A088739 - OEIS

%I #14 Feb 16 2015 16:17:02

%S 4,6,4,9,10,4,14,15,4,6,4,21,22,4,25,26,9,4,6,4,33,34,35,4,38,39,4,6,

%T 4,9,46,4,49,10,51,4,6,55,4,57,58,4,62,9,4,65,6,4,69,10,4,74,15,4,77,

%U 6,4,9,82,4,85,86,87,4,6,91,4,93,94,95,4,14,9,4,6,4,15,106,4,10,111,4

%N Smallest semiprime divisor of n-th composite number.

%C This is to semiprimes A001358 as A052369 is to primes A000040. - _Jonathan Vos Post_, Jan 11 2011

%H Zak Seidov, <a href="/A088739/b088739.txt">Table of n, a(n) for n = 1..1000</a>

%t semiPrimeQ[n_] := Plus @@ Last /@ FactorInteger@ n == 2; f[n_] := Min@ Select[ Divisors@ n, semiPrimeQ] /. {\[Infinity] -> 0}; f /@ Rest@ Select[ Range@ 112, !PrimeQ@ # &]

%t Reap[Do[If[! PrimeQ[n], Sow[Select[Divisors[n], PrimeOmega[#] == 2 &][[1]]]],{n, 4, 1200}]][[2, 1]] (* _Zak Seidov_, Mar 18 2013 *)

%Y Cf. A088740, A002808, A001358.

%K nonn

%O 1,1

%A _Reinhard Zumkeller_, Oct 12 2003