cp's OEIS Frontend

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

%I A179931 #31 Aug 02 2019 21:32:22
%S A179931 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 A179931 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 A179931 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 A179931 a(n) = gcd(sigma(n), sigma_2(n)).
%C A179931 A006530(a(n)) = A082066(n). - _Reinhard Zumkeller_, Jul 10 2011, the latter A-number corrected by _Antti Karttunen_, May 22 2017.
%C A179931 Not multiplicative: a(2)*a(19) <> a(38). - _R. J. Mathar_, Oct 08 2011
%H A179931 Reinhard Zumkeller, <a href="/A179931/b179931.txt">Table of n, a(n) for n = 1..10000</a>
%p A179931 A179931 := proc(n) igcd( numtheory[sigma](n), numtheory[sigma][2](n)) ; end proc:
%p A179931 seq(A179931(n),n=1..100) ;
%t A179931 Table[GCD @@ Map[DivisorSigma[#, n] &, {1, 2}], {n, 100}] (* _Michael De Vlieger_, May 22 2017 *)
%o A179931 (PARI) a(n)=gcd(sigma(n),sigma(n,2)) \\ _Charles R Greathouse IV_, Feb 14 2013
%Y A179931 Cf. A179930, A000203, A001157, A082066, A009194.
%K A179931 nonn
%O A179931 1,3
%A A179931 _N. J. A. Sloane_, Jul 09 2011, following a suggestion from _R. J. Mathar_