A088739 Smallest semiprime divisor of n-th composite number.

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, 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, 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

Offset: 1

Author

Reinhard Zumkeller, Oct 12 2003

Keywords

• nonn

Comments

This is to semiprimes A001358 as A052369 is to primes A000040. - Jonathan Vos Post, Jan 11 2011

Links

• Zak Seidov, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A088740, A002808, A001358.

Programs

Mathematica

semiPrimeQ[n_] := Plus @@ Last /@ FactorInteger@ n == 2; f[n_] := Min@ Select[ Divisors@ n, semiPrimeQ] /. {\[Infinity] -> 0}; f /@ Rest@ Select[ Range@ 112, !PrimeQ@ # &]
Reap[Do[If[! PrimeQ[n], Sow[Select[Divisors[n], PrimeOmega[#] == 2 &][[1]]]],{n, 4, 1200}]][[2, 1]] (* Zak Seidov, Mar 18 2013 *)