A071189 - OEIS

%I #23 Mar 24 2024 03:55:31

%S 1,3,2,7,2,2,2,3,13,2,2,2,2,2,2,31,2,3,2,2,2,2,2,2,31,2,2,2,2,2,2,3,2,

%T 2,2,7,2,2,2,2,2,2,2,2,2,2,2,2,3,3,2,2,2,2,2,2,2,2,2,2,2,2,2,127,2,2,

%U 2,2,2,2,2,3,2,2,2,2,2,2,2,2,11,2,2,2,2,2,2,2

%N Smallest prime factor of sum of divisors of n.

%H Antti Karttunen, <a href="/A071189/b071189.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A020639(A000203(n)).

%F a(n) = 2 if and only if n is in A028983. - _Amiram Eldar_, Mar 24 2024

%t Table[FactorInteger[DivisorSigma[1,n]][[1,1]],{n,90}] (* _Harvey P. Dale_, May 15 2011 *)

%o (PARI) A071189(n) = if(1==n, n, my(f = factor(sigma(n))); vecmin(f[, 1])); \\ _Antti Karttunen_, Jul 24 2017

%o (PARI) first(n) = {my(v = vector(n, i, 2), sq = List()); for(i=1, sqrtint(n), listput(sq, i^2); listput(sq, 2*i^2)); listsort(sq); v[1]=1; for(i=2, #sq, if(sq[i]>n,break); v[sq[i]] = factor(sigma(sq[i]))[, 1]~[1]);v} \\ _David A. Corneth_, Jul 24 2017

%Y Cf. A000203, A020639, A028982, A028983, A071187, A071190.

%K nonn

%O 1,2

%A _Reinhard Zumkeller_, May 15 2002