OFFSET
1,3
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Moebius Transform.
Eric Weisstein's World of Mathematics, Totient Function.
FORMULA
G.f.: -Sum_{k>=1} mu(7 * k) * x^k / (1 - x^k)^2, where mu() is the Moebius function (A008683).
From Amiram Eldar, Dec 17 2022: (Start)
Multiplicative with a(7^e) = 7^e, and a(p^e) = (p-1)*p^(e-1) if p != 7.
Dirichlet g.f.: zeta(s-1)/(zeta(s)*(1-1/7^s)).
Sum_{k=1..n} a(k) ~ (49/(16*Pi^2)) * n^2. (End)
MATHEMATICA
Array[EulerPhi[7 #]/6 &, 79] (* Michael De Vlieger, Dec 16 2022 *)
PROG
(PARI) a(n) = eulerphi(7*n)/6;
(PARI) my(N=80, x='x+O('x^N)); Vec(-sum(k=1, N, moebius(7*k)*x^k/(1-x^k)^2))
CROSSREFS
KEYWORD
nonn,easy,mult
AUTHOR
Seiichi Manyama, Dec 16 2022
STATUS
approved