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A074754
Number of integers k such that sigma(k) divides n.
2
1, 1, 2, 2, 1, 3, 2, 3, 2, 1, 1, 6, 2, 3, 3, 3, 1, 5, 1, 3, 3, 1, 1, 10, 1, 2, 2, 5, 1, 5, 3, 5, 2, 1, 2, 9, 1, 2, 4, 5, 1, 8, 1, 3, 3, 1, 1, 13, 2, 1, 2, 3, 1, 7, 1, 8, 3, 1, 1, 12, 1, 4, 4, 5, 2, 3, 1, 3, 2, 3, 1, 18, 1, 2, 3, 3, 2, 6, 1, 7, 2, 1, 1, 15, 1, 1, 2, 4, 1, 10, 4, 2, 5, 1, 1, 19, 1, 5, 2, 3, 1
OFFSET
1,3
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
Sequence in context: A241605 A128180 A209279 * A322529 A349526 A329949
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Sep 28 2002
STATUS
approved