A023884 Average of divisors except itself is an integer.

2, 3, 5, 6, 7, 9, 11, 13, 15, 17, 19, 23, 25, 29, 30, 31, 33, 37, 41, 43, 44, 47, 49, 51, 53, 59, 61, 67, 69, 71, 73, 79, 81, 83, 87, 89, 91, 97, 101, 103, 107, 109, 113, 114, 117, 120, 121, 123, 124, 127, 131, 133, 135, 137, 139, 141, 149, 151, 157, 159

Offset: 1

Author

Olivier Gérard

Keywords

• nonn

Comments

All the prime numbers (A000040) are in this sequence. But there are other numbers (A023885). - Daniel Lignon, Feb 26 2015

Links

• Daniel Lignon, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000040, A023885.

Programs

Maple

q:=n->(irem(numtheory[sigma](n)-n,numtheory[tau](n)-1)=0):
select(q,[$2..159])[]; # Lorenzo Sauras Altuzarra, Feb 01 2023

Mathematica

Select[Range[2000],IntegerQ[Mean[Most[Divisors[#]]]]&] (* Daniel Lignon, Feb 26 2015 *)

PARI

isok(n) = (n != 1) && !((sigma(n) - n) % (numdiv(n)-1)); \\ Michel Marcus, Mar 01 2015