A071190 Greatest prime factor of sum of divisors of n, for n >= 2; a(1) = 1.

1, 3, 2, 7, 3, 3, 2, 5, 13, 3, 3, 7, 7, 3, 3, 31, 3, 13, 5, 7, 2, 3, 3, 5, 31, 7, 5, 7, 5, 3, 2, 7, 3, 3, 3, 13, 19, 5, 7, 5, 7, 3, 11, 7, 13, 3, 3, 31, 19, 31, 3, 7, 3, 5, 3, 5, 5, 5, 5, 7, 31, 3, 13, 127, 7, 3, 17, 7, 3, 3, 3, 13, 37, 19, 31, 7, 3, 7, 5, 31

Offset: 1

Author

Reinhard Zumkeller, May 15 2002

Keywords

• nonn

Comments

a(n) is the greatest noncomposite factor of the sum of divisors of n. (Cf. A008578.) - Omar E. Pol, Feb 11 2018

Links

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

Crossrefs

Cf. A000203, A006530, A071188, A071189.

Programs

Mathematica

Table[FactorInteger[DivisorSigma[1,n]][[-1,1]],{n,80}] (* Harvey P. Dale, Nov 18 2014 *)

PARI

a(n) = if (n==1, 1, my(f = factor(sigma(n))); f[#f~,1]); \\ Michel Marcus, Sep 21 2014

Formula

a(n) = A006530(A000203(n)).

Extensions

Name clarified by David A. Corneth and Omar E. Pol, Feb 11 2018