A230608 - OEIS

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A230608

Numbers with abundancy 4 <= sigma(n)/n < 5.

27720, 30240, 32760, 50400, 55440, 60480, 65520, 75600, 83160, 85680, 90720, 95760, 98280, 100800, 105840, 110880, 115920, 120120, 120960, 128520, 131040, 138600, 141120, 143640, 151200, 163800, 166320, 171360, 176400, 180180, 181440, 184800, 191520, 194040

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OFFSET

1,1

COMMENTS

A subsequence of A023198 (numbers with abundancy >= 4). It differs from A023198 from a(31093) on: The term A023198(31093) = 122522400 = A023199(5) = A215264(1) is not in this sequence. It excludes all terms of A215264, but also the 5-perfect numbers A046060, which are neither in this sequence nor in A215264. [Corrected by M. F. Hasler, Dec 05 2013]

A108775(a(n)) = 4.

There are 31092 terms less than 122522399. - T. D. Noe, Dec 04 2013

LINKS

Jaroslav Krizek, Table of n, a(n) for n = 1..1000

Eric Weisstein's World of Mathematics, Abundancy

Eric Weisstein's World of Mathematics, Abundant Number

EXAMPLE

27720 is in sequence because sigma(27720) / 27720 = 112320 / 27720 = 4.0519....

MATHEMATICA

Select[Range[200000], 4 <= DivisorSigma[1, #]/# < 5 &] (* T. D. Noe, Dec 04 2013 *)

CROSSREFS

Cf. A000203, A023198, A023199, A108775.