OFFSET
1,2
COMMENTS
This sequence is infinite. The smallest values of n for which sigma(n)/usigma(n) > 2, 3 and 4 are a(7), a(19), and a(44).
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..56
EXAMPLE
sigma(72)=195, usigma(72)=90, and their ratio 195/90=13/6 is higher than sigma(m)/usigma(m) for all m<72, thus 72 is in this sequence.
MATHEMATICA
usigma[n_] := If[n == 1, 1, Times @@ (1 + Power @@@ FactorInteger[n])];
a={}; rmax=0; Do[r=DivisorSigma[1, n]/usigma[n]; If[r>rmax, AppendTo[a, n]; rmax=r], {n, 3000}]; a
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Apr 28 2017
STATUS
approved