OFFSET
1,6
COMMENTS
See Comments on denominators in A326690.
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..16384
Antti Karttunen, Data supplement: n, a(n) computed for n = 1..65537
Wikipedia, Giuga number
FORMULA
a(p) = 0 if p is a prime.
a(g) = 1 if g is a known Giuga number (see my 2nd comment in A007850).
EXAMPLE
-1/1, 0/1, 0/1, 1/4, 0/1, 2/3, 0/1, 3/8, 2/9, 3/5, 0/1, 3/4, 0/1, 4/7, 7/15, 7/16, 0/1, 7/9, 0/1, 13/20, 3/7, 6/11, 0/1, 19/24, 4/25, 7/13, 8/27, 17/28, 0/1, 1/1
MATHEMATICA
PrimeFactors[n_] := Select[Divisors[n], PrimeQ];
g[n_] := Numerator[Sum[1/p, {p, PrimeFactors[n]}] - 1/n];
Table[ g[n], {n, 100}]
PROG
(PARI) a(n) = numerator(sumdiv(n, d, isprime(d)/d) - 1/n); \\ Michel Marcus, Jul 19 2019
CROSSREFS
KEYWORD
sign,frac
AUTHOR
Jonathan Sondow, Jul 18 2019
STATUS
approved