A023885 - OEIS

%I #32 Oct 09 2023 01:44:38

%S 6,9,15,25,30,33,44,49,51,69,81,87,91,114,117,120,121,123,124,133,135,

%T 141,159,164,169,170,177,182,207,213,217,244,247,249,252,259,267,270,

%U 273,276,282,284,289,301,303,306,310,320,321,339,343,345

%N Nonprimes whose average of proper divisors is an integer.

%H Jon E. Schoenfield, <a href="/A023885/b023885.txt">Table of n, a(n) for n = 1..10000</a> (first 1000 terms from Vincenzo Librandi)

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

%e 6 is a term: its proper divisors are 1, 2, and 3, and their average is (1+2+3)/3 = 6/3 = 2.

%t Select[Range[400],!PrimeQ[#]&&IntegerQ[Mean[Most[ Divisors[ #]]]]&] (* _Harvey P. Dale_, Jun 28 2011 *)

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

%Y Cf. A023884.

%K nonn

%O 1,1

%A _Olivier Gérard_