A069926 Number of k, 1<=k<=n, such that k divides sigma(k).

1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3

Offset: 1

Author

Benoit Cloitre, May 05 2002

Keywords

• easy
• nonn

Comments

a(n) is the number of multiply-perfect numbers (A007691) <= n. - Robert Israel, Aug 29 2018

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A007691.

Programs

PARI

for(n=1,150,print1(sum(i=1,n,if(sigma(i)%(i),0,1)),","))

Formula

a(n) = Card(k: 1<=k<=n : sigma(k) == 0 (mod k) ).