A020486 - OEIS

A020486

Average of squares of divisors is an integer: numbers k such that sigma_0(k) divides sigma_2(k).

1, 3, 4, 5, 7, 11, 12, 13, 15, 17, 19, 20, 21, 23, 25, 27, 28, 29, 31, 33, 35, 37, 39, 41, 43, 44, 47, 48, 49, 51, 52, 53, 55, 57, 59, 60, 61, 65, 67, 68, 69, 71, 73, 75, 76, 77, 79, 83, 84, 85, 87, 89, 91, 92, 93, 95, 97, 100, 101, 103, 105, 107, 108, 109, 111, 112, 113, 115, 116

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OFFSET

1,2

COMMENTS

If sigma_2(k)/sigma_0(k) is a square then k is an RMS-number (A140480). - Ctibor O. Zizka, Jul 14 2008

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1000 from T. D. Noe)

FORMULA

A001157(k) mod A000005(k) = 0. - Reinhard Zumkeller, Jan 15 2013

MATHEMATICA

Select[Range[150], Divisible[DivisorSigma[2, #], DivisorSigma[0, #]]&] (* Harvey P. Dale, May 03 2011 *)

PROG