OFFSET
1,1
COMMENTS
2 is the only squarefree term in this sequence. - Amiram Eldar, Jul 13 2024
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..114 (terms 1..69 from Michel Marcus)
EXAMPLE
For k=2, sigma(k)/k = 3/2 with both numerator and denominator prime.
MATHEMATICA
a247086Q[n_] := Block[{f = DivisorSigma[1, n]/n}, And[PrimeQ@ Numerator@ f, PrimeQ@ Denominator@ f]]; a247086[n_] := Select[Range@ n, a247086Q@ # &]; a247086[10^6] (* Michael De Vlieger, Jan 11 2015 *)
PROG
(PARI) isok(n) = my(ab = sigma(n)/n); isprime(numerator(ab)) && isprime(denominator(ab));
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Jan 10 2015
STATUS
approved