A023885 Nonprimes whose average of proper divisors is an integer.

6, 9, 15, 25, 30, 33, 44, 49, 51, 69, 81, 87, 91, 114, 117, 120, 121, 123, 124, 133, 135, 141, 159, 164, 169, 170, 177, 182, 207, 213, 217, 244, 247, 249, 252, 259, 267, 270, 273, 276, 282, 284, 289, 301, 303, 306, 310, 320, 321, 339, 343, 345

Offset

1,1

Links

Jon E. Schoenfield, Table of n, a(n) for n = 1..10000 (first 1000 terms from Vincenzo Librandi)

Example

4 is not a term: its proper divisors are 1 and 2, and their average is (1+2)/2 = 3/2.
6 is a term: its proper divisors are 1, 2, and 3, and their average is (1+2+3)/3 = 6/3 = 2.

Mathematica

Select[Range[400], !PrimeQ[#]&&IntegerQ[Mean[Most[ Divisors[ #]]]]&] (* Harvey P. Dale, Jun 28 2011 *)

Prog

(PARI) isok(n) = (n != 1) && !isprime(n) && !((sigma(n) - n) % (numdiv(n)-1)); \\ Michel Marcus, Mar 28 2021

Crossrefs

Cf. A023884.

Sequence in context: A075278 A316048 A102971 * A031209 A271826 A242183

Adjacent sequences: A023882 A023883 A023884 * A023886 A023887 A023888

Keyword

nonn

Author

Olivier Gérard

Status

approved