A071189 Smallest prime factor of sum of divisors of n.

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, 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, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2, 2, 11, 2, 2, 2, 2, 2, 2, 2

Offset

1,2

Links

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

Formula

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

a(n) = 2 if and only if n is in A028983. - Amiram Eldar, Mar 24 2024

Mathematica

Table[FactorInteger[DivisorSigma[1, n]][[1, 1]], {n, 90}] (* Harvey P. Dale, May 15 2011 *)

Prog

(PARI) A071189(n) = if(1==n, n, my(f = factor(sigma(n))); vecmin(f[, 1])); \\ Antti Karttunen, Jul 24 2017
(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

Crossrefs

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

Sequence in context: A085594 A085587 A326607 * A137822 A300845 A344766

Adjacent sequences: A071186 A071187 A071188 * A071190 A071191 A071192

Keyword

nonn

Author

Reinhard Zumkeller, May 15 2002

Status

approved