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A023885
Nonprimes whose average of proper divisors is an integer.
2
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
KEYWORD
nonn
STATUS
approved