A179931 a(n) = gcd(sigma(n), sigma_2(n)).

1, 1, 2, 7, 2, 2, 2, 5, 13, 2, 2, 14, 2, 2, 4, 31, 2, 13, 2, 42, 4, 2, 2, 10, 31, 2, 20, 14, 2, 4, 2, 21, 4, 2, 4, 91, 2, 10, 4, 10, 2, 4, 2, 42, 26, 2, 2, 62, 57, 93, 4, 14, 2, 20, 4, 10, 20, 10, 2, 84, 2, 2, 26, 127, 4, 4, 2, 42, 4, 4, 2, 65, 2, 2, 62, 14, 4, 4, 2, 62, 121, 2, 2, 28, 4, 2, 20, 10, 2, 26, 4, 42, 4, 2, 4, 42, 2, 57, 26, 217

Offset: 1

Author

N. J. A. Sloane, Jul 09 2011, following a suggestion from R. J. Mathar

Keywords

• nonn

Comments

A006530(a(n)) = A082066(n). - Reinhard Zumkeller, Jul 10 2011, the latter A-number corrected by Antti Karttunen, May 22 2017.

Not multiplicative: a(2)*a(19) <> a(38). - R. J. Mathar, Oct 08 2011

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A179930, A000203, A001157, A082066, A009194.

Programs

Maple

A179931 := proc(n) igcd( numtheory[sigma](n), numtheory[sigma][2](n)) ; end proc:
seq(A179931(n),n=1..100) ;

Mathematica

Table[GCD @@ Map[DivisorSigma[#, n] &, {1, 2}], {n, 100}] (* Michael De Vlieger, May 22 2017 *)

PARI

a(n)=gcd(sigma(n),sigma(n,2)) \\ Charles R Greathouse IV, Feb 14 2013