OFFSET
1,3
LINKS
FORMULA
sum(k=1, n, a(k)) seems to be asymptotic to c*n*log(n) with c=0.7...
G.f.: sum(k>=1, 1/(1-x^sigma(k))).
a(n) = Sum_{k=1..n} (1 - ceiling(n/sigma(k)) + floor(n/sigma(k))). - Wesley Ivan Hurt, Apr 21 2023
MATHEMATICA
Table[Length[Select[Range[n], Divisible[n, DivisorSigma[1, #]]&]], {n, 1, 100}] (* Vaclav Kotesovec, Feb 16 2019 *)
PROG
(PARI) a(n)=sum(i=1, n, if(n%sigma(i), 0, 1))
(PARI) a(n)=if(n<1, 0, polcoeff(sum(k=1, n, 1/(1-x^sigma(k)), x*O(x^sigma(n))), n))
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Sep 28 2002
STATUS
approved