login
a(n) = gcd(sigma(n), sigma_2(n)).
7

%I #31 Aug 02 2019 21:32:22

%S 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,

%T 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,

%U 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

%N a(n) = gcd(sigma(n), sigma_2(n)).

%C A006530(a(n)) = A082066(n). - _Reinhard Zumkeller_, Jul 10 2011, the latter A-number corrected by _Antti Karttunen_, May 22 2017.

%C Not multiplicative: a(2)*a(19) <> a(38). - _R. J. Mathar_, Oct 08 2011

%H Reinhard Zumkeller, <a href="/A179931/b179931.txt">Table of n, a(n) for n = 1..10000</a>

%p A179931 := proc(n) igcd( numtheory[sigma](n), numtheory[sigma][2](n)) ; end proc:

%p seq(A179931(n),n=1..100) ;

%t Table[GCD @@ Map[DivisorSigma[#, n] &, {1, 2}], {n, 100}] (* _Michael De Vlieger_, May 22 2017 *)

%o (PARI) a(n)=gcd(sigma(n),sigma(n,2)) \\ _Charles R Greathouse IV_, Feb 14 2013

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

%K nonn

%O 1,3

%A _N. J. A. Sloane_, Jul 09 2011, following a suggestion from _R. J. Mathar_