A046666 a(n) = n - (smallest prime dividing n).

0, 0, 2, 0, 4, 0, 6, 6, 8, 0, 10, 0, 12, 12, 14, 0, 16, 0, 18, 18, 20, 0, 22, 20, 24, 24, 26, 0, 28, 0, 30, 30, 32, 30, 34, 0, 36, 36, 38, 0, 40, 0, 42, 42, 44, 0, 46, 42, 48, 48, 50, 0, 52, 50, 54, 54, 56, 0, 58, 0, 60, 60, 62, 60, 64, 0, 66, 66, 68, 0, 70, 0

Offset: 2

Author

N. J. A. Sloane

Keywords

• nonn

Comments

From Reinhard Zumkeller, Jul 08 2010: (Start)

a(n)=0 iff n is not composite;

for composite n: a(n) = max(m: m < n and gcd(m,n) > 1). (End)

Links

• Antti Karttunen, Table of n, a(n) for n = 2..16385

Crossrefs

Cf. A020639, A061228.

Programs

Maple

with(numtheory):
a:= n-> n-min(factorset(n)[]):
seq(a(n), n=2..100);  # Alois P. Heinz, Jan 07 2015

Mathematica

Table[n-First@(First/@FactorInteger[n]),{n,2,200}] (* Vladimir Joseph Stephan Orlovsky, Apr 08 2011 *)

PARI

a(n) = n - factor(n)[1,1]; \\ Michel Marcus, Jan 07 2015

Formula

a(n) = n - A020639(n). - Michel Marcus, Jan 07 2015