OFFSET
1,4
COMMENTS
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..10000
G. P. Michon, Multiplicative functions.
FORMULA
Multiplicative function with a(p^e)=-(-2)^(e-1) for any prime p and any positive exponent e.
a(n) = n/2 when n is a power of 4 (A000302).
Dirichlet g.f.: Product_{p prime} ((p^s + 1)/(p^s + 2)). - Amiram Eldar, Oct 26 2023
MAPLE
A162512 := proc(n)
local a, f;
a := 1;
for f in ifactors(n)[2] do
a := -a*(-2)^(op(2, f)-1) ;
end do:
return a;
end proc:
seq(A162512(n), n=1..100) ; # R. J. Mathar, May 20 2017
MATHEMATICA
b[n_] := (-1)^(PrimeOmega[n] - PrimeNu[n]);
a[n_] := a[n] = If[n == 1, 1, -Sum[b[n/d] a[d], {d, Most@ Divisors[n]}]];
Array[a, 100] (* Jean-François Alcover, Feb 17 2020 *)
PROG
(PARI) a(n) = my(f=factor(n)); for(i=1, #f~, f[i, 1]=-(-2)^(f[i, 2]-1); f[i, 2]=1); factorback(f); \\ Michel Marcus, May 20 2017
(Python)
from sympy import factorint
from operator import mul
def a(n):
f=factorint(n)
return 1 if n==1 else reduce(mul, [-(-2)**(f[i] - 1) for i in f]) # Indranil Ghosh, May 20 2017
(Scheme) (define (A162512 n) (if (= 1 n) n (* (- (expt -2 (- (A067029 n) 1))) (A162512 (A028234 n))))) ;; Antti Karttunen, May 20 2017, after the given multiplicative formula.
CROSSREFS
KEYWORD
easy,mult,sign
AUTHOR
Gerard P. Michon, Jul 05 2009, Jul 06 2009
STATUS
approved