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A109510 Values of n such that sigma(n)-2n (the abundance of n) is a (nonnegative) square. 2
6, 12, 28, 70, 88, 108, 168, 220, 348, 496, 550, 580, 726, 748, 798, 910, 918, 952, 1032, 1120, 1200, 1240, 1504, 1644, 1672, 1840, 1888, 2140, 2205, 2392, 2470, 2472, 2814, 3190, 3222, 3580, 3804, 4030, 4648, 4732, 4758, 5032, 5670, 5830 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Abundance

EXAMPLE

12 is in the sequence since sigma(12) - 2*12 = 28 - 24 = 4 = 2^2 is a square.

MATHEMATICA

Select[Range[6000], IntegerQ@Sqrt[DivisorSigma[1, #] - 2 #] &] (* Amiram Eldar, Jun 22 2019 *)

PROG

(PARI) isok(n) = issquare(sigma(n) - 2*n); \\ Michel Marcus, Jun 22 2019

CROSSREFS

Cf. A000203, A000290, A023196, A033880.

Cf. A000396 (perfect numbers: a subsequence).

Sequence in context: A325812 A323652 A223346 * A034715 A294730 A079390

Adjacent sequences:  A109507 A109508 A109509 * A109511 A109512 A109513

KEYWORD

nonn

AUTHOR

Eric W. Weisstein, Jun 30 2005

EXTENSIONS

Name clarified by Amiram Eldar, Jun 22 2019

STATUS

approved

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Last modified July 6 05:58 EDT 2020. Contains 335475 sequences. (Running on oeis4.)