossrefs

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.

Sequence in context: A053815 A194446 A251758 * A326584 A166333 A322966

Adjacent sequences: A250477 A250478 A250479 * A250481 A250482 A250483

Keyword

nonn

Author

Antti Karttunen & Michel Lagneau, Dec 09 2014

Status

approved