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

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.

Cf. A005100 (deficient numbers with abundancy 1 <= a < 2),

Cf. A204829 (numbers with abundancy 2 <= a < 3),

Cf. A204828 (abundant numbers with abundancy 3 <= a < 4).

Cf. A215264 (abundant numbers with abundancy > 5).

Sequence in context: A079321 A251235 A023198 * A204831 A345153 A190111

Adjacent sequences: A230605 A230606 A230607 * A230609 A230610 A230611

Keyword

nonn

Author

Jaroslav Krizek, Nov 29 2013

Extensions

Corrected and edited by M. F. Hasler, Dec 05 2013

Status

approved