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A285906
Numbers n such that sigma(n)/usigma(n) > sigma(m)/usigma(m) for all m < n, where sigma(n) is the sum of divisors of n (A000203) and usigma(n) is the sum of unitary divisors of n (A034448).
5
1, 4, 8, 16, 32, 64, 72, 144, 216, 288, 432, 864, 1728, 2592, 3456, 3600, 5184, 7200, 10800, 21600, 43200, 64800, 86400, 108000, 129600, 216000, 259200, 324000, 432000, 518400, 529200, 648000, 1058400, 2116800, 3175200, 4233600, 5292000, 6350400, 10584000
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
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
Sequence in context: A298356 A036313 A121986 * A172042 A352918 A145108
KEYWORD
nonn
AUTHOR
Amiram Eldar, Apr 28 2017
STATUS
approved