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A071189
Smallest prime factor of sum of divisors of n.
8
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
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
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, May 15 2002
STATUS
approved