OFFSET
1,5
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..10000
FORMULA
From Amiram Eldar, Nov 17 2023: (Start)
a(n) = Sum_{d|n} Kronecker(-21, d).
Multiplicative with a(p^e) = 1 if Kronecker(-17, p) = 0 (p = 3 or 7), a(p^e) = (1+(-1)^e)/2 if Kronecker(-17, p) = -1, and a(p^e) = e+1 if Kronecker(-17, p) = 1.
Asymptotic mean: Limit_{m->oo} (1/m) * Sum_{k=1..m} a(k) = 4*Pi/(3*sqrt(21)) = 0.914068... . (End)
MATHEMATICA
a[n_] := If[n < 0, 0, DivisorSum[n, KroneckerSymbol[-21, #] &]]; Table[a[n], {n, 1, 100}] (* G. C. Greubel, Jul 17 2018 *)
PROG
(PARI) my(m = -21); direuler(p=2, 101, 1/(1-(kronecker(m, p)*(X-X^2))-X))
(PARI) a(n) = sumdiv(n, d, kronecker(-21, d)); \\ Amiram Eldar, Nov 17 2023
CROSSREFS
KEYWORD
nonn,easy,mult
AUTHOR
STATUS
approved