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

Author

Olivier Gérard

Keywords

• nonn

Examples

			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.
		

Links

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

Crossrefs

Cf. A023884.

Programs

Mathematica

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

PARI

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